A 38 Million Year Old Neptune-Sized Planet in the Kepler Field

TL;DR

This paper reports the discovery of a young, Neptune-sized exoplanet in the Kepler field, with detailed analysis confirming its membership in a 38-million-year-old stellar cluster, making it the youngest such planet with a precise age.

Contribution

It identifies Kepler 1627A as part of the young $ ext{δ}$ Lyr cluster and characterizes its planet Kepler 1627Ab, providing the youngest age estimate for a Kepler-discovered exoplanet with detailed stellar and planetary analysis.

Findings

Kepler 1627A is part of the 38 Myr $ ext{δ}$ Lyr cluster.

Kepler 1627Ab is a Neptune-sized planet, youngest with a precise age.

Transit profile shows asymmetry and possible correlation with light curve slope.

Abstract

Kepler 1627A is a G8V star previously known to host a 3.8 Earth-radius planet on a 7.2 day orbit. The star was observed by the Kepler space telescope because it is nearby (d=329 pc) and it resembles the Sun. Here we show using Gaia kinematics, TESS stellar rotation periods, and spectroscopic lithium abundances that Kepler 1627 is a member of the 38 $\pm$ 6 Myr old $\delta$ Lyr cluster. To our knowledge, this makes Kepler 1627Ab the youngest planet with a precise age yet found by the prime Kepler mission. The Kepler photometry shows two peculiarities: the average transit profile is asymmetric, and the individual transit times might be correlated with the local light curve slope. We discuss possible explanations for each anomaly. More importantly, the $\delta$ Lyr cluster is one of about 10$^3$ coeval groups whose properties have been clarified by Gaia. Many other exoplanet hosts are candidate members of these clusters; these memberships can be verified with the trifecta of Gaia, TESS, and ground-based spectroscopy.