Kepler-90: Giant transit-timing variations reveal a super-puff

TL;DR

This study analyzes large transit timing variations in the Kepler-90 system to precisely determine planetary masses and orbits, revealing a super-puff planet and insights into system stability and dynamical history.

Contribution

It provides a unique solution to orbital parameters of Kepler-90g and h using large TTVs, and uncovers the system's stability and non-resonant configuration over billions of years.

Findings

Kepler-90g is a super-puff with low density.

The system's planets are near 2:3 resonance but not locked.

The system remains stable over billions of years.

Abstract

Exoplanet Transit Timing Variations (TTVs) caused by gravitational forces between planets can be used to determine planetary masses and orbital parameters. Most of the observed TTVs are small and sinusoidal in time, leading to degeneracies between the masses and orbital parameters. Here we report a TTV analysis of Kepler-90g and Kepler-90h, which exhibit large TTVs up to 25 hours. With optimization, we find a unique solution which allows us to constrain all of the orbital parameters. The best fit masses for Kepler-90g and 90h are $15.0^{+0.9}_{-0.8}$ $M_{\bigoplus}$ (Earth mass) and $203^{+5}_{-5}M_{\bigoplus}$, respectively, with Kepler-90g having an unusually low apparent density of $0.15\pm 0.05\, {\rm g\,cm^{-3}}$. The uniqueness of orbital parameter solution enables a long-term dynamical integration, which reveals that although their periods are close to 2:3 orbital resonance, they are not locked in resonance, and the configuration is stable over billions of years. The dynamical history of the system suggests that planet interactions are able to raise the eccentricities and break the resonant lock after the initial formation.