Physics, Earth & Space Icon Physics, Earth & Space

How Solar Systems Are Formed

Editor’s note: As a series at ENV, we are pleased to present "Exoplanets." Daniel Bakken is an engineer who teaches astronomy at the college level, and an entrepreneur in compound semiconductor crystal growth. In a series of articles he critically examines recent claims about exoplanets beyond our solar system, asking whether our own planet Earth is a rarity, or common, in the cosmos. For previous articles in the series, see here.

In this series so far, with our new knowledge of planetary formation, illuminated by the ability to test relevant theories against the many new star systems discovered, we have gained new understanding of our solar system, its formation, and its relative commonality, or rarity. With Earth as the only known example so far of a Class I world, we can better understand what a system including such a world looks like, along with its early history.

The solar system formed through a process described by a model, the nebular hypothesis, that was introduced long ago. The early Sun had a disc of material orbiting it. Near the Sun the lighter gases hydrogen and helium were depleted due to the rising temperature. Grains of heavier elements formed in this environment, and coalesced to form the planetesimals that would further aggregate into the future planets. A similar process also occurred in the outer solar system, but outside a frost line where certain substances could condense into ices. In our solar system this frost line fell between the modern-day orbits of Mars and Jupiter. It is easy to see why the large planets are outside this line; they could accumulate more volatiles, like hydrogen, helium, water, methane, ammonia, etc., and also use their gravity to sweep up the lighter gases to become the gas giants.1

exoplanet2.jpgThis model was thought to be able to explain other planetary systems, until we discovered that it wasn’t common. The nebular hypothesis model is still largely accepted, but we have many more details to add. We now understand planetary migration after formation, and as noted above, it must have operated in Earth’s solar system as well. Jupiter likely formed further out than its present orbit. As it migrated inward, it passed through a 2:1 orbital resonance with Saturn, then both came to their present distances, and their orbits circularized.2 Uranus and Neptune also migrated inwards from their points of formation, and likely through their gravitational interactions actually exchanged places in their orbits. That is, Uranus may have formed further out, but became the seventh planet, with Neptune the eighth.3

This updated model is called the Nice model, after the town in France where it was developed. A further update, now called Nice II, includes the asteroids, and more refinements of the terrestrial planets’ orbits. Nice II is also known as the "jumping Jupiter" model, as Jupiter is now thought to have moved much more rapidly past the 2:1 resonance with Saturn, going from less than 2 to more than 2.3:1. This model shows that Jupiter migrated inwards modestly, with Saturn migrating outwardly by about 20 percent.4 Further refinements, referred to as the "Grand Tack scenario," have contributed an earlier view of the solar system in which Jupiter may have migrated inwards towards the Earth’s orbit, then back outward. This may help answer some questions about of the properties of the asteroid belt and its composition.5 Yet there are still many ad-hoc parameters that have to be worked out.6 Nevertheless, we can see the complexity of these models increasing with their ability to model our solar system. Even small changes lead to very different outcomes, and this may help to explain why so many other planetary systems are quite different from ours. With regard to exoplanets, one important observation is the occurrence of gas giant planets in tight orbits around their host stars. These planets must have formed much further from their star, and migrated inwards due to these migration processes. During this migration, any potential terrestrial planets would have been ejected, or driven into the star.7

It appears the orbits of the inner planets in our solar system have been remarkably stable and circular. Long-term climate stability of the Earth is directly related to this property of the solar system’s history. Being in the habitable zone is not sufficient for a planet to continuously keep liquid water on its surface. The planet’s atmosphere and climate must be stable for several billion years. Yet to achieve this balance in the long term, the processes are "still poorly known."8

The long-term circularity of the gas giants’ orbits also plays a direct role in a terrestrial planet’s stability, and they must have assumed their positions in our solar system very early. The relative masses of the giant planets and their positions are very conducive to the Earth’s maintaining a very circular orbit, and allowing its long term stability.9 This is the most striking property of the solar system compared to other planetary systems being discovered. Even the other smaller planets play a role in the Earth’s climate stability. For example, if Saturn had twice its mass, or had been close to a major mean motion resonance, or even if Venus weren’t in the solar system, these all would have led to the Earth having a much greater eccentricity, or non-circular orbit, with catastrophic implications for habitability.10

Next up: A stable atmosphere.

References:

(1) Gonzalez, "Setting the Stage for Habitable Planets, 51-52.

(2) Ibid., 52.

(3) Ibid.

(4) Ibid., 48.

(5) Ibid.

(6) Ibid.

(7) Peter Ward and Donald Brownlee, The Life and Death of Planet Earth: How the New Science of Astrobiology Charts the Ultimate Fate of Our World (New York: Times Books, 2002), 193.

(8) Forget, "On the Probability of Habitable Planets," 180.

(9) Gonzalez, "Setting the Stage for Habitable Planets," 53.

(10) Ibid.