A test of the high-eccentricity migration scenario for close-in planets

Abstract

In the high-eccentricity migration (HEM) scenario, close-in planets reach the vicinity of the central star on high-eccentricity orbits that become circularized---with a concomitant decrease in the semimajor axis---through a tidal interaction with the star. Giant planets that arrive with periastron distances that are smaller than the Roche limit $a_{\mathrm{R}}$ lose their gaseous envelopes, resulting in an inner edge to the surviving planets' distribution. The observational evidence for this effect, while extensive, is nonetheless somewhat ambiguous because of the effect of tidal orbital decay. Here we consider another key prediction of the HEM scenario---the existence of a spatial eccentricity gradient near the location where the circularization time becomes comparable to the planet's age for typical parameters. Previous studies already found evidence for this gradient and demonstrated that its properties are consistent with the circularization process being dominated by tidal dissipation in the planet (encapsulated by the tidal quality factor $Q_{\mathrm{p}}^{\prime }$). Our work extends these treatments by constructing explicit model distributions for comparison with the data and by carrying out backward-in-time integrations using observed system parameters. We show that circularization generally occurs outside the distribution's inner edge (which defines the boundary of the so-called sub-Jovian desert) and that typically $Q_{\mathrm{p}}^{\prime }\approx 10^6$ in the circularization zone (to within a factor of 3). We also find tentative evidence for an eccentricity gradient in lower-mass planets, indicating that formation through HEM may be relevant down to Neptune scales.

Under review in AJ

References in corpus (8)

• Tidal Evolution of Close-in Extra-Solar Planets
• The Mass of KOI-94d and a Relation for Planet Radius, Mass, and Incident Flux
• On the tidal evolution of Hot Jupiters on inclined orbits
• Gas disks to gas giants: Simulating the birth of planetary systems
• On The Origins Of Eccentric Close-in Planets
• A population of planetary systems characterized by short-period, Earth-sized planets
• Planets on the Edge
• On the origin of dynamically isolated hot Earths

Cited by in corpus (6)

• Mapping the exo-Neptunian landscape. A ridge between the desert and savanna
• On the origin of dynamically isolated hot Earths
• Dynamical Evolution of Closely Packed Multiple Planetary Systems Subject to Atmospheric Mass-Loss
• Sub-Jovian desert of exoplanets at its boundaries. Parameter dependence along the main sequence
• Planetary companions orbiting the M dwarfs GJ 724 and GJ 3988. A CARMENES and IRD collaboration
• The Neptunian ridge as a natural outcome of high-eccentricity tidal migration