The Odd Architecture of the Inner Solar System

The architecture of the inner solar system (arrangement of the four inner terrestrial planets) has puzzled astronomers for decades. Mars has only 10% of the mass of Earth, but standard protoplanetary disk models predict that Mars should be as massive as the Earth because Mars' position in the protoplanetary disk should have allowed it to grow as large as the Earth.

Hansen began his simulation with planetesimals in the blue annulus (Figure 4‑27), which led to the formation of planets with similar locations, eccentricities, and masses to the four terrestrial planets.[1] The majority of material formed Venus and Earth, and a smaller fraction of planetesimals from within the annulus leaked out and formed the much smaller Mercury and Mars. The radiometric dating of Mars supports Hansen’s scenario. Further confirmation was provided by Izidoro, who analyzed a variable density disk and showed that the timescale of formation with Hansen’s annulus predicted that Mars finished forming long before Earth, as observed data indicates.[2] Since Hanson’s model successfully formed the inner solar system architecture, scientists have attempted to determine how a gap or low-density region could have formed between 1 and 2 AU. Two popular explanations for the small Mars are the Grand Tack scenario and the pebble accretion model. Both are dependent on the existence of Jupiter prior to the inner solar system, and this might not be a good assumption (see Section 5-2).

Walsh and Morbidelli proposed the Grand Tack scenario as a mechanism that might have formed the gap between 1 and 2 AU.[3] The Grand Tack Scenario has Jupiter (and Saturn) migrating into the inner solar system to 1.5 or 2 AU and then back out again to its present position at 5 AU. In the process, it would have depleted the protoplanetary disk of planetesimals in the region between 1 and 3 AU and caused the formation of the inner annulus.^[4] However, the inward and outward movement of the giant planets requires a high level of fine-tuning, which makes it an unattractive natural solution. Others have proposed that a gap naturally formed in the young circumsolar disk. However, most planetary scientists do not think that this would happen.

The pebble accretion model is another proposed explanation for the geometry of the inner solar system. Pebble accretion is hypothesized to work in two ways. First, with streaming instability (described in next chapter) dust and pebbles develop concentrations in certain parts of the disk and gravitationally collapse toward each other, forming planetesimals. Second, accretion is that it rapidly increases the size of some planetesimals and planets but not others. Planetesimals or planets latch onto the pebbles by gravity as they pass by, thus increasing the size of larger planetesimals. If Mars for some reason latched onto fewer pebbles, then this would explain why it is smaller. Levison simuulated a pebble accretion model in which a smaller Mars did not accrete pebbles migrating from the outer to inner solar system. Levison’s model also prevented pebbles from passing Jupiter into the inner solar system. These two modifications produced planets that match the inner solar system architecture with a small Mars. Although Levison’s model produced the inner solar system, other researchers have not supported his model outcomes. Gerbig (2019) showed that pebbles and planetesimals form everywhere in the disk and that this occurs between 10,000 and 1,000,000 years after the initial formation of the disk.[5] Izidoro et al. (2019) simulated planet formation with pebble accretion and had Jupiter and Saturn intercept inward moving pebbles. [6]

"Jupiter and Saturn are the great architects of the solar system. The cores of Jupiter and Saturn probably formed early and regulated the pebble flux to the terrestrial region. They first intercepted and consumed part of the pebbles drifting inwards but eventually reached pebble isolation disconnecting the inner and outer solar system. As consequence of this process, terrestrial protoplanetary embryos got starved and only grew –at most– to about Mars-mass.”

Even with interception of pebbles by Jupiter and Saturn, Izidoro found that multiple super earths (5 times the size of Earth) typically formed in the inner solar system. None of Izidoro’s planet formation simulations formed the architecture of the solar system.

"In light of what we just discussed about our current view of solar system formation, none of our simulations comes close to form our solar system. Perhaps our closest approximation is produced in Model-III with Rockypeb = 0.1 cm and S peb = 1 (see top-left of Figure 4‑27). This figure suggests that if just a few seeds form near the snowline, they can grow much more than the inner planetary seeds. In some of these simulations, the more massive cores have a few Earth-masses and sit around 2-3 AU [7]

Not only is the architecture of the inner solar system odd with a small Mars, but the overall architecture of the solar system with small inner planets and giant outer planets is also odd (as mentioned by Raymond earlier in this chapter). Unlike Brad Hanson’s theoretical work based on computer simulations of planetesimals, Lauren Weiss, an observational astronomer at the University of Montreal, observed exoplanet systems with multiple exoplanets in order to determine whether it is normal to have different sized planets or similar sized planets in a planetary system.[8] She is part of the California-Kepler Survey team, which is a telescope that observes exoplanets. In her work, she focused on 355 multi-planet systems. Prior to observation of exoplanets, scientists did not think that the overall architecture of the solar system with small inner planets and large outer planets was necessarily odd. However, Weiss observed that planetary systems tend to have planets of similar sizes, and that planetary systems especially to not have large differences in size from one planet to the next. She called her observations “planets in a pod” (Figure 4-26) where planets have similar sizes in each system, and they have regular spacing, such as the peas in a pod.

Hanson’s research disrupted our preconceived ideas about our own circumsolar disk, which we assumed was relatively uniform with distance from the sun. Weiss’s research disrupted our concept of planetary systems, which we assumed were like our own solar system with vastly different planet sizes. We didn’t realize that our disk was theoretically odd or that our solar system is unusual in the general population of planetary systems. As described in the next chapter, isotope data indicates that the inner and outer planets in the solar system formed independently. This would explain why the planets in the inner and outer solar system have such drastically different sizes. The solar system violates Lauren Weiss’s “planets in a pod” scenario even worse than just two populations of planets. Mars is the smallest inner planet, and it is next to the largest outer planet, Jupiter. In addition, there is a gap in planets between Mars and Jupiter. Scientists love to disrupt the status quo with innovative research and data analysis, and Hanson and Weiss certainly did that. It is an active research area, so expect papers to continue to come out that either raise or answer questions about our solar system.

[1] Hansen, Brad MS. "Formation of the terrestrial planets from a narrow annulus." The Astrophysical Journal 703, no. 1 (2009): 1131.

[2] Izidoro, Local Mass Depletion.

[3] Walsh, Kevin J., Alessandro Morbidelli, Sean N. Raymond, D. P. O’brien, and A. M. Mandell. "Populating the asteroid belt from two parent source regions due to the migration of giant planets—“The Grand Tack”." Meteoritics & Planetary Science 47, no. 12 (2012): 1941-1947.

[4] Izidoro, André, N. Haghighipour, O. C. Winter, and M. Tsuchida. "Terrestrial planet formation in a protoplanetary disk with a local mass depletion: A successful scenario for the formation of Mars." The Astrophysical Journal 782, no. 1 (2014): 31.

[5] Gerbig, Konstantin, Christian T. Lenz, and Hubert Klahr. "Linking planetesimal and dust content in protoplanetary disks via a local toy model." arXiv preprint arXiv:1908.02608 (2019).

[6] Izidoro, André, Bertram Bitsch, Sean N. Raymond, Anders Johansen, Alessandro Morbidelli, Michiel Lambrechts, and Seth A. Jacobson. "Formation of planetary systems by pebble accretion and migration: Hot super-Earth systems from breaking compact resonant chains." arXiv preprint arXiv:1902.08772 (2019).

[7] Izidoro, Formation

[8] Naud Marie-Eve. 2018. Planets around other Stars are like Peas in a Pod. Institute for Research on Exoplanets. University of Montreal. http://www.exoplanetes.umontreal.ca/planets-around-other-stars-are-like-peas-in-a-pod/?lang=en