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

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 (Earth mass) and 203 +5 −5 M , respectively, with Kepler-90g having an unusually low apparent density of 0.15 ± 0.05 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.

WASP-107b's density is even lower: a case study for the physics of planetary gas envelope accretion and orbital migration

“…While runaway accretion was likely launched at 1 AU, the envelope growth of this super-puffy Neptune was probably stunted as a result of its migration to the inner disk. This scenario could also explain the formation of the super-puff Kepler-90g found at 0.7 AU (Liang et al 2020). In-situ formation, on the other hand, is ruled out for such a high gas-to-core mass ratio as it would require higher metallicities than allowed by the large radius of this planet.…”

Grain Growth in Escaping Atmospheres: Implications for the Radius Inflation of Super-Puffs

“…The red and blue dashed lines show the contour of R(10 −8 bar)/R(10 −2 bar) = 1.5 and 1.3, respectively, and the white unsustainable region indicate the phase space yielding τ loss < 0.1 Gyr via Parker wind mass loss. In the bottom panel, we also plot the super-puff candidates listed in Chachan et al (2020) with adding Kepler-90g (Liang et al 2020), where the shortened planet name (e.g., Kepler-51b as K-51b) is denoted.…”

“…Observational efforts in the last decade revealed the ubiquity of low-mass exoplanets with sizes between Earth and Neptune-super-Earths and sub-Neptunes-in this galaxy (e.g., Mayor et al 2011;Fressin et al 2013;Dressing & Charbonneau 2015;Fulton & Petigura 2018). The Kepler mission discovered several low-mass planets whose sizes are comparable to gas giants (e.g., Lissauer et al 2011;Jontof-Hutter et al 2014;Ofir et al 2014;Masuda 2014;Mills et al 2016;Hadden & Lithwick 2017;Vissapragada et al 2020;Liang et al 2020). They are called super-puffs because of their extremely low bulk density of ρ p 0.1 g cm −3 .…”