Did Fomalhaut, HR 8799, and HL Tauri Form Planets via the Gravitational Instability? Placing Limits on the Required Disk Masses

TL;DR

This study assesses whether gravitational instability can form giant planets in systems like Fomalhaut, HR 8799, and HL Tau by placing limits on the disk masses needed, using improved cooling time calculations.

Contribution

It introduces a new technique for accurately determining disk cooling times, refining the mass limits for planet formation via disk fragmentation.

Findings

Disk mass needed is about 0.1 solar masses for Fomalhaut b, HL Tau, and HR 8799's outer planet.

Shorter cooling times make disk fragmentation more feasible for these planets.

Inner HR 8799 planets likely did not form in situ by disk fragmentation.

Abstract

Disk fragmentation resulting from the gravitational instability has been proposed as an efficient mechanism for forming giant planets. We use the planet Fomalhaut b, the triple-planetary system HR 8799, and the potential protoplanet associated with HL Tau to test the viability of this mechanism. We choose the above systems since they harbor planets with masses and orbital characteristics favored by the fragmentation mechanism. We do not claim that these planets must have formed as the result of fragmentation, rather the reverse: if planets can form from disk fragmentation, then these systems are consistent with what we should expect to see. We use the orbital characteristics of these recently discovered planets, along with a new technique to more accurately determine the disk cooling times, to place both lower and upper limits on the disk surface density--and thus mass--required to form these objects by disk fragmentation. Our cooling times are over an order of magnitude shorter than those of Rafikov (2005),which makes disk fragmentation more feasible for these objects. We find that the required mass interior to the planet's orbital radius is ~0.1 Msun for Fomalhaut b, the protoplanet orbiting HL Tau, and the outermost planet of HR 8799. The two inner planets of HR 8799 probably could not have formed in situ by disk fragmentation.