Formation of Planets

There are many questions about planet formation. Two of the keys to an understanding of planet formation are the relics remaining from the early solar system and the analysis of isotopes on earth, other planets and asteroids, and the relic meteorites. A recent paper in Science based on analysis of isotopes indicates that the inner planetesimals formed early in the disk, and radioactive aluminum 26 dried out the planetesimals that formed Earth.  In case you didn't watch it before, the following video is primarily about this chapter. 

Scientists analyze chondrite meteorites to try to understand the great dichotomy between the small inner planets and the giant outer planets (Figure 5-2).

Studies of isotopes in materials from the inner and outer solar system indicate that the inner solar system remained independent of the outer solar system. The inner solar system has non carbonaceous planets, meteorites, and asteroids while the outer solar system has carbonaceous meteorites, asteroids, and planets [1] (Figure 5-3). There is also a bimodal distribution of asteroids in the asteroid belt with most of the non-carbonaceous asteroids in the inner part of the asteroid belt and the carbonaceous asteroids are mostly in the outer part of the asteroid belt. Bennu, the asteroid that Osirix-Rex sampled, is a carbonaceous asteroid, which means it formed from the pool of material in the outer solar system. Earth's isotope profile is like the non-carbonaceous meteorites and asteroids. Water on earth also came from the inner solar system. The difference in isotopes indicates that that inner and outer planets formed from two pools of material, with the inner planets forming from materials in the inner solar system.

"Geochemical and astronomical evidence demonstrates that planet formation occurred in two spatially and temporally separated reservoirs. The origin of this dichotomy is unknown.... Measurements of meteorites have shown that the inner and outer Solar System formed from two distinct reservoirs of material. Existing models have proposed that these were split by Jupiter forming first, which would open a gap in the protoplanetary disc...."  [2]

"There is some physical mechanism in the disk around the young Sun that kept these families separated," said Lichtenberg, an astrophysicist at the University of Oxford" [3]

The asteroid belt is outside the orbit of Mars. Although the asteroid belt looks like it has a high mass in Figure 5-4, the entire asteroid belt has only 4% of the mass of the moon. The lack of material in the asteroid belt indicates that "Significant mass loss from the belt likely took place at a very early stage in Solar System evolution." [4] Three hypothesized models of solar system formation attempt to explain the lack of material in the circumsolar disk between Earth's orbit and the outside of the asteroid belt. Two models attribute the gap to Jupiter, either migration by Jupiter into the inner solar system (Grand Tack) or gravitational perturbation of the region (Nice model) by Jupiter from its current orbit. In the Grand Tack hypothesis, Jupiter and Saturn moved into the inner solar system and scattered planetesimals outward and then moved into the outer solar system and scattered inner and outer solar system planetesimals inwards, thus resulting in the bimodal distribution in the asteroid belt and the overall lack of mass in the asteroid belt.[5] In the "Nice" model (Nice, France), Jupiter excited the asteroid belt zone by gravity and depleted it of material. There is also the low mass asteroid belt hypothesis, which is that there was a gap in the disk in the region between Earth and the outside of the asteroid belt, but the cause is unknown. See the MOJO 10/11 video below for more details on these hypotheses.

The following video might be a little out of date, but it describes some of the issues associated with planet formation

ALMA observations of disks such as the 100,000-year-old HL Tauri disk indicate that planets can form within 100,000 years of the formation of the star, which is faster than astronomers had expected prior to the ALMA observations, and which indicates that rapid formation of inner solar system planets might have taken place. Lichtenberg stated, 

"Astronomical observations of these disks provide evidence for rapid dust coagulation (1), show ringed substructure (2), and indicate a decrease in total dust mass with disk age, to below the total masses of fully assembled exoplanetary systems. This suggests that planet formation starts early."  [6]

If the earth and terrestrial planets formed early in the solar system, prior to Jupiter, then this might solve the problem of the dry Earth, which should have been a wet planet with a high fraction of water and other volatiles, based on its distance from the sun. [7][8] If the planetesimals in the inner solar system formed quickly, < 0.3 million years after the beginning of the solar system, then aluminum-26 would have still been highly radioactive. The rapid decay of aluminum-26 would have generated tremendous heat and dried out planetesimals. The amount of drying would have been dependent on the size of planetesimals. The larger planetesimals, greater than 30 km radius would have become hotter and would have become completely dehydrated. Because the planetesimals were small and had minimal gravity, the water would have been lost and escaped into interstellar space. [9] [10][11] Lichtenberg proposed that two bursts of planet formation took place as the snowline migrated from the inner to the outer solar system, with the inner planets forming first. He did not consider that there might have been a preexisting gap in the protoplanetary disk.

"The first planetesimal burst incorporated a substantial amount of radioactive aluminum-26, which heated the inner terrestrial protoplanets from the inside," Lichtenberg said. "This degassed the initial water abundances and made the inner terrestrial planets dry. This did not not happen with the outer solar system, which formed over a longer time interval," [12]

The rest of this section describes mechanisms that might have formed the planetesimals in the inner solar system. Dust particles (water ice and minerals) gather in the midplane dust layer (Figure 5‑5). Possible planetesimal formation mechanisms include (1) accretion, (2) disk fragmentation, and (3) streaming instability and pebble accretion.  

Accretion begins in the midplane dust layer when small dust particles stick together by Van Der Waals (electrostatic) forces; however, simulations indicate that the first stage of accretion is cut short at earth’s orbital distance because of the meter-sized barrier problem. After particles reach one cm in diameter, they decouple from the gas field and encounter wind drag in the gas field because of the differential velocities of gas and particles. S. J. Weidenschilling (1977) found that particle accretion could not form objects larger than 1 m at earth’s orbit.  His calculations showed that one-meter diameter objects drift in towards the sun at a velocity of 100 m/s due to gas drag.  At that speed, objects fall into the sun in about 1,000 years.  Although accretion can form planets in the outer solar system, where the disk rotates slowly and gas drag is less intense, accretion is not likely to have formed planets in the inner solar system. 

The second possible mode of planetesimal formation is disk fragmentation, also called gravitational instability. In order for this to happen the midplane dust layer must become so concentrated that gravity takes over and breaks apart the midplane dust layer into units that become 100 km diameter planetesimals. 

The third proposed mechanism is streaming instability. The first part of the following MOJO video describes streaming instability. In this scenario, streams of dust lead to clumping of pebbles (1 mm - 10 cm), which then collapse together by gravity and form a planetesimal.  The planetesimal continues to grow as it attracts more pebbles. The problem with this theory in the inner solar system is that the calculated gas to solids ratio is too high. Possibly, if the inner solar system was wet, with water in the midplane, the gas to solids ratio would have been higher than that calculated by assuming a dry inner solar system. In the streaming instability model, the process forms planetesimals of 100 km size without going through the accretion stage; thus, this scenario is also supported by the sizes of asteroids in the asteroid belt. https://youtu.be/yXq1i3HlumA.

By whatever mechanism planetesimals formed, models indicate that planetesimals naturally grow into planets in the inner solar system. The larger planetesimals grow faster than small planetesimals because they capture more material than smaller planetesimals. Eventually, they start to capture objects by gravity and enter the oligarchic stage where one planetary embryo begins to absorb all the others and eventually grows into a planet. During the last phase, the planetary embryos and planets smash into each other. Earth probably had 5 of these giant impacts, with the last one forming the moon (Section 5-3)

In the outer solar system, the large planets captured the gas in their orbital range once they reached a size 10 times larger than the earth. The gas giants, Jupiter and Saturn, had to capture the gas before the gas dissipated, which is generally about 3 million years from the time the gas cloud collapses and initially forms a protostar. 

A major problem in solar system formation models is that models and observations indicate that inward orbital migration of large planets is common in planetary systems (see MOJO video 10/11), but it is likely that the most planets formed in their current positions in the solar system. One migration probably took place. Scientists think that Neptune moved from a position in the mid solar system to the outer solar system 3.9 billion years ago.  As it moved through asteroid belts, it threw the asteroids inward toward the sun. The activity peaked 3.9 billion years ago during the Late Heavy Bombardment, when many asteroids impacted the earth. 

There can also be interactions between planets in which closely orbiting planets interact gravitationally where one planet is kicked out of the planetary system, and the other acquires an eccentric orbit. This apparently did not happen in the solar system because all the planets have relatively circular orbits.

References

[1] Kruijer, Thomas S., Thorsten Kleine, and Lars E. Borg. "The great isotopic dichotomy of the early Solar System." Nature Astronomy 4, no. 1 (2020): 32-40. The great isotopic dichotomy of the early Solar System | Nature Astronomy 

[2] Lichtenberg, Tim, Joanna Dra̧żkowska, Maria Schönbächler, Gregor J. Golabek, Thomas O. Hands. "Bifurcation of planetary building blocks during Solar System formation." Science 371, 6527 (2021): 365-70. Bifurcation of planetary building blocks during Solar System formation | Science (sciencemag.org) 

[3] Hays, Brooks, Two-step formation explains our solar system's dichotomy of planets. UPI Science News. Jan 21, 2021. Accessed on Jan 24, 2021 at <Two-step formation explains our solar system's dichotomy of planets - UPI.com >

[4] Burbine, Thomas H., and Richard C. Greenwood. "Exploring the Bimodal Solar System via Sample Return from the Main Asteroid Belt: The Case for Revisiting Ceres." Space science reviews 216, no. 4 (2020): 1-27. 

[5]  Burbine and Greenwood, Bimodal

[6] Lichtenberg, Bifurcation.

[7] Kuchner, Marc J. "Volatile-rich Earth-mass planets in the habitable zone." The Astrophysical Journal Letters 596, no. 1 (2003): L105. 

[8] Tian, Feng, and Shigeru Ida. "Water contents of Earth-mass planets around M dwarfs." Nature Geoscience 8, no. 3 (2015): 177-180. 

[9] Lichtenberg, Tim, Gregor J. Golabek, Taras V. Gerya, and Michael R. Meyer. "The effects of short-lived radionuclides and porosity on the early thermo-mechanical evolution of planetesimals." Icarus 274 (2016): 350-365. 

[10] Lichtenberg, Tim, Gregor J. Golabek, Remo Burn, Michael R. Meyer, Yann Alibert, Taras V. Gerya, and Christoph Mordasini. "A water budget dichotomy of rocky protoplanets from 26 Al-heating." Nature Astronomy 3, no. 4 (2019): 307-313. 

[11] Lichtenburg, bifurcation.

[12] Hays, Two-step

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Arrokoth planetesimal in Kuiper belt (beyond Neptune). Credit: NASA/Johns Hopkins/Roman Tkachenko