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Forming Ultra-Short-Period Planets Via Disk Migration in a Sub-Keplerian Disk

Presentation #101.03 in the session “Evolution and Migration in Exoplanet Systems: Sub-Neptunes and Super-Earths”.

Published onJun 01, 2021
Forming Ultra-Short-Period Planets Via Disk Migration in a Sub-Keplerian Disk

Ultra-short-period planets (USPs) reside interior to the expected truncation radius for a typical T Tauri disk, requiring extra explanations for their current orbital locations beyond simple disk migration. In particular, once a planet migrates close to the disk truncation radius, Type I torques will go to zero or switch direction depending on the stellar and disk conditions, and the result is that the planet is expected to stop migration and become trapped. Smaller planets will become trapped at larger distances exterior to the gap, at odds with the observation correlation that USPs tend to have smaller physical radii (<2 Earth radii). In this presentation, we explain how for suitable disk parameters, magnetically-driven sub-Keplerian gas flow in the inner disk can naturally counteract these effects and subsequently produce USPs at their observed orbital radii. The sub-Keplerian gas flow provides a headwind to small planets, providing a strong torque which can overcome the effects of outwards Type I migration in the inner disk. For suitable disk and planet parameters, the torques due to the sub-Keplerian gas flow lead to inwards migration on a rapid timescale. This mechanism plus the variability of T Tauri stars results in a plausible mechanism to take small planets from 0.05 - 0.1 AU to 0.01-0.02 AU, consistent with the currently observed USP properties.

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