Migration of Extrasolar Planets: Effects from X-Wind Accretion Disks

TL;DR

This paper investigates how magnetic effects in X-wind accretion disks influence the migration and formation of extrasolar planets, revealing new migration mechanisms, disk truncation effects, and conditions favorable for in situ giant planet formation.

Contribution

It introduces a subkeplerian migration mechanism driven by magnetic disk properties and explores its impact on planetary migration and formation, differing from traditional models.

Findings

Subkeplerian flow drives a new planet migration mechanism.

Inner disk truncation explains observed planetary orbital periods.

Enhanced midplane density supports in situ giant planet formation.

Abstract

Magnetic fields are dragged in from the interstellar medium during the gravitational collapse that forms star/disk systems. Consideration of mean field magnetohydrodynamics (MHD) in these disks shows that magnetic effects produce subkeplerian rotation curves and truncate the inner disk. This letter explores the ramifications of these predicted disk properties for the migration of extrasolar planets. Subkeplerian flow in gaseous disks drives a new migration mechanism for embedded planets and modifies the gap opening processes for larger planets. This subkeplerian migration mechanism dominates over Type I migration for sufficiently small planets (m_P < 1 M_\earth) and/or close orbits (r < 1 AU). Although the inclusion of subkeplerian torques shortens the total migration time by only a moderate amount, the mass accreted by migrating planetary cores is significantly reduced. Truncation of the inner disk edge (for typical system parameters) naturally explains final planetary orbits with periods P=4 days. Planets with shorter periods P=2 days can be explained by migration during FU-Ori outbursts, when the mass accretion rate is high and the disk edge moves inward. Finally, the midplane density is greatly increased at the inner truncation point of the disk (the X-point); this enhancement, in conjunction with continuing flow of gas and solids through the region, supports the in situ formation of giant planets.