Zodiacal Exoplanets in Time (ZEIT) VI: a three-planet system in the Hyades cluster including an Earth-sized planet

TL;DR

This paper reports the discovery of a three-planet system in the Hyades cluster, including an Earth-sized planet, providing valuable insights into planetary evolution in young stellar environments.

Contribution

It presents the first robust detection of an Earth-sized planet in a young cluster and characterizes a three-planet system around a Hyades star, enabling follow-up studies.

Findings

Discovery of an Earth-sized planet in a young cluster.

Identification of two larger planets likely a mini-Neptune and super-Earth.

Potential for precise radial velocity follow-up due to star's brightness and low variability.

Abstract

Planets in young clusters are powerful probes of the evolution of planetary systems. Here we report the discovery of three planets transiting K2-136 (EPIC 247589423), a late K dwarf in the Hyades (~800 Myr) cluster, and robust detection limits for additional planets in the system. The planets were identified from their K2 light curves, as part of our survey of young clusters and star forming regions. The smallest planet has a radius comparable to Earth (0.99 +/- 0.05 Earth radii), making it one of the few Earth-sized planets with a known, young age. The two larger planets are likely a mini-Neptune and a super-Earth, with radii of 2.91+/-0.11 and 1.45+/-0.10 Earth radii, respectively. The predicted radial velocity signals from these planets are between 0.4 and 2 m/s, achievable with modern precision RV spectrographs. Because the target star is bright (V=11.2) and has relatively low-amplitude stellar variability for a young star (2-6 mmag), EPIC 247589423 hosts the best planets known in a young open cluster for precise radial velocity follow-up, enabling a robust test of earlier claims that young planets are less dense than their older counterparts.