Indeed, an understanding of the evolution of the gaseous component of planet-forming disks can provide insights into the processes governing planet formation. In the context of giant planet formation, both the total disk masses (which are dominated by the gaseous component), and the lifetime of gas in the giant planet region of the disk can be used to constrain the dominant mode(s) of giant planet formation. That is, since the gravitational instability mode of giant planet formation requires disk masses that are a fair fraction of the mass of the star (Md — 0.1 M*), gravitational instability may be a relatively uncommon mode of planet formation if disks are typically less massive. The gravitational instability mode also operates on very short timescales (^1 Myr), and so is not particularly constrained by a short lifetime for the gas. In contrast, the core accretion mode of giant planet formation can make giant planets efficiently with only modest mass disks (Md — 0.01 Mq). However, it operates more leisurely, taking some 1-10 Myrs to form giant planets, so a short lifetime for the gas could limit the prevalence of this mode of giant planet formation.
The lifetime of gas in the terrestrial planet region of the disk is also of interest. This is because residual gas in this region of the disk can affect the outcome of terrestrial planet formation, i.e., the masses and eccentricities of planets, and their consequent habitability. For example, in the picture of terrestrial planet formation described by Kominami & Ida (2002), only a narrow range in residual gas column density is likely to lead to planets with Earth-like masses and eccentricities. If the gas column density in the terrestrial planet region is much larger than 1 g cm~2 at the epoch when lunar-mass protoplanets assemble to form terrestrial planets (at ages of a few Myr), gravitational gas drag is strong enough to circularize the orbits of the lunar-mass protoplanets, making it difficult for them to collide and build up planets with Earth-like masses, i.e., masses that would be large enough to support gaseous atmospheres. Conversely, if the gas column density in the terrestrial planet region is much less than 1 g cm~2, Earth-mass planets can be produced, but gravitational gas drag is too weak to circularize the orbit of the planet. As a result, only a narrow range of gas column densities ^1gcm~2 is expected to produce terrestrial planets with the Earth-like masses and low eccentricities that we associate with habitability on Earth.
These considerations motivate the characterization of the gaseous component of disks over a range of radii, in both the giant and terrestrial planet regions of the disk (120 AU) and over a range of masses—from the large gas masses characteristic of giant planet formation (> 1Mj), down to the residual gas masses of interest for the outcome of terrestrial planet formation.
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