hypothesis on formation of planets

Planet Formation

The Milky Way alone probably contains hundreds of billions of planets, based on the thousands of exoplanets we’ve already identified. These planets share a history and origin with their host stars, and none of the star systems observed so far resemble the Solar System. Modern studies of planet formation include comparing exoplanetary systems, identification of protoplanetary disks around newborn stars, and computer models to trace the creation of planets from their origins in interstellar dust and gas.

Center for Astrophysics | Harvard & Smithsonian astronomers study the formation of planets:

Looking for complex organic molecules in protoplanetary disks. Astronomers use the Atacama Large Millimeter/submillimeter Array (ALMA), the CfA’s Submillimeter Array (SMA) , and other instruments capable of identifying light absorbed by these molecules. Knowing how these materials are distributed in a protoplanetary disk helps us determine how life could arise on planets. Complex Organic Molecules Discovered in Infant Star System

Searching for newborn planets in protoplanetary disks. Protoplanets gather material onto themselves from the protoplanetary disk, creating a gap. The position and size of these gaps can reveal if the new planets are rocky or gaseous, how massive they are tell us, and whether they migrate from their initial positions. However, not every gap contains a planet, so careful observation is needed to determine what’s going on. Disk Gaps Don't Always Signal Planets

Searching also for Earth-like planets in infant star systems. While exoplanet hunts mostly find planets close in to their stars, studies of protoplanets are more likely to identify planets farther out. That’s helpful if we want to find planets like ours, which orbits in the “habitable zone” where liquid water can potentially exist on the planet’s surface. A Planet Is Forming in an Earth-like Orbit around a Young Star

Modeling planet formation to infer exoplanet structure and composition. Exoplanets are too small for us to take images of them, much less study details of their interiors. However, theoretical models fill in that knowledge, using data from real planetary systems. That helps us understand “super-Earths”, a common type of rocky world larger than Earth that doesn’t exist in the Solar System. Earth-like Planets Have Earth-like Interiors

Studying the chemical composition of comets to understand planet formation. Comets are relics of the protoplanetary disk that gave birth to the Solar System. Astronomers compare the molecules observed in protoplanetary to those on the surface of comets to understand the primordial chemistry of newborn star systems. Astronomers Discover Traces of Methyl Chloride around Infant Stars and Nearby Comet

Participating in space probe missions to asteroids. Like comets, asteroids are leftovers from the formation of the Solar System, carrying chemical traces of the protoplanetary disk. The Origins, Spectral Interpretation, Resource Identification, Security, Regolith, Explorer (OSIRIS-REx) is designed to study the near-Earth asteroid Bennu and return a sample of its surface material to Earth. Asteroid Mission Will Carry Student X-ray Experiment

Protoplanetary Disks

Stars form from cold interstellar molecular clouds . As they collapse into protostars under the force of gravity, the remaining matter forms a spinning disk . Eventually the star stops accreting matter, leaving the disk in orbit around it. The leftover gas and dust inside that protoplanetary disk become the ingredients for planet formation.

For that reason, the chemical composition of the protoplanetary disk determines the composition of the eventual planets that form from it. Since the newborn star and its planets had a common origin in the cloud that made them, their history and composition are linked. Organic molecules present in the original molecular cloud become part of the protoplanetary disk and planets that form from it.

Astronomers have identified a large number of protoplanetary disks, including some with gaps that might reveal the presence of a planet being formed. The structure of these disks provides clues to where planets form, and whether they change orbits after formation.  

This artist's illustration compares the interior structures of Earth (left) with the exoplanet Kepler-93b (right). Even though the exoplanet is four times Earth's mass, research shows rocky planets all likely have the same internal structure.

Learning From Exoplanets and Their Host Stars

None of the thousands of exoplanetary systems discovered so far resemble the Solar System. While this is mostly because of the way we detect exoplanets, our understanding of planet formation has to account for the diversity of worlds we observe. Many exoplanets orbit closer to their host star than Mercury orbits the Sun, and quite a few star systems contain giant planets in the inner part of the system. In addition, several systems have multiple planets in orbits that are much closer together than any two planets in the Solar System.

Additionally, many known exoplanets orbit red dwarf stars, which are much less hot than the Sun. A number of these stars are also very active, producing violent flares that would likely affect the planet. Some other stars may have been too active during their early life to allow planets to form.

However, even the best observations of protoplanets and exoplanets aren’t good enough to reveal exactly how planets form and migrate. For that reason, astronomers develop computer simulations to understand how the protoplanetary disk produces the types of planets we observe, and how atoms and molecules are distributed to create the kinds of worlds we observe in the Solar System and beyond.

• How do stars and planets form and evolve?
• What conditions are necessary for life?
• Computational Astrophysics
• Planetary Systems
• Stellar Astronomy
• Radio and Geoastronomy
• Solar, Stellar, and Planetary Sciences

Related News

The case of the missing jupiters: gas giant planets are a no-show around small red stars, astronomers find potentially volcano-covered earth-size world, alien planet found spiraling to its doom around an aging star, it's a planet: new evidence of baby planet in the making, astronomers detect carbon dioxide on planet for the first time with jwst, case solved: missing carbon monoxide was hiding in the ice, incoming postdoc awarded 51 pegasi b fellowship, telescope to help tell the story of the universe, exoplanets in debris disks, astronomers may have discovered the first planet outside of our galaxy, from molecular cores to planet forming disks (c2d), telescopes and instruments, the submillimeter array - maunakea, hi.

• school Campus Bookshelves
• menu_book Bookshelves
• perm_media Learning Objects
• login Login
• how_to_reg Request Instructor Account
• hub Instructor Commons
• Download Page (PDF)
• Download Full Book (PDF)
• Periodic Table
• Physics Constants
• Scientific Calculator
• Reference & Cite
• Tools expand_more
• Readability

selected template will load here

This action is not available.

8.2: Origin of the Solar System—The Nebular Hypothesis

• Last updated
• Save as PDF
• Page ID 6889

• Chris Johnson, Matthew D. Affolter, Paul Inkenbrandt, & Cam Mosher
• Salt Lake Community College via OpenGeology

Our solar system formed at the same time as our Sun as described in the nebular hypothesis. The nebular hypothesis is the idea that a spinning cloud of dust made of mostly light elements, called a nebula, flattened into a protoplanetary disk, and became a solar system consisting of a star with orbiting planets [ 12 ]. The spinning nebula collected the vast majority of material in its center, which is why the sun Accounts for over 99% of the mass in our solar system.

Planet Arrangement and Segregation

As our solar system formed, the nebular cloud of dispersed particles developed distinct temperature zones. Temperatures were very high close to the center, only allowing condensation of metals and silicate minerals with high melting points. Farther from the Sun, the temperatures were lower, allowing the condensation of lighter gaseous molecules such as methane, ammonia, carbon dioxide, and water [ 13 ]. This temperature differentiation resulted in the inner four planets of the solar system becoming rocky, and the outer four planets becoming gas giants.

Both rocky and gaseous planets have a similar growth model. Particles of dust, floating in the disc were attracted to each other by static charges and eventually, gravity. As the clumps of dust became bigger, they interacted with each other—colliding, sticking, and forming proto-planets. The planets continued to grow over the course of many thousands or millions of years, as material from the protoplanetary disc was added. Both rocky and gaseous planets started with a solid core. Rocky planets built more rock on that core, while gas planets added gas and ice. Ice giants formed later and on the furthest edges of the disc, accumulating less gas and more ice. That is why the gas-giant planets Jupiter and Saturn are composed of mostly hydrogen and helium gas, more than 90%. The ice giants Uranus and Neptune are composed of mostly methane ices and only about 20% hydrogen and helium gases.

The planetary composition of the gas giants is clearly different from the rocky planets. Their size is also dramatically different for two reasons: First, the original planetary nebula contained more gases and ices than metals and rocks. There was abundant hydrogen, carbon, oxygen, nitrogen, and less silicon and iron, giving the outer planets more building material. Second, the stronger gravitational pull of these giant planets allowed them to collect large quantities of hydrogen and helium, which could not be collected by the weaker gravity of the smaller planets.

Jupiter’s massive gravity further shaped the solar system and growth of the inner rocky planets. As the nebula started to coalesce into planets, Jupiter’s gravity accelerated the movement of nearby materials, generating destructive collisions rather than constructively gluing material together [ 14 ]. These collisions created the asteroid belt, an unfinished planet, located between Mars and Jupiter. This asteroid belt is the source of most meteorites that currently impact the Earth. Study of asteroids and meteorites help geologist to determine the age of Earth and the composition of its core, mantle, and crust. Jupiter’s gravity may also explain Mars’ smaller mass, with the larger planet consuming material as it migrated from the inner to the outer edge of the solar system [ 15 ].

Pluto and Planet Definition

The outermost part of the solar system is known as the Kuiper belt, which is a scattering of rocky and icy bodies. Beyond that is the Oort cloud, a zone filled with small and dispersed ice traces. These two locations are where most comets form and continue to orbit, and objects found here have relatively irregular orbits compared to the rest of the solar system. Pluto, formerly the ninth planet, is located in this region of space. The XXVIth General Assembly of the International Astronomical Union (IAU) stripped Pluto of planetary status in 2006 because scientists discovered an object more massive than Pluto, which they named Eris. The IAU decided against including Eris as a planet, and therefore, excluded Pluto as well. The IAU narrowed the definition of a planet to three criteria:

• Enough mass to have gravitational forces that force it to be rounded
• Not massive enough to create a fusion
• Large enough to be in a cleared orbit, free of other planetesimals that should have been incorporated at the time the planet formed. Pluto passed the first two parts of the definition, but not the third. Pluto and Eris are currently classified as dwarf planets

12. Montmerle T, Augereau J-C, Chaussidon M, et al (2006) Solar System Formation and Early Evolution: the First 100 Million Years. In: From Suns to Life: A Chronological Approach to the History of Life on Earth. Springer New York, pp 39–95

13. Martin RG, Livio M (2012) On the evolution of the snow line in protoplanetary discs. Mon Not R Aston Soc Lett 425:L6–L9

14. Petit J-M, Morbidelli A, Chambers J (2001) The Primordial Excitation and Clearing of the Asteroid Belt. Icarus 153:338–347. https://doi.org/10.1006/icar.2001.6702

15. Walsh KJ, Morbidelli A, Raymond SN, et al (2011) A low mass for Mars from Jupiter’s early gas-driven migration. Nature 475:206–209

• school Campus Bookshelves
• menu_book Bookshelves
• perm_media Learning Objects
• login Login
• how_to_reg Request Instructor Account
• hub Instructor Commons
• Download Page (PDF)
• Download Full Book (PDF)
• Periodic Table
• Physics Constants
• Scientific Calculator
• Reference & Cite
• Tools expand_more
• Readability

selected template will load here

This action is not available.

14.4: Formation of the Solar System

• Last updated
• Save as PDF
• Page ID 64740

Learning Objectives

By the end of this section, you will be able to:

• Describe the motion, chemical, and age constraints that must be met by any theory of solar system formation
• Summarize the physical and chemical changes during the solar nebula stage of solar system formation
• Explain the formation process of the terrestrial and giant planets
• Describe the main events of the further evolution of the solar system

As we have seen, the comets , asteroids , and meteorites are surviving remnants from the processes that formed the solar system. The planets, moons, and the Sun, of course, also are the products of the formation process, although the material in them has undergone a wide range of changes. We are now ready to put together the information from all these objects to discuss what is known about the origin of the solar system.

Observational Constraints

There are certain basic properties of the planetary system that any theory of its formation must explain. These may be summarized under three categories: motion constraints, chemical constraints, and age constraints. We call them constraints because they place restrictions on our theories; unless a theory can explain the observed facts, it will not survive in the competitive marketplace of ideas that characterizes the endeavor of science. Let’s take a look at these constraints one by one.

There are many regularities to the motions in the solar system. We saw that the planets all revolve around the Sun in the same direction and approximately in the plane of the Sun’s own rotation. In addition, most of the planets rotate in the same direction as they revolve, and most of the moons also move in counterclockwise orbits (when seen from the north). With the exception of the comets and other trans-neptunian objects, the motions of the system members define a disk or Frisbee shape. Nevertheless, a full theory must also be prepared to deal with the exceptions to these trends, such as the retrograde rotation (not revolution) of Venus.

In the realm of chemistry, we saw that Jupiter and Saturn have approximately the same composition—dominated by hydrogen and helium. These are the two largest planets, with sufficient gravity to hold on to any gas present when and where they formed; thus, we might expect them to be representative of the original material out of which the solar system formed. Each of the other members of the planetary system is, to some degree, lacking in the light elements. A careful examination of the composition of solid solar-system objects shows a striking progression from the metal-rich inner planets, through those made predominantly of rocky materials, out to objects with ice-dominated compositions in the outer solar system. The comets in the Oort cloud and the trans-neptunian objects in the Kuiper belt are also icy objects, whereas the asteroids represent a transitional rocky composition with abundant dark, carbon-rich material.

As we saw in Other Worlds: An Introduction to the Solar System, this general chemical pattern can be interpreted as a temperature sequence: hot near the Sun and cooler as we move outward. The inner parts of the system are generally missing those materials that could not condense (form a solid) at the high temperatures found near the Sun. However, there are (again) important exceptions to the general pattern. For example, it is difficult to explain the presence of water on Earth and Mars if these planets formed in a region where the temperature was too hot for ice to condense, unless the ice or water was brought in later from cooler regions. The extreme example is the observation that there are polar deposits of ice on both Mercury and the Moon; these are almost certainly formed and maintained by occasional comet impacts.

As far as age is concerned, we discussed that radioactive dating demonstrates that some rocks on the surface of Earth have been present for at least 3.8 billion years, and that certain lunar samples are 4.4 billion years old. The primitive meteorites all have radioactive ages near 4.5 billion years. The age of these unaltered building blocks is considered the age of the planetary system. The similarity of the measured ages tells us that planets formed and their crusts cooled within a few tens of millions of years (at most) of the beginning of the solar system. Further, detailed examination of primitive meteorites indicates that they are made primarily from material that condensed or coagulated out of a hot gas; few identifiable fragments appear to have survived from before this hot-vapor stage 4.5 billion years ago.

The Solar Nebula

All the foregoing constraints are consistent with the general idea, introduced in Other Worlds: An Introduction to the Solar System, that the solar system formed 4.5 billion years ago out of a rotating cloud of vapor and dust—which we call the solar nebula —with an initial composition similar to that of the Sun today. As the solar nebula collapsed under its own gravity, material fell toward the center, where things became more and more concentrated and hot. Increasing temperatures in the shrinking nebula vaporized most of the solid material that was originally present.

At the same time, the collapsing nebula began to rotate faster through the conservation of angular momentum (see the Orbits and Gravity and Earth, Moon, and Sky chapters). Like a figure skater pulling her arms in to spin faster, the shrinking cloud spun more quickly as time went on. Now, think about how a round object spins. Close to the poles, the spin rate is slow, and it gets faster as you get closer to the equator. In the same way, near the poles of the nebula, where orbits were slow, the nebular material fell directly into the center. Faster moving material, on the other hand, collapsed into a flat disk revolving around the central object (Figure 14.11). The existence of this disk-shaped rotating nebula explains the primary motions in the solar system that we discussed in the previous section. And since they formed from a rotating disk, the planets all orbit the same way.

Picture the solar nebula at the end of the collapse phase, when it was at its hottest. With no more gravitational energy (from material falling in) to heat it, most of the nebula began to cool. The material in the center, however, where it was hottest and most crowded, formed a star that maintained high temperatures in its immediate neighborhood by producing its own energy. Turbulent motions and magnetic fields within the disk can drain away angular momentum, robbing the disk material of some of its spin. This allowed some material to continue to fall into the growing star, while the rest of the disk gradually stabilized.

The temperature within the disk decreased with increasing distance from the Sun, much as the planets’ temperatures vary with position today. As the disk cooled, the gases interacted chemically to produce compounds; eventually these compounds condensed into liquid droplets or solid grains. This is similar to the process by which raindrops on Earth condense from moist air as it rises over a mountain.

Let’s look in more detail at how material condensed at different places in the maturing disk (Figure 14.12). The first materials to form solid grains were the metals and various rock-forming silicates. As the temperature dropped, these were joined throughout much of the solar nebula by sulfur compounds and by carbon- and water-rich silicates, such as those now found abundantly among the asteroids. However, in the inner parts of the disk, the temperature never dropped low enough for such materials as ice or carbonaceous organic compounds to condense, so they were lacking on the innermost planets.

Far from the Sun, cooler temperatures allowed the oxygen to combine with hydrogen and condense in the form of water (H 2 O) ice. Beyond the orbit of Saturn, carbon and nitrogen combined with hydrogen to make ices such as methane (CH 4 ) and ammonia (NH 3 ). This sequence of events explains the basic chemical composition differences among various regions of the solar system.

Example 14.1: Rotation of the Solar Nebula

We can use the concept of angular momentum to trace the evolution of the collapsing solar nebula. The angular momentum of an object is proportional to the square of its size (diameter) divided by its period of rotation ( D 2 P ) Appendix F tells us has a radius of about 40 AU?

We are given that the final diameter of the solar nebula is about 80 AU. Noting the initial state before the collapse and the final state at Pluto’s orbit, then

P final P initial = ( D final D initial ) 2 = ( 80 10,000 ) 2 = ( 0.008 ) 2 = 0.000064 P final P initial = ( D final D initial ) 2 = ( 80 10,000 ) 2 = ( 0.008 ) 2 = 0.000064

With P initial equal to 1,000,000 years, P final , the new rotation period, is 64 years. This is a lot shorter than the actual time Pluto takes to go around the Sun, but it gives you a sense of the kind of speeding up the conservation of angular momentum can produce. As we noted earlier, other mechanisms helped the material in the disk lose angular momentum before the planets fully formed.

Exercise \(\PageIndex{1}\)

What would the rotation period of the nebula in our example be when it had shrunk to the size of Jupiter’s orbit?

The period of the rotating nebula is inversely proportional to D 2 D 2 . As we have just seen, P final P initial = ( D final D initial ) 2 . P final P initial = ( D final D initial ) 2 . Initially, we have P initial = 10 6 yr and D initial = 10 4 AU. Then, if D final is in AU, P final (in years) is given by P final = 0.01 D final 2 . P final = 0.01 D final 2 . If Jupiter’s orbit has a radius of 5.2 AU, then the diameter is 10.4 AU. The period is then 1.08 years.

Formation of the Terrestrial Planets

The grains that condensed in the solar nebula rather quickly joined into larger and larger chunks, until most of the solid material was in the form of planetesimals, chunks a few kilometers to a few tens of kilometers in diameter. Some planetesimals still survive today as comets and asteroids. Others have left their imprint on the cratered surfaces of many of the worlds we studied in earlier chapters. A substantial step up in size is required, however, to go from planetesimal to planet.

Some planetesimals were large enough to attract their neighbors gravitationally and thus to grow by the process called accretion . While the intermediate steps are not well understood, ultimately several dozen centers of accretion seem to have grown in the inner solar system. Each of these attracted surrounding planetesimals until it had acquired a mass similar to that of Mercury or Mars. At this stage, we may think of these objects as protoplanets —“not quite ready for prime time” planets.

Each of these protoplanet s continued to grow by the accretion of planetesimals. Every incoming planetesimal was accelerated by the gravity of the protoplanet, striking with enough energy to melt both the projectile and a part of the impact area. Soon the entire protoplanet was heated to above the melting temperature of rocks. The result was planetary differentiation , with heavier metals sinking toward the core and lighter silicates rising toward the surface. As they were heated, the inner protoplanets lost some of their more volatile constituents (the lighter gases), leaving more of the heavier elements and compounds behind.

Formation of the Giant Planets

In the outer solar system, where the available raw materials included ices as well as rocks, the protoplanets grew to be much larger, with masses ten times greater than Earth. These protoplanets of the outer solar system were so large that they were able to attract and hold the surrounding gas. As the hydrogen and helium rapidly collapsed onto their cores, the giant planets were heated by the energy of contraction. But although these giant planets got hotter than their terrestrial siblings, they were far too small to raise their central temperatures and pressures to the point where nuclear reactions could begin (and it is such reactions that give us our definition of a star). After glowing dull red for a few thousand years, the giant planets gradually cooled to their present state (Figure 14.13).

The collapse of gas from the nebula onto the cores of the giant planets explains how these objects acquired nearly the same hydrogen-rich composition as the Sun. The process was most efficient for Jupiter and Saturn; hence, their compositions are most nearly “cosmic.” Much less gas was captured by Uranus and Neptune, which is why these two planets have compositions dominated by the icy and rocky building blocks that made up their large cores rather than by hydrogen and helium. The initial formation period ended when much of the available raw material was used up and the solar wind (the flow of atomic particles) from the young Sun blew away the remaining supply of lighter gases.

Further Evolution of the System

All the processes we have just described, from the collapse of the solar nebula to the formation of protoplanets, took place within a few million years. However, the story of the formation of the solar system was not complete at this stage; there were many planetesimals and other debris that did not initially accumulate to form the planets. What was their fate?

The comets visible to us today are merely the tip of the cosmic iceberg (if you’ll pardon the pun). Most comets are believed to be in the Oort cloud, far from the region of the planets. Additional comets and icy dwarf planets are in the Kuiper belt, which stretches beyond the orbit of Neptune. These icy pieces probably formed near the present orbits of Uranus and Neptune but were ejected from their initial orbits by the gravitational influence of the giant planets.

In the inner parts of the system, remnant planetesimals and perhaps several dozen protoplanets continued to whiz about. Over the vast span of time we are discussing, collisions among these objects were inevitable. Giant impacts at this stage may have stripped Mercury of part of its mantle and crust, reversed the rotation of Venus, and broke off part of Earth to create the Moon (all events we discussed in other chapters).

Smaller-scale impacts also added mass to the inner protoplanets. Because the gravity of the giant planets could “stir up” the orbits of the planetesimals, the material impacting on the inner protoplanets could have come from almost anywhere within the solar system. In contrast to the previous stage of accretion, therefore, this new material did not represent just a narrow range of compositions.

As a result, much of the debris striking the inner planets was ice-rich material that had condensed in the outer part of the solar nebula. As this comet-like bombardment progressed, Earth accumulated the water and various organic compounds that would later be critical to the formation of life. Mars and Venus probably also acquired abundant water and organic materials from the same source, as Mercury and the Moon are still doing to form their icy polar caps.

Gradually, as the planets swept up or ejected the remaining debris, most of the planetesimals disappeared. In two regions, however, stable orbits are possible where leftover planetesimals could avoid impacting the planets or being ejected from the system. These regions are the asteroid belt between Mars and Jupiter and the Kuiper belt beyond Neptune. The planetesimals (and their fragments) that survive in these special locations are what we now call asteroids, comets, and trans-neptunian objects.

Astronomers used to think that the solar system that emerged from this early evolution was similar to what we see today. Detailed recent studies of the orbits of the planets and asteroids, however, suggest that there were more violent events soon afterward, perhaps involving substantial changes in the orbits of Jupiter and Saturn. These two giant planets control, through their gravity, the distribution of asteroids. Working backward from our present solar system, it appears that orbital changes took place during the first few hundred million years. One consequence may have been scattering of asteroids into the inner solar system, causing the period of “heavy bombardment” recorded in the oldest lunar craters.

Universe Today

Space and astronomy news

How Are Planets Formed?

How did the Solar System’s planets come to be? The leading theory is something known as the “protoplanet hypothesis”, which essentially says that very small objects stuck to each other and grew bigger and bigger — big enough to even form the gas giants, such as Jupiter.

But how the heck did that happen? More details below.

Birthing the Sun

About 4.6 billion years ago, as the theory goes, the location of today’s Solar System was nothing more than a loose collection of gas and dust — what we call a nebula. (Orion’s Nebula is one of the most famous examples you can see in the night sky.)

Then something happened that triggered a pressure change in the center of the cloud, scientists say. Perhaps it was a supernova exploding nearby, or a passing star changing the gravity. Whatever the change, however, the cloud collapsed and created a disc of material, according to NASA .

The center of this disc saw a great increase in pressure that eventually was so powerful that hydrogen atoms loosely floating in the cloud began to come into contact. Eventually, they fused and produced helium, kickstarting the formation of the Sun.

The Sun was a hungry youngster — it ate up 99% of what was swirling around, NASA says — but this still left 1% of the disc available for other things. And this is where planet formation began.

Time of chaos

The Solar System was a really messy place at this time, with gas and dust and debris floating around. But planet formation appears to have happened relatively rapidly. Small bits of dust and gas began to clump together. The young Sun pushed much of the gas out to the outer Solar System and its heat evaporated any ice that was nearby.

Over time, this left rockier planets closer to the Sun and gas giants that were further away. But about four billion or so years ago, an event called the “late heavy bombardment” resulted in small bodies pelting the bigger members of the Solar System. We almost lost the Earth when a Mars-sized object crashed into it, as the theory goes.

What caused this is still under investigation, but some scientists believe it was because the gas giants were moving around and perturbing smaller bodies at the fringe of the Solar System. At any rate, in simple terms, the clumping together of protoplanets (planets in formation) eventually formed the planets.

We can still see leftovers of this process everywhere in the Solar System. There is an asteroid belt between Mars and Jupiter that perhaps would have coalesced into a planet had Jupiter’s gravity not been so strong. And we also have comets and asteroids that are sometimes considered referred to as “building blocks” of our Solar System.

We’ve described in detail what happened in our own Solar System, but the important takeaway is that many of these processes are at work in other places. So when we speak about exoplanet systems — planets beyond our Solar System — it is believed that a similar sequence of events took place. But how similar is still being learned.

Making the case

One major challenge to this theory, of course, is no one (that we know of!) was recording the early history of the Solar System. That’s because the Earth wasn’t even formed yet, so it was impossible for any life — let alone intelligent life — to keep track of what was happening to the planets around us.

There are two major ways astronomers get around this problem. The first is simple observation. Using powerful telescopes such as the Atacama Large Millimeter/submillimeter Array (ALMA), astronomers can actually observe dusty discs around young planets. So we have numerous examples of stars with planets being born around them.

The second is using modelling. To test their observational hypotheses, astronomers run computer modelling to see if (mathematically speaking) the ideas work out. Often they will try to use different conditions during the simulation, such as perhaps a passing star triggering changes in the dust cloud. If the model holds after many runs and under several conditions, it’s more likely to be true.

That said, there still are some complications. We can’t use modelling yet to exactly predict how the planets of the Solar System ended up where they were. Also, in fine detail our Solar System is kind of a messy place, with phenomena such as asteroids with moons.

And we need to have a better understanding of external factors that could affect planet formation, such as supernovae (explosions of old, massive stars.) But the protoplanet hypothesis is the best we’ve got — at least for now.

We have written many articles about the protoplanet hypothesis for Universe Today. Here’s an article about how the Solar System was formed , and here’s an article about protoplanets . We’ve also recorded a series of episodes of Astronomy Cast about every planet in the Solar System. Start here, Episode 49: Mercury .

Share this:

• Click to share on Facebook (Opens in new window)
• Click to share on Twitter (Opens in new window)
• Click to share on Reddit (Opens in new window)

One Reply to “How Are Planets Formed?”

If the “asteroid” between Mars and Jupiter you’re referring to is Ceres, many planetary scientists argue it did coalesce into a planet, albeit a small one, smaller than it would have been if not for the influence of Jupiter. Ceres is in hydrostatic equilibrium, meaning rounded by its own gravity, is geologically layered into core, mantle, and crust, and may even have cryovolcanism and a subsurface ocean. These all are features of planets.

How many exoplanet system discoveries send planetary scientists back to the drawing board when it comes to understanding planet formation? Very likely, there is more than one way to form a planetary system. We still do not know how hot Jupiters ended up in close orbits around their stars or how giant planets ended up in very distant orbits from their stars. Once we are able to study exoplanet systems as a whole rather than just seeing one planet orbiting a star, a whole new range of possibilities regarding planet formation is likely to be considered.

Comments are closed.

Historical Geology

A free online textbook for Historical Geology courses

Nebular theory and the formation of the solar system

In the beginning….

How and when does the story of Earth begin? A logical place to start is with the formation of the planet, but as you’ll soon see, the formation of the planet is part of a larger story, and that story implies some backstory before the story, too. The purpose of this case study is to present our best scientific understanding of the formation of our solar system from a presolar nebula, and to put that nebula in context too.

Nebular theory

The prevailing scientific explanation for the origin of the Earth does a good job of not only explaining the Earth’s formation, but the Sun and all the other planets too. Really, it’s not “the Earth’s origin story” alone so much as it is the origin story of the whole solar system . Not only that, but our Sun is but one star among a hundred million in our galaxy, and our galaxy is one of perhaps a hundred million in the universe. So the lessons we learn by studying our own solar system can likely be applied more generally to the formation of other solar systems elsewhere, including those long ago, in galaxies far, far away. The vice versa is also true: Our understanding of our own solar system’s origin story is being refined as we learn more about exoplanets, some of which defy what we see in our own system; “ hot Jupiters ” and “ super-Earths ,” for instance, are features we see in other star systems but not our own.

When we use powerful telescopes to stare out into the galaxy, we observe plenty of other stars, but we observe other things too, including fuzzy looking features called nebulae. A nebula is a big cloud of gas and dust in space. It’s not as bright as a star because it’s not undergoing thermonuclear fusion, with the tremendous release of energy that accompanies that process. An example of a nebula that you are likely to be able to see is in the constellation Orion. Orion’s “belt,” three stars in a row, is a readily identifiable feature in the northern hemisphere’s night sky in winter. A smaller trio of light spots “dangle” from the belt; this is Orion’s sword scabbard. A cheap pair of binoculars will let you examine these objects for yourself; you will discover that the middle point of light in this smaller trio is not a star. It is a nebula called Messier 42.

Nebulae like Messier 42 are common features of the galaxy, but not as common as stars. Nebulae appear to be short-lived features, as matter is often attracted to other matter. All that stuff distributed in that tremendous volume of space is not as stable as it would be if it were all to be drawn together into a few big clumps. Particles pull together with their neighboring particles under the influence of various forces, including “static cling” or electrostatic attraction. This is the same force that makes tiny dust motes clump up into dust bunnies under your couch!

Now, electrostatic force is quite strong for pulling together small particles over small distances, but if you want to make big things like planets and stars out from a nebula, you’re going to need gravity to take over at some point. Gravity is a rather weak force. After all: every time you take a step, you’ve overcoming the gravitational pull of the entire Earth. But gravity can work very efficiently over distance, if the masses involved are large enough. So static cling was the initial organizer, until the “space dust bunnies” got large enough, then gravity was able to take over, attracting mass to mass. The net result is that the gajillions of tiny pieces of the nebula were drawn together, swirling into a denser and denser amalgamation. The nebula began to spin, flattening out from top to bottom, and flattening out into a spinning disk, something between a Frisbee and a fried egg in shape:

Once a star forms in the center, astronomers call the ring of debris around it a protoplanetary disk. Two important processes that helped organize the protoplanetary disk further were condensation and accretion.

Condensation is the process where gaseous matter sticks together to make liquid or solid matter. We have evidence of condensation in the form of small spherical objects with internal layering, kind of like “space hailstones.” These are chondrules, and they represent the earliest objects formed in our solar system. (Occasionally, we are lucky enough to find chondrules that have survived until the present day, entombed inside certain meteorites of the variety called chondrites.)

Chondrules glommed onto other chondrules, and stuck themselves together into primordial “rocks,” building up larger and larger objects. Eventually, these objects got to be big enough to pull their mass into an round shape, and we would be justified to dub them “planetesimals.” Planetesimals gobbled up nearby asteroids, and smashed into other planetesimals, merging and growing through time through the process of accretion. The kinetic force of these collisions heated the rocky and metallic material of the planetesimals, and their temperature also went up as radioactive decay heated them from within. Once warm, denser material could sink to their middles, and lighter-weight elements and compounds rose up to their surface. So not only were they maturing into spheroidal shapes, but they were also differentiating internally, separating into layers organized by density.

Meteorites that show metallic compositions represent “core” material from these planetesimals; core material that we would never get to glimpse had not their surrounding rocky material been blasted off. Iron meteorites such as the Canyon Diablo meteorite below (responsible for Arizona’s celebrated Meteor Crater) therefore are evidence of differentiation of planetesimals into layered bodies, followed by disaggregation: a polite way of saying they were later violently ripped apart by energetic collisions.

If you were to somehow weigh the nebula before condensation and accretion, and again 4.6 billion years later, we’d find the mass to be the same. Rather than being dispersed in a diffuse cloud of uncountable atoms, the condensation and accretion of the nebula resulted in exactly the same amount of stuff, but organized into a smaller and smaller number of bigger and bigger objects. The biggest of these was the Sun, comprising about 99.86% of all the mass in the solar system. Four-fifths of the remaining 0.14% makes up the planet Jupiter.  Saturn, Neptune, and Uranus are huge gas giants as well. The inner rocky planets (including Earth) make up a tiny, tiny fraction of the total mass of the whole solar system – but of course, just because they are relatively small, that doesn’t mean they are unimportant!

The process of accretion continues into the present day, though at a slower pace than the earliest days of the solar system. One place you can observe this is in the asteroid belt, where there are certain asteroids that are basically nothing more than a big 3D pile of space rocks, held together under their own gravity. Consider the asteroid called Itokawa 25143, for instance:

Only about half a kilometer long, and only a few hundred meters wide, Itokawa doesn’t even have enough gravity to pull itself into a sphere. If you were to land on the surface of Itokawa and kick a soccer-ball-sized boulder, it would readily fly off into space, as the force of your kick would be much higher than the force of gravity causing it to stay put.

Another example of accretion continuing to this day is meteorite impacts. Every time a chunk of rock in space intersects the Earth, its mass is added to that of the planet. In that instant, the solar system gets a little bit cleaner (fewer leftover bits rattling around) and the planet gets a little more massive. A spectacular example of this occurred in 1994 with Comet Shoemaker-Levy 9, a  comet which had only been discovered the previous year. Jupiter’s immense gravity broke the comet into chunks, and then swallowed them up one after another. Astronomers on Earth watched with fascination as the comet chunks, some more than a kilometer across, slammed into Jupiter’s atmosphere at 60 km/second (~134,000 mph), creating a 23,700°C fireball and enormous impact scars that were as large as the entire Earth. These scars lasted for months.

This incredibly dramatic event perhaps raises the hair on our necks, seeing the violence and power of cosmic collisions. It’s a reminder that Earthlings are not safe from accretionary impacts even today – as the dinosaurs found out. For the purposes of our current discussion, though, bear in mind that the collision was really a merger between the masses of Comet Shoemaker-Levy 9 and the planet Jupiter, and after the dust settled, the solar system had one fewer object left off by itself, and Jupiter gained a bit more mass. This is the overall trend of the accretion of our solar system from the presolar nebula: under gravity’s influence, the available mass becomes more and more concentrated through time.

Did I get it?

Your answer:

Correct answer:

Your Answers

A star is born

Because the Sun is so massive, it is able to achieve tremendous pressures in its interior. These pressures are so high, they can actually force two atoms into the same space , overcoming their immense repulsion for one another, and causing their two nuclei to merge. As two atoms combine to make one more massive atom, energy is released. This process is thermonuclear fusion. Once it begins, stars begin to give off light.

The ability of stars to make big atoms from small ones is key to understanding the history of our solar system and our planet. Planet Earth is made of a wide variety of chemical elements, both lightweight and heavy. All of these elements must have been present in the nebula, in order for them to be included in Earth’s “starting mixture.” Elements formed in the Sun today stay in the Sun, fusing low-weight atoms into heavier atoms. So all the elements on Earth today came from a pre-Sun star. We can go outside on a spring day and enjoy the Sun’s warmth, but the carbon that makes up the skin that basks in that warmth was forged in the heart of another star, a star that’s gone now, a star that blew up.

This exploding star was the source of the nebula where we began this case study: it’s the backstory that occurred before the opening scene. Our solar system is like a “haunted house,” where billions of years ago, there was a vibrant, healthy main-sequence star right here, in this part of the galaxy. Perhaps it had planets orbiting it. Perhaps some of those planets harbored life. We’ll never know: the explosion wiped the slate clean, and “reset” the solar system for the iteration in which we live. The ghostly remnants of this time before our own still linger, in the very stuff we’re made from. This long-dead star fused hydrogen to build the carbon in our bodies, the iron in our blood, the oxygen we breathe, and the silicon in the rocks of our planet.

This is an incredible realization to embrace: everything you know, everything you trust, everything you are , is stardust.

Age of the solar system

So just when did all this happen? An estimate for the age of the solar system can be made using isotopes of the element lead (Pb). There are several isotopes of lead, but for the purposes of figuring out the age of the solar system, consider these four: 208 Pb, 207 Pb, 206 Pb, and 204 Pb.

208 Pb, 207 Pb, 206 Pb are all radiogenic: that is to say, they stable “daughter” isotopes that are produced from the radioactive “parent” isotopes. Each is produced from a different parent, at a different rate:

204 Pb is, as far as we know, non-radiogenic. It’s relevant to this discussion because it can serve as a ‘standard’ that can allow us to compare the other lead isotopes to one another. Just as if we wanted to compare the currencies of Namibia, Indonesia, and Chile, we might reference all three to the U.S. dollar. The dollar would serve as a standard of comparison, allowing us to better see the value of the Namibian currency relative to the Indonesian currency and the Chilean currency. That’s what 204 Pb is doing for us here.

This is a plot showing the modeled evolution of our three radiogenic lead isotopes relative to 204 Pb. It is constrained by terrestrial lead samples at the young end, and projected back in time in accordance with our measurements of how quickly these three isotopes of lead are produced by their radioactive parents. Of course, if we go back far enough in time, we run out of samples to evaluate. The Earth’s rock cycle has destroyed all its earliest rocks. They’ve been metamorphosed, or weathered, or melted – perhaps many times over! What would be really nice is to find some rocks from the early end of these curves – some samples that could verify these projections back in time are accurate.

Such samples do exist! But they are not from the Earth so much as “from the Earth’s starting materials.” If the nebular theory is correct, then a few leftover scraps of the planet’s starting materials are found in the solar system’s asteroids. Every now and again, bits of these space rocks fall to earth, and if they survive their passage through the atmosphere, we may be lucky enough to collect them, and analyze them. We call these space rocks “meteors” as they streak through the atmosphere, heating through friction and oxidizing as they fall. Those that make it all the way to Earth’s surface are known as “meteorites.” They can be often be distinguished by their scalloped fusion crust, as with this sample:

Meteorites come in several varieties, including rocky and metallic versions. It is very satisfying that when measurements of these meteorites’ lead isotopes are added to the plot above, they all fall exactly where our understanding of lead isotope production would have them: at the start of each of these model evolution curves. Each lead isotope system tells the same answer for the age of the Earth, acting like three independent witnesses corroborating one another’s testimony. And the answer they all give is 4.6 billion years ago (4.6 Ga). That’s what 208 Pb says. That’s what 207 Pb says. And that’s what 206 Pb says. They all agree, and they agree with the predicted curves based on terrestrial (Earth rock) measurements. This agreement gives us great confidence in this number. The Earth, and meteorites (former asteroids), and the solar system of which they are all a part, began about 4.6 billion years ago…

…But what came before that?

The implications of meteorites

In 1969, a meteorite fell through Earth’s atmosphere and broke up over Mexico. A great many pieces of this meteorite were recovered and made available for scientific analysis. It turned out to be a carbonaceous chondrite, the largest of its kind ever documented. It was named the Allende ( “eye-YEN-day” ) meteorite, for the tiny Chihuahuan village closest to the center of the area over which its fragments were scattered.

One of the materials making up Allende’s chondrules was the calcium feldspar called anorthite. Anorthite is an extraordinarily common mineral in Earth’s crust, but the Allende anorthite was different. For some reason, it has a large amount of magnesium in it. When geochemists determined what kind of magnesium this was, they were surprised to find that it was mostly 26 Mg, an uncommon isotope. The abundances of 25 Mg and 24 Mg were found to be about the same level as Earth rocks, but 26 Mg was elevated by about 1.3%.  And after all, magnesium doesn’t even “belong” in a feldspar. The chemical formula of anorthite is CaAl 2 Si 2 O 8 – there’s no “Mg” spot in there. Why was this odd 26 Mg in this chondritic anorthite?

One way to make 26 Mg is the break-down of radioactive 26 Al. The problem with this idea is that there is no 26 Al around today . It’s an example of an extinct isotope: an atom of aluminum so unstable that it falls apart extremely rapidly. The half-life is only 717,000 years. But because these chondrules condensed in the earliest days of the solar system, there may well have been plenty of 26 Al around at that point for them to incorporate. And Al, of course, is a key part of anorthite’s Ca Al 2 Si 2 O 8 crystal structure.

So the idea is that weird extra 26 Mg in the chondrule’s anorthite could be explained by suggesting it wasn’t always 26 Mg: Instead, it started off as 26 Al ,and it belonged in that crystal’s structure. However, over a short amount of time, it all fell apart, and that left the 26 Mg behind to mark where it had once been. If this interpretation is true, it has shocking implications for the story of our solar system.

To understand why, we first need to ask, what came before the nebula? What was the ‘pre-nebula’ situation? Where did the nebula come from, anyhow?

It turns out that nebulae are generated when old stars of a certain size explode.

These explosions are called supernovae (the plural of supernova). The “nova” part of the name comes from the fact that they are very bright in the night sky – an indication of how energetic the explosion is. They look like “new” stars to the casual observer. Supernovae occur when a star has exhausted its supply of lightweight fuel, and it runs out of small atoms that can be fused together under normal conditions. The outward-directed force ceases, and gravitationally-driven inward-directed forces suddenly dominate, collapsing the star in upon itself. This jacks up the pressures to unbelievably high levels, and is responsible for the nuclear fusion of big atoms – every atom heavier than iron is made instantaneously in the fires of the supernova.

That suite of freshly-minted atoms included a bunch of unstable isotopes, including 26 Al.

And here’s the kicker: If the 26 Al was made in a supernova, started decaying immediately, and yet enough was around that a significant portion of it could be woven into the Allende chondrules’ anorthite, that implies a very short amount of time between the obliteration of our Sun’s predecessor, and the first moments of our own. Specifically, the 717,000 year half-life of 26 Al suggests that this “transition between solar systems” played out in less than 5 million years, conceivably in only 2 million years.

That is very, very quickly.

In summary, the planet Earth is part of a solar system centered on the Sun. This solar system, with its star, its classical planets, its dwarf planets, and its “leftover” comets and asteroids, formed from a nebula full of elements in the form of gas and dust. Over time, these many very small pieces stuck together to make bigger concentrations of mass, eventually culminating in a star and a bunch of planets that orbit it. Asteroids (and asteroids that fall to Earth, called meteorites), are leftovers from this process. The starting nebula itself formed from the destruction of a previous star that had exploded in a supernova. The transition from the pre-Sun star to our solar system took place shockingly rapidly.

Further reading

Marcia Bjornerud’s book Reading the Rocks . Basic Books, 2005: 226 pages.

Jennifer A. Johnson (2019), “ Populating the periodic table: Nucleosynthesis of the elements ,” Science. 01 Feb 2019 : 474-478.

Lee, T., D. A. Papanastassiou, and G. J. Wasserburg (1976), Demonstration of 26 Mg excess in Allende and evidence for 26 Al , Geophysical Research Letters , 3(1), 41-44.

______________

PDF of this page

Chapter Contents

• 1 In the beginning…
• 2 Nebular theory
• 3 A star is born
• 4 Age of the solar system
• 5 The implications of meteorites
• 7 Further reading

21.6 New Perspectives on Planet Formation

Learning objectives.

By the end of this section, you will be able to:

• Explain how exoplanet discoveries have revised our understanding of planet formation
• Discuss how planetary systems quite different from our solar system might have come about

Traditionally, astronomers have assumed that the planets in our solar system formed at about their current distances from the Sun and have remained there ever since. The first step in the formation of a giant planet is to build up a solid core, which happens when planetesimals collide and stick. Eventually, this core becomes massive enough to begin sweeping up gaseous material in the disk, thereby building the gas giants Jupiter and Saturn.

How to Make a Hot Jupiter

The traditional model for the formation of planets works only if the giant planets are formed far from the central star (about 5–10 AU), where the disk is cold enough to have a fairly high density of solid matter needed to make the planet cores. It cannot explain the hot Jupiters , which are located very close to their stars where there is no solid matter because it would be completely vaporized. It also cannot explain the elliptical orbits we observe for some exoplanets because the orbit of a protoplanet, whatever its initial shape, will quickly become circular through interactions with the surrounding disk of material and will remain that way as the planet grows by sweeping up additional matter.

So we have two options: either we find a new model for forming planets close to the searing heat of the parent star, or we find a way to change the orbits of planets originally formed in colder regions far from their parent starts, so that they can then travel inward after they form. Most research now supports the latter explanation.

There are two ways that giant planets could travel inward. Calculations show that if a planet forms while a substantial amount of gas remains in the disk, then some of the planet’s orbital angular momentum can be transferred to the disk. As it loses momentum (through a process that reminds us of the effects of friction), the planet will spiral inward. This process can transport giant planets, initially formed in cold regions of the disk, closer to the central star—thereby producing hot Jupiters. Gravitational interactions between planets in the chaotic early solar system can also cause a newly formed giant planet to change its orbit to be more elliptical, carrying it into the hotter regions close to the star. Once it is in that more crowded region, the same forces that we mentioned earlier (that remind us of friction) can then cause its orbit to become circular again, but in the hotter region much closer to the star.

In some cases, we can use the combination of transit plus Doppler measurements to determine whether the planets orbit in the same plane and in the same direction as the star. For the first few cases, things seemed to work just as we anticipated: like the solar system, the gas giant planets orbited in their star’s equatorial plane and in the same direction as the spinning star.

Then, some startling discoveries were made of gas giant planets that orbited at right angles or even in the opposite sense as the spin of the star. How could this happen? Again, there must have been interactions between planets. It’s possible that before the system settled down, two planets came close together, so that one was kicked into an unusual orbit. Or perhaps a passing star perturbed the system after the planets were newly formed.

Forming Planetary Systems

When the Milky Way Galaxy was young, the stars that formed did not contain many heavy elements like iron. Several generations of star formation and star death were required to enrich the interstellar medium for subsequent generations of stars. Since planets seem to form “inside out,” starting with the accretion of the materials that can make the rocky cores with which planets start, astronomers wondered when in the history of the Galaxy, planet formation would turn on.

The star Kepler-444 has shed some light on this question. This is a tightly packed system of five planets—the smallest comparable in size to Mercury and the largest similar in size to Venus. All five planets were detected with the Kepler spacecraft as they transited their parent star. All five planets orbit their host star in less than the time it takes Mercury to complete one orbit about the Sun. Remarkably, the host star Kepler-444 is more than 11 billion years old and formed when the Milky Way was only 2 billion years old. So the heavier elements needed to make rocky planets must have already been available then. This ancient planetary system sets the clock on the beginning of rocky planet formation to be relatively soon after the formation of our Galaxy.

Kepler data demonstrate that while rocky planets inside Mercury’s orbit are missing from our solar system, they are common around other stars, like Kepler-444. When the first systems packed with close-in rocky planets were discovered, we wondered why they were so different from our solar system. When many such systems were discovered, we began to wonder if it was our solar system that was different. This led to speculation that additional rocky planets might once have existed close to the Sun in our solar system.

There is some evidence from the motions in the outer solar system that Jupiter may have migrated inward long ago. If correct, then gravitational perturbations from Jupiter could have dislodged the orbits of close-in rocky planets, causing them to fall into the Sun. Consistent with this picture, astronomers now think that Uranus and Neptune probably did not form at their present distances from the Sun but rather closer to where Jupiter and Saturn are now. The reason for this idea is that density in the disk of matter surrounding the Sun at the time the planets formed was so low outside the orbit of Saturn that it would take several billion years to build up Uranus and Neptune. Yet we saw earlier in the chapter that the disks around protostars survive only a few million years.

Therefore, scientists have developed computer models demonstrating that Uranus and Neptune could have formed near the current locations of Jupiter and Saturn, and then been kicked out to larger distances through gravitational interactions with their neighbors. All these wonderful new observations illustrate how dangerous it can be to draw conclusions about a phenomenon in science (in this case, how planetary systems form and arrange themselves) when you are only working with a single example.

Exoplanets have given rise to a new picture of planetary system formation—one that is much more chaotic than we originally thought. If we think of the planets as being like skaters in a rink, our original model (with only our own solar system as a guide) assumed that the planets behaved like polite skaters, all obeying the rules of the rink and all moving in nearly the same direction, following roughly circular paths. The new picture corresponds more to a roller derby, where the skaters crash into one another, change directions, and sometimes are thrown entirely out of the rink.

Habitable Exoplanets

While thousands of exoplanets have been discovered in the past two decades, every observational technique has fallen short of finding more than a few candidates that resemble Earth ( Figure 21.27 ). Astronomers are not sure exactly what properties would define another Earth. Do we need to find a planet that is exactly the same size and mass as Earth? That may be difficult and may not be important from the perspective of habitability. After all, we have no reason to think that life could not have arisen on Earth if our planet had been a little bit smaller or larger. And, remember that how habitable a planet is depends on both its distance from its star and the nature of its atmosphere. The greenhouse effect can make some planets warmer (as it did for Venus and is doing more and more for Earth).

We can ask other questions to which we don’t yet know the answers. Does this “twin” of Earth need to orbit a solar-type star, or can we consider as candidates the numerous exoplanets orbiting K- and M-class stars? (In the summer of 2016, astronomers reported the discovery of a planet with at least 1.3 times the mass of Earth around the nearest star, Proxima Centauri , which is spectral type M and located 4.2 light years from us.) We have a special interest in finding planets that could support life like ours, in which case, we need to find exoplanets within their star’s habitable zone, where surface temperatures are consistent with liquid water on the surface. This is probably the most important characteristic defining an Earth-analog exoplanet.

The search for potentially habitable worlds is one of the prime drivers for exoplanet research in the next decade. Astronomers are beginning to develop realistic plans for new instruments that can even look for signs of life on distant worlds (examining their atmospheres for gases associated with life, for example). If we require telescopes in space to find such worlds, we need to recognize that years are required to plan, build, and launch such space observatories. The discovery of exoplanets and the knowledge that most stars have planetary systems are transforming our thinking about life beyond Earth. We are closer than ever to knowing whether habitable (and inhabited) planets are common. This work lends a new spirit of optimism to the search for life elsewhere, a subject to which we will return in Life in the Universe .

As an Amazon Associate we earn from qualifying purchases.

This book may not be used in the training of large language models or otherwise be ingested into large language models or generative AI offerings without OpenStax's permission.

Want to cite, share, or modify this book? This book uses the Creative Commons Attribution License and you must attribute OpenStax.

Access for free at https://openstax.org/books/astronomy-2e/pages/1-introduction

• Authors: Andrew Fraknoi, David Morrison, Sidney Wolff
• Publisher/website: OpenStax
• Book title: Astronomy 2e
• Publication date: Mar 9, 2022
• Location: Houston, Texas
• Book URL: https://openstax.org/books/astronomy-2e/pages/1-introduction
• Section URL: https://openstax.org/books/astronomy-2e/pages/21-6-new-perspectives-on-planet-formation

© Jan 23, 2024 OpenStax. Textbook content produced by OpenStax is licensed under a Creative Commons Attribution License . The OpenStax name, OpenStax logo, OpenStax book covers, OpenStax CNX name, and OpenStax CNX logo are not subject to the Creative Commons license and may not be reproduced without the prior and express written consent of Rice University.

share this!

January 12, 2023

This article has been reviewed according to Science X's editorial process and policies . Editors have highlighted the following attributes while ensuring the content's credibility:

fact-checked

peer-reviewed publication

trusted source

Confirming the theory behind the formation of planets, stars and black holes

by John Greenwald, Princeton University

The first laboratory realization of the long-standing but never-before confirmed theory of the puzzling formation of planets, stars and supermassive black holes by swirling surrounding matter has been produced at PPPL. This breakthrough confirmation caps more than 20 years of experiments at PPPL, the national laboratory devoted to the study of plasma science and fusion energy.

The puzzle arises because matter orbiting around a central object does not simply fall into it, due to what is called the conservation of angular momentum that keeps planets and the rings of Saturn from tumbling from their orbits. That's because the outward centrifugal force balances out the inward pull of gravity on the orbiting matter. However, the clouds of dust and plasma called accretion disks that swirl around and collapse into celestial bodies do so in defiance of the conservation of angular momentum.

Puzzle solution

The solution to this puzzle, a theory known as the Standard Magnetorotational Instability (SMRI), was first proposed in 1991 by then-University of Virginia theorists Steven Balbus and John Hawley. They built on the fact that in a fluid that conducts electricity, whether the fluid be plasma or liquid metal , magnetic fields behave like springs connecting different sections of the fluid.

This allows ubiquitous Alfvén waves, named after Nobel Prize winner Hannes Alfvén, to create a back-and-forth force between the inertia of the swirling fluid and the springiness of the magnetic field , causing angular momentum to be rapidly transferred between different sections of the disk.

This powerful instability shifts the plasma toward a more stable configuration, the SMRI theory says. The shift pushes the orbit-conserving angular momentum outward toward the rim of the disk, freeing inner sections to collapse over millions of years into the encircled celestial bodies, creating the planets and stars that come out at night. The process has been verified numerically, but never demonstrated experimentally or observationally until now.

"This has remained theoretical until now," said physicist Yin Wang, lead author of two recent papers, one in September in Physical Review Letters ( PRL ) and a Nature Communications paper published in August that details the combined experimental, numerical, and theoretical confirmation. Recent results produced on the novel MRI device developed at the laboratory, "have successfully detected the signature of SMRI," Wang said. Coauthors of the papers include physicists Erik Gilson and Fatima Ebrahimi of PPPL.

'Great news'

"This is great news," said theory co-developer Steven Balbus. "To now be able to study this in the laboratory is a wonderful development, both for astrophysics and for the field of magnetohydrodynamics more generally.

The MRI device, initially conceived by physicists Hantao Ji of PPPL and Jeremy Goodman of Princeton, both coauthors of these papers, consists of two concentric cylinders that spin at different speeds, creating a flow that mimics a swirling accretion disk. The experiment spun galinstan, a liquid metal alloy enclosed in a magnetic field. The caps that seal the top and bottom of the cylinders rotate at an intermediate speed, contributing to the experimental effect.

Physicists now plan new experimental and numerical studies to further characterize the reported SMRI. One study will test the crucial outward shift of angular momentum by measuring the velocity of the swirling liquid metal together with the dimensions of the magnetic field and the correlations between them.

"These studies will advance the emerging field of interdisciplinary laboratory astrophysics," Wang said. "They illustrate how astrophysics can be done in laboratories to help solve problems that space telescopes and satellite missions can't handle on their own, a major achievement for laboratory research."

Yin Wang et al, Identification of a non-axisymmetric mode in laboratory experiments searching for standard magnetorotational instability, Nature Communications (2022). DOI: 10.1038/s41467-022-32278-0

Journal information: Physical Review Letters , Nature Communications

Provided by Princeton University

Explore further

Feedback to editors

Fossilized dinosaur eggshells can preserve amino acids, the building blocks of proteins, over millions of years

US announces tough tap water standards for 'forever chemicals'

Physicists discover a novel quantum state in an elemental solid

2 hours ago

Quantum crystal of frozen electrons—the Wigner crystal—is visualized for the first time

New strategy for assessing the applicability of reactions

3D mouth of an ancient jawless fish suggests they were filter-feeders, not scavengers or hunters

Single-crop farming has potential to harm bees, study finds

New spectrometer helps identify alternative catalyst materials for affordable hydrogen fuel cells

New molecular device unlocks potential for targeted drug delivery and self-healing materials

3 hours ago

Certified timber harvesting of tropical forests proves beneficial for gorillas and elephants

Relevant physicsforums posts, w boson and weak decay, liquid scintillation counting: external source to determine efficiency.

Apr 9, 2024

Could one preserve polarized neutrons from decaying with a high B-field?

Apr 6, 2024

MCNP for the ionization chamber

Apr 3, 2024

PseudoVector Mesons

Mar 31, 2024

FRIB creates five new isotopes (Tm-182,-183; Yb-186,-187; Lu-190)

More from High Energy, Nuclear, Particle Physics

Related Stories

Novel experiment validates widely speculated mechanism behind the formation of stars

Feb 5, 2019

Using magnetic and electric fields to emulate black hole and stellar accretion disks

Aug 31, 2022

Exploring the source of stars and planets in a laboratory

Oct 23, 2020

Laboratory experiments probe the formation of stars and planets

Nov 5, 2018

New findings bring physicists closer to understanding the formation of planets and stars

Jan 14, 2019

Why does inside of solar system not spin faster? Old mystery has possible new solution

Jul 6, 2022

Recommended for you

A return to roots: Lab builds its first stellarator in 50 years and opens the door for research into new plasma physics

Apr 2, 2024

Plasma fusion: Adding just enough fuel to the fire

Mar 28, 2024

New research on tungsten unlocks potential for improving fusion materials

Mar 13, 2024

Researchers generate super-fast electrons with table-top laser systems

Study shows inverting fusion plasmas improves performance

Mar 12, 2024

One way to improve a fusion reaction: Use weaknesses as strengths

Mar 5, 2024

Let us know if there is a problem with our content

Use this form if you have come across a typo, inaccuracy or would like to send an edit request for the content on this page. For general inquiries, please use our contact form . For general feedback, use the public comments section below (please adhere to guidelines ).

Please select the most appropriate category to facilitate processing of your request

Thank you for taking time to provide your feedback to the editors.

Your feedback is important to us. However, we do not guarantee individual replies due to the high volume of messages.

E-mail the story

Your email address is used only to let the recipient know who sent the email. Neither your address nor the recipient's address will be used for any other purpose. The information you enter will appear in your e-mail message and is not retained by Phys.org in any form.

Newsletter sign up

Get weekly and/or daily updates delivered to your inbox. You can unsubscribe at any time and we'll never share your details to third parties.

More information Privacy policy

Donate and enjoy an ad-free experience

We keep our content available to everyone. Consider supporting Science X's mission by getting a premium account.

E-mail newsletter

• Scholarly Community Encyclopedia
• Log in/Sign up

Video Upload Options

• MDPI and ACS Style
• Chicago Style

The history of scientific thought about the Formation and evolution of the Solar System begins with the Copernican Revolution. The first recorded use of the term "Solar System" dates from 1704.

1. Contemporary View

The most widely accepted theory of planetary formation, known as the nebular hypothesis, maintains that 4.6 billion years ago, the Solar System formed from the gravitational collapse of a giant molecular cloud which was light years across. Several stars, including the Sun, formed within the collapsing cloud. The gas that formed the Solar System was slightly more massive than the Sun itself. Most of the mass collected in the centre, forming the Sun; the rest of the mass flattened into a protoplanetary disc, out of which the planets and other bodies in the Solar System formed.

There are, however, arguments against this hypothesis.

2. Formation Hypothesis

French philosopher and mathematician René Descartes was the first to propose a model for the origin of the Solar System in his Le Monde (ou Traité de lumière) which he wrote in 1632 and 1633 and for which he delayed publication because of the Inquisition and it was published only after his death in 1664. In his view, the Universe was filled with vortices of swirling particles and the Sun and planets had condensed from a particularly large vortex that had somehow contracted, which explained the circular motion of the planets and was on the right track with condensation and contraction. However, this was before Newton's theory of gravity and we now know matter does not behave in this fashion. [ 1 ]

The vortex model of 1944, [ 1 ] formulated by German physicist and philosopher Baron Carl Friedrich von Weizsäcker, which harkens back to the Cartesian model, involved a pattern of turbulence-induced eddies in a Laplacian nebular disc. In it a suitable combination of clockwise rotation of each vortex and anti-clockwise rotation of the whole system can lead to individual elements moving around the central mass in Keplerian orbits so there would be little dissipation of energy due to the overall motion of the system but material would be colliding at high relative velocity in the inter-vortex boundaries and in these regions small roller-bearing eddies would coalesce to give annular condensations. It was much criticized as turbulence is a phenomenon associated with disorder and would not spontaneously produce the highly ordered structure required by the hypothesis. As well, it does not provide a solution to the angular momentum problem and does not explain lunar formation nor other very basic characteristics of the Solar System. [ 2 ]

The Weizsäcker model was modified [ 1 ] in 1948 by Dutch theoretical physicist Dirk Ter Haar, in that regular eddies were discarded and replaced by random turbulence which would lead to a very thick nebula where gravitational instability would not occur. He concluded the planets must have formed by accretion and explained the compositional difference (solid and liquid planets) as due to the temperature difference between the inner and outer regions, the former being hotter and the latter being cooler, so only refractories (non-volatiles) condensed in the inner region. A major difficulty is that in this supposition turbulent dissipation takes place in a time scale of only about a millennium which does not give enough time for planets to form.

The nebular hypothesis was first proposed in 1734 by Emanuel Swedenborg [ 3 ] and later elaborated and expanded upon by Immanuel Kant in 1755. A similar theory was independently formulated by Pierre-Simon Laplace in 1796. [ 4 ]

In 1749, Georges-Louis Leclerc, Comte de Buffon conceived the idea that the planets were formed when a comet collided with the Sun, sending matter out to form the planets. However, Laplace refuted this idea in 1796, showing that any planets formed in such a way would eventually crash into the Sun. Laplace felt that the near-circular orbits of the planets were a necessary consequence of their formation. [ 5 ] Today, comets are known to be far too small to have created the Solar System in this way. [ 5 ]

In 1755, Immanuel Kant speculated that observed nebulae may in fact be regions of star and planet formation. In 1796, Laplace elaborated by arguing that the nebula collapsed into a star, and, as it did so, the remaining material gradually spun outward into a flat disc, which then formed the planets. [ 5 ]

2.1. Alternative Theories

However plausible it may appear at first sight, the nebular hypothesis still faces the obstacle of angular momentum; if the Sun had indeed formed from the collapse of such a cloud, the planets should be rotating far more slowly. The Sun, though it contains almost 99.9 percent of the system's mass, contains just 1 percent of its angular momentum. [ 6 ] This means that the Sun should be spinning much more rapidly.

Tidal theory

Attempts to resolve the angular momentum problem led to the temporary abandonment of the nebular hypothesis in favour of a return to "two-body" theories. [ 5 ] For several decades, many astronomers preferred the tidal or near-collision hypothesis put forward by James Jeans in 1917, in which the planets were considered to have been formed due to the approach of some other star to the Sun. This near-miss would have drawn large amounts of matter out of the Sun and the other star by their mutual tidal forces, which could have then condensed into planets. [ 5 ] However, in 1929 astronomer Harold Jeffreys countered that such a near-collision was massively unlikely. [ 5 ] Objections to the hypothesis were also raised by the American astronomer Henry Norris Russell, who showed that it ran into problems with angular momentum for the outer planets, with the planets struggling to avoid being reabsorbed by the Sun. [ 7 ]

The Chamberlin-Moulton model

Forest Moulton in 1900 had also shown that the nebular hypothesis was inconsistent with observations because of the angular momentum. Moulton and Chamberlin in 1904 originated the planetesimal hypothesis [ 8 ] (see Chamberlin–Moulton planetesimal hypothesis). Along with many astronomers of the day they came to believe the pictures of "spiral nebulas" from the Lick Observatory were direct evidence of forming solar systems. These turned out to be galaxies instead but the Shapley-Curtis debate about these was still 16 years in the future. One of the most fundamental issues in the history of astronomy was distinguishing between nebulas and galaxies.

Moulton and Chamberlin suggested that a star had passed close to the Sun early in its life to cause tidal bulges and that this, along with the internal process that leads to solar prominences, resulted in the ejection of filaments of matter from both stars. While most of the material would have fallen back, part of it would remain in orbit. The filaments cooled into numerous, tiny, solid fragments, ‘planetesimals’, and a few larger protoplanets. This model received favourable support for about 3 decades but passed out of favour by the late '30s and was discarded in the '40s by the realization it was incompatible with the angular momentum of Jupiter, but a part of it, planetesimal accretion, was retained. [ 1 ]

Lyttleton's scenario [ 1 ]

In 1937 and 1940, Ray Lyttleton postulated that a companion star to the Sun collided with a passing star. Such a scenario was already suggested and rejected by Henry Russell in 1935. Lyttleton showed terrestrial planets were too small to condense on their own so suggested one very large proto-planet broke in two because of rotational instability, forming Jupiter and Saturn, with a connecting filament from which the other planets formed. A later model, from 1940 and 1941, involves a triple star system, a binary plus the Sun, in which the binary merges and later breaks up because of rotational instability and escapes from the system leaving a filament that formed between them to be captured by the Sun. Objections of Lyman Spitzer apply to this model also. [ clarification needed ]

Band-structure model

In 1954, 1975, and 1978 [ 9 ] Swedish astrophysicist Hannes Alfvén included electromagnetic effects in equations of particle motions, and angular momentum distribution and compositional differences were explained. In 1954 he first proposed the band structure in which he distinguished an A-cloud, containing mostly helium, but with some solid- particle impurities ("meteor rain"), a B-cloud, with mostly hydrogen, a C-cloud, having mainly carbon, and a D-cloud, made mainly of silicon and iron. Impurities in the A-cloud form Mars and the Moon (later captured by Earth), in the B-cloud they condense into Mercury, Venus, and Earth, in the C-cloud they condense into the outer planets, and Pluto and Triton may have formed from the D-cloud.

Interstellar cloud theory

In 1943, the Soviet astronomer Otto Schmidt proposed that the Sun, in its present form, passed through a dense interstellar cloud, emerging enveloped in a cloud of dust and gas, from which the planets eventually formed. This solved the angular momentum problem by assuming that the Sun's slow rotation was peculiar to it, and that the planets did not form at the same time as the Sun. [ 5 ] Extensions of the model, together forming the Russian school, include Gurevich and Lebedinsky (in 1950), Safronov (in 1967,1969), Safronov and Vityazeff (in 1985), Safronov and Ruskol (in 1994), and Ruskol (in 1981), among others [ 10 ] However, this hypothesis was severely dented by Victor Safronov who showed that the amount of time required to form the planets from such a diffuse envelope would far exceed the Solar System's determined age. [ 5 ]

Ray Lyttleton modified the theory by showing that a 3rd body was not necessary and proposing that a mechanism of line accretion described by Bondi and Hoyle in 1944 would enable cloud material to be captured by the star (Williams and Cremin, 1968, loc. cit.)

Hoyle's hypothesis

In this model [ 1 ] (from 1944) the companion went nova with ejected material captured by the Sun and planets forming from this material. In a version a year later it was a supernova. In 1955 he proposed a similar system to Laplace, and with more mathematical detail in 1960. It differs from Laplace in that a magnetic torque occurs between the disk and the Sun, which comes into effect immediately or else more and more matter would be ejected resulting in a much too massive planetary system, one comparable to the Sun. The torque causes a magnetic coupling and acts to transfer angular momentum from the Sun to the disk. The magnetic field strength would have to be 1 gauss. The existence of torque depends on magnetic lines of force being frozen into the disk (a consequence of a well-known MHD (magnetohydrodynamic) theorem on frozen-in lines of force). As the solar condensation temperature when the disk was ejected could not be much more than 1000 degrees K., a number of refractories must be solid, probably as fine smoke particles, which would grow with condensation and accretion. These particles would be swept out with the disk only if their diameter at the Earth's orbit was less than 1 meter so as the disk moved outward a subsidiary disk consisting of only refractories remains behind where the terrestrial planets would form. The model is in good agreement with the mass and composition of the planets and angular momentum distribution provided the magnetic coupling is an acceptable idea, but not explained are twinning, the low mass of Mars and Mercury, and the planetoid belts. It was Alfvén who formulated the concept of frozen-in magnetic field lines.

Kuiper's theory

Gerard Kuiper (in 1944) [ 1 ] argued, like Ter Haar, that regular eddies would be impossible and postulated that large gravitational instabilities might occur in the solar nebula, forming condensations. In this, the solar nebula could be either co-genetic with the Sun or captured by it. Density distribution would determine what could form: either a planetary system or a stellar companion. The 2 types of planets were assumed to be due to the Roche limit. No explanation was offered for the Sun's slow rotation which Kuiper saw as a larger G-star problem.

Whipple's theory

In Fred Whipple's 1948 scenario [ 1 ] a smoke cloud about 60,000 AU in diameter and with 1 solar mass ( M ☉ ) contracts and produces the Sun. It has a negligible angular momentum thus accounting for the Sun's similar property. This smoke cloud captures a smaller one with a large angular momentum. The collapse time for the large smoke and gas nebula is about 100 million years and the rate is slow at first, increasing in later stages. The planets would condense from small clouds developed in, or captured by, the 2nd cloud, the orbits would be nearly circular because accretion would reduce eccentricity due to the influence of the resisting medium, orbital orientations would be similar because the small cloud was originally small and the motions would be in a common direction. The protoplanets might have heated up to such high degrees that the more volatile compounds would have been lost and the orbital velocity decreases with increasing distance so that the terrestrial planets would have been more affected. The weaknesses of this scenario are that practically all the final regularities are introduced as a priori assumptions and most of the hypothesizing was not supported by quantitative calculations. For these reasons it did not gain wide acceptance.

• Williams, I.O., Cremin, A.W. 1968. A survey of theories relating to the origin of the solar system. Qtly. Rev. RAS 9: 40–62. ads.abs.harvard.edu/abs
• Woolfson, Michael Mark, The Origin and Evolution of universe and the Solar System, Taylor and Francis, 2000 ; completely considered that collision of the two suns produce the solar system and universe in the entire 100,00 years of the evolution.
• Swedenborg, Emanuel. 1734, (Principia) Latin: Opera Philosophica et Mineralia (English: Philosophical and Mineralogical Works), (Principia, Volume 1)
• See, T. J. J. (1909). "The Past History of the Earth as Inferred from the Mode of Formation of the Solar System". Proceedings of the American Philosophical Society 48 (191): 119–128. 
• Michael Mark (1993). "The Solar System: Its Origin and Evolution". Journal of the Royal Astronomical Society 34: 1–20. Bibcode: 1993QJRAS..34....1W. "Physics Department, University of New York".  http://adsabs.harvard.edu/abs/1993QJRAS..34....1W
• Woolfson, Michael Mark (1984). "Rotation in the Solar System". Philosophical Transactions of the Royal Society of London 313 (1524): 5. doi:10.1098/rsta.1984.0078. Bibcode: 1984RSPTA.313....5W.  https://dx.doi.org/10.1098%2Frsta.1984.0078
• Benjamin Crowell (1998–2006). "5". Conservation Laws. lightandmatter.com. ISBN 0-9704670-2-8. http://www.lightandmatter.com/html_books/2cl/ch05/ch05.html. 
• Sherrill, T.J. 1999. A Career of Controversy: the Anomaly of T.J.J. See. J. Hist. Astrn. ads.abs.harvard.edu/abs/1999JHA.
• Alfvén, H. 1978. Band Structure of the Solar System. In Origin of the Solar System, S.F. Dermot, ed, pp. 41–48. Wiley.
• Williams, I.O., Cremin, A.W. 1968. A survey of theories relating to the origin of the solar system. Qtly. Rev. RAS 9: 40–62. ads.abs.harvard.edu/abs.

• Terms and Conditions
• Privacy Policy
• Advisory Board

Help | Advanced Search

Astrophysics > Earth and Planetary Astrophysics

Title: four-of-a-kind comprehensive atmospheric characterisation of the hr 8799 planets with vlti/gravity.

Abstract: With four companions at separations from 16 to 71 au, HR 8799 is a unique target for direct imaging, presenting an opportunity for the comparative study of exoplanets with a shared formation history. Combining new VLTI/GRAVITY observations obtained within the ExoGRAVITY program with archival data, we perform a systematic atmospheric characterisation of all four planets. We explore different levels of model flexibility to understand the temperature structure, chemistry and clouds of each planet using both petitRADTRANS atmospheric retrievals and fits to self-consistent radiative-convective equilibrium models. Using Bayesian Model Averaging to combine multiple retrievals, we find that the HR 8799 planets are highly enriched in metals, with [M/H] $\gtrsim$1, and have stellar to super-stellar C/O ratios. The C/O ratio increases with increasing separation from $0.55^{+0.12}_{-0.10}$ for d to $0.78^{+0.03}_{-0.04}$ for b, with the exception of the innermost planet which has a C/O ratio of $0.87\pm0.03$. By retrieving a quench pressure and using a disequilibrium chemistry model we derive vertical mixing strengths compatible with predictions for high-metallicity, self-luminous atmospheres. Bayesian evidence comparisons strongly favour the presence of HCN in HR 8799 c and e, as well as CH$_{4}$ in HR 8799 c, with detections at $>5\sigma$ confidence. All of the planets are cloudy, with no evidence for patchiness. The clouds of c, d and e are best fit by silicate clouds lying above a deep iron cloud layer, while the clouds of the cooler HR 8799 b are more likely composed of Na$_{2}$S. With well defined atmospheric properties, future exploration of this system is well positioned to unveil further detail in these planets, extending our understanding of the composition, structure, and formation history of these siblings.

Submission history

Access paper:.

• Other Formats

References & Citations

• Google Scholar
• Semantic Scholar

BibTeX formatted citation

Bibliographic and Citation Tools

Code, data and media associated with this article, recommenders and search tools.

• Institution

arXivLabs: experimental projects with community collaborators

arXivLabs is a framework that allows collaborators to develop and share new arXiv features directly on our website.

Both individuals and organizations that work with arXivLabs have embraced and accepted our values of openness, community, excellence, and user data privacy. arXiv is committed to these values and only works with partners that adhere to them.

Have an idea for a project that will add value for arXiv's community? Learn more about arXivLabs .

2 places named Nineveh, not 7, in path of totality for April 8 eclipse | Fact check

The claim: Eclipse crosses seven cities named Ninevah, crosses 2017 eclipse path in 'Little Egypt, Illinois’

A March 29 Facebook post ( direct link , archive link ) shows a map of the U.S. with the paths of totality for the total solar eclipses in 2017 and 2024 and the annular eclipse in 2023.

“Not only does the solar eclipse April 8th go thru 7 cities called NINEVAH, it crosses the one from 2017 in a town called LITTLE EGYPT, IL (sic),” reads part of the post's caption, which goes on to make the case that those locations show a biblical connection to the eclipse event.

The post was shared more than 1,800 times in a week.

Fact check roundup :  Total solar eclipse sparks many misleading flat Earth claims online. Here's what's true and false

More from the Fact-Check Team: How we pick and research claims | Email newsletter | Facebook page

Our rating: Partly false

This attempt to link the eclipse to biblical locations whiffs on the details. There are two Ninevehs, not seven, inside the April 8 eclipse's path of totality that is shown in the post. The other five Ninevahs will just see a partial eclipse, like nearly all of the continental U.S. The eclipse paths do cross in “Little Egypt,” but that's a large region, not a town.

Two Ninevehs lie in April 8 path of totality; no Illinois town named 'Little Egypt'

The first total solar eclipse to pass over the U.S. since 2017 takes place April 8 . Its path of totality – where the sun will be completely blocked as the moon moves between it and Earth – crosses 13 states from Texas t o Maine .

Fact check : A 'sex magic ritual'? No, NASA rockets are to study atmosphere during eclipse

The image shown in the post is a clear reference to the April 8 path of totality – with a thin yellow line marking the 115-mile-wide swath – along with totality paths from two past ecl ipses .

The Eclipse2024.org website lists municipalities that will experience the total solar eclipse and includes seven places across the U.S. named “Nineveh.” But only two fall in the path of totality: Nineveh, Indiana , and Nineveh, Ohio . Five others – in New York , Virginia , Pennsylvania , Texas and Kentucky – will see a partial eclipse.

Four more Ninevahs are not listed on the eclipse site but do appear on the National Weather Service's list , with two in Missouri , and one in Pennsylvania and New York . None are in the totality path, a searchable U.S. eclipse map published by the NWS in Texas shows.

The post also wrongly claims the April 8 path of totality intersects with the one from the 2017 total solar eclipse in a town named Little Egypt, Illinois. There is no such town. Rather, “Little Egypt” is the nickname for a section of Illinois that includes Carbondale and its surrounding area . Its definition has ranged from the southern half of the state to a group of fewer than 20 counties, according to Mississippi Valley Traveler .

The 2017 and 2024 totality paths cross in an area that spans parts of Missouri and Kentucky, as well as portions of the "Little Egypt" region in Illinois, according to nationaleclipse.com .

The April 8 eclipse has spawned a significant amount of misinformation online. Among the claims USA TODAY has debunked are assertions that the Federal Emergency Management Agency and the National Guard had never before been mobilized for a solar eclipse and that the Earth, moon and sun are not scheduled to be aligned that day .

USA TODAY reached out to the Facebook user who shared the post but did not immediately receive a response.

Lead Stories also debunked a version of the claim.

Our fact-check sources:

• National Weather Service (Internet Archive), accessed April 5, NWS List of Ninevehs
• Eclipse2024, accessed April 5, Eclipse Community Page for Nineveh, Indiana
• Eclipse2024, accessed April 5, The 2024 eclipse in Nineveh, Ohio, USA
• Eclipse2024, accessed April 5, The 2024 eclipse in Nineveh, New York, USA
• Eclipse2024, accessed April 5, The 2024 eclipse in Nineveh, Virginia, USA
• Eclipse2024, accessed April 5, The 2024 eclipse in Nineveh, Texas, USA
• Eclipse2024, accessed April 5, The 2024 eclipse in Nineveh, Pennsylvania, USA
• Eclipse2024, accessed April 5, The 2024 eclipse in Nineveh, Kentucky, USA
• National Weather Service (Internet Archive), accessed April 5, Nineveh MO Adair Co
• National Weather Service (Internet Archive), accessed April 5, Nineveh MO Lincoln Co
• National Weather Service (Internet Archive), accessed April 5, Nineveh PA Greene Co
• National Weather Service (Internet Archive), accessed April 5, Nineveh Junction NY
• Mississippi Valley Traveler, Dec. 8, 2018, Illinois’ Little Egypt
• Carbondale, Illinois, accessed April 5, Meet Our Town
• National Eclipse, accessed April 5, Eclipse Maps
• CBS News, April 5, NASA estimates 99 percent of Americans will see the sun covered, at least partially, during Monday's

Thank you for supporting our journalism. You can subscribe to our print edition, ad-free app or e-newspaper here .

USA TODAY is a verified signatory of the International Fact-Checking Network, which requires a demonstrated commitment to nonpartisanship, fairness and transparency. Our fact-check work is supported in part by a grant from Meta .