An Independent Planet Search In The Kepler Dataset. II. An extremely low-density super-Earth mass planet around Kepler-87

TL;DR

This study confirms two long-period planets in the Kepler-87 system through TTV analysis, revealing a super-Earth with extremely low density and identifying additional smaller candidates, providing insights into planetary composition and system architecture.

Contribution

First detailed TTV analysis of Kepler-87 system confirming planet masses and densities, including the lowest-density super-Earth, and discovering additional small planetary candidates.

Findings

Kepler-87 c is the lowest-density super-Earth known.

Strong anti-correlated TTVs confirm two long-period planets.

Additional small planetary candidates detected in the system.

Abstract

[ABRIDGED]: Aims: two candidates in the KOI 1574 system are relatively long-period (about 114d and 191d) and in 5:3 resonance. We therefore search for TTVs in this particularly promising system. Methods: The full Kepler data was used, allowing to search for TTVs as well as for additional transit-like signals. Results: We detect strong anti-correlated TTVs of the 114d and 191d signals, dynamically confirming them as members of the same system. Dynamical simulations reproducing the observed TTVs allow us to also determine the masses of the planets. KOI 1574.01 (hereafter Kepler-87 b) was found to have a radius of 13.49 +/- 0.55 R_earth and a mass of 324.2 +/- 8.8M_earth, and KOI 1574.02 (Kepler-87 c) was found to have a radius of 6.14 +/- 0.29R_earth and a mass of 6.4 +/- 0.8M_earth. Both planets have low densities of 0.729 and 0.152 g cm^-3, respectively, which is non-trivial for such cold and old (7-8 Gyr) planets. Specifically, Kepler-87 c is the lowest- density planet in the super-Earth mass range. Both planets are thus particularly amenable to modeling and planetary structure studies, and also present an interesting case were ground-based photometric follow-up of Kepler planets is very desirable. Finally, we also detect two more short period super-Earth sized planetary (< 2R_earth) candidates in the system, making the relatively high multiplicity of this system notable against the general paucity of multiple systems in the presence of giant planets like Kepler-87 b.