Constraining the Oblateness of Kepler Planets

TL;DR

This study uses Kepler light curves to measure the oblateness of exoplanets and brown dwarfs, providing the first tentative detection of an oblate exoplanet and setting constraints on others.

Contribution

First tentative detection of an oblate exoplanet outside the solar system and new constraints on the oblateness of several Kepler planet candidates.

Findings

Tentative detection of oblateness in Kepler 39b (KOI-423.01).

Constraints on oblateness of HAT-P-7b, KOI-686.01, and KOI-197.01.

Implication that brown dwarf Q'-value must be much larger to prevent tidal spin-down.

Abstract

We use Kepler short cadence light curves to constrain the oblateness of planet candidates in the Kepler sample. The transits of rapidly rotating planets that are deformed in shape will lead to distortions in the ingress and egress of their light curves. We report the first tentative detection of an oblate planet outside of the solar system, measuring an oblateness of $0.22 \pm 0.11$ for the 18 $M_J$ mass brown dwarf Kepler 39b (KOI-423.01). We also provide constraints on the oblateness of the planets (candidates) HAT-P-7b, KOI-686.01, and KOI-197.01 to be < 0.067, < 0.251, and < 0.186, respectively. Using the Q'-values from Jupiter and Saturn, we expect tidal synchronization for the spins of HAT-P-7b, KOI-686.01 and KOI-197.01, and for their rotational oblateness signatures to be undetectable in the current data. The potentially large oblateness of KOI-423.01 (Kepler 39b) suggests that the Q'-value of the brown dwarf needs to be two orders of magnitude larger than that of the solar system gas giants to avoid being tidally spun-down.