Ultra-short-period Planets from Secular Chaos

Abstract

Abstract Over 100 rocky planets orbiting Sun-like stars in very short orbital periods (≲1 day) have been discovered by the Kepler mission. The origin of these planets, known as ultra-short-period (USP) planets, remains elusive. Here, we propose that most of these planets, originally at periods of ∼5–10 days, reach their current orbits via high-eccentricity migration. In a scaled-down version of the dynamics that may have been experienced by their high-mass analogs, the hot Jupiters, these planets reach high eccentricities via chaotic secular interactions with their companion planets and then undergo orbital circularization due to dissipation from tides raised on the planet. This proposal is motivated by the following observations: planetary systems observed by Kepler often contain several super-Earths with non-negligible eccentricities and inclinations, possibly extending beyond ∼au distances; by contrast, only a small fraction of USP planets have known transiting companions, which are generally not closely spaced, and we argue that most of them should have companions with periods ≳10 days. The proposed scenario naturally explains the observation that most USP planets have significantly more distant transiting companions compared to their counterparts at slightly longer periods (1–3 days). Our model predicts that USP planets should have: (i) spin–orbit angles, and inclinations relative to outer planets, in the range of ∼10–50°; (ii) several outer planetary companions extending beyond ∼1 au distances. Both of these predictions may be tested by TESS and its follow-up observations.