Why Super-Jupiters Are More Likely To Migrate Outward? (Planetary Science)

Dempsey and colleagues in their recent paper showed that, high masses planets like super-jupiters are more likely to migrate outward. This is because eccentric disks have more extended outer gaps and weaker outer torques.

Planetary migration occurs when a planet or other body in orbit around a star interacts with a disk of gas or planetesimals, resulting in the alteration of its orbital parameters, especially its semi-major axis. Planetary migration is the most likely explanation for hot Jupiters: exoplanets with Jovian masses but orbits of only a few days. There are many different mechanisms by which planets’ orbits can migrate like disk migration, tidal migration, planetesimal-driven migration etc.

Out of all these mechanisms, disk migration arises from the gravitational force exerted by a sufficiently massive body embedded in a disk on the surrounding disk’s gas, which perturbs its density distribution. Type II migration is a subtype of disk migration. In type II, it is hypothesized that planets open gaps that are empty of gas. As a result, gas does not cross a planet’s orbit, and the planet is forced to migrate in lockstep with the disk’s inward accretion flow. But this hypothesis is at odds with hydrodynamical simulations, which find that inward gas flow is largely unimpeded by a deep gap. Furthermore, recent simulations show that the giant planets are of about one Jupiter mass migrate inward at a rate differs from the Type II prediction.

Now, Dempsey and colleagues by running 2D hydrodynamical simulations showed that at higher masses, planets migrate outward. Their study recently appeared in Arxiv.

“Our result differs from previous ones because of our longer simulation times, lower viscosity, and our boundary conditions that allow the disk to reach viscous steady state.”

They used the staggered-mesh GPU accelerated code FARGO3D. The code is run in the frame centered on the star and co-rotating with the planet.

Profiles of surface density, eccentricity, and (cumulative) deposited torque in some nearly circular disks (top panels) and eccentric disks (bottom). Eccentric disks have more extended outer gaps and weaker outer torques. The latter forces their total torque, ∆T ≡ Tdep|rrp, to be negative. Simulations are labeled such that e.g., ”q8a30h5” corresponds to q = 8 × 10¯3, α = 30 × 10¯3, and h = 5 × 10¯2 © Dempsey et al.

They showed that the transition from inward to outward migration coincides with the known transition from circular to eccentric disks that occurs for planets more massive than a few Jupiters. In an eccentric disk, the torque on the outer disk weakens due to two effects: the planet launches weaker waves, and those waves travel further before damping. As a result, the torque on the inner disk dominates, and the planet pushes itself outward.

“In the circular disks, the gaps are nearly symmetric relative to the planet’s position, while in the eccentric disks, the outer half of the gap becomes significantly wider than the inner one. In addition, the circular disks have a modest density pile-up outside of the planet’s orbit, and the eccentric ones have a deficit.”

“Our results suggest that the many super-Jupiters observed by direct-imaging at large distances from the star may have gotten there by outward migration.”

— concluded authors of the study

Featured image: Artist impression of super jupiter © respective owner

Reference: Adam M. Dempsey, Diego J. Muñoz, Yoram Lithwick, “Super-Jupiters Migrate Outward”, Arxiv, pp. 1-10, 2021. https://arxiv.org/abs/2105.05277

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