The properties of planets that form via disc instability
Investigating the properties of giant planets that form via direct gravitational collapse.
A previous News Item highlighted research that indicated that wide orbit, gas giant planets probably formed via direct gravitational collapse, rather than via the bottom up growth of solids to form a core that, if sufficiently massive, can then attract a massive gaseous envelope. In a new paper (Hall et al. 2017) we use high-resolution, numerical simulations to probe the properties of the objects that might form via direct gravitational collapse (also referred to as disc instability). As with earlier work, we find that these objects tend to form on wide orbits, and that their masses are typically greater than that of Jupiter. This is consistent with these being the source of the directly imaged exoplanets that we are finding on wide orbits.
We also find it is typical for multiple objects to form in each disc. What this means that they can then dynamically interact with each other, as illustrated in the figure below. This has been largely ignored in earlier work, and can quite significantly influence the outcome. Some of the objects can be ejected to become free-floating planets, or they can be scattered onto even wider orbits than where they formed. Their orbital eccentricities can be pumped to higher values (i.e., the orbits are non-circular). Some also spiral in towards the central star, but are typically destroyed and so this is unlikely to play a significant role in the formation of close-in exoplanets.
What this research suggests is that it is important to consider dynamical interactions during the formation process in order to properly understand the properties of objects that might form via direct gravitational collapse.