Zodiacal Exoplanets in Time (ZEIT) VIII: A Two Planet System in Praesepe from K2 Campaign 16

TL;DR

This paper reports the discovery of a two-planet system around a young star in the Praesepe cluster, providing insights into early planetary system evolution and demonstrating the detection capabilities of K2 in young clusters.

Contribution

First confirmation of a multi-planet system in a young open cluster using K2 data, highlighting the potential for studying planetary formation in early stages.

Findings

Two mini-Neptune planets with periods of 5.84 and 19.66 days.

Planets are in a 2-planet system in Praesepe, 650 Myr old.

Radial velocity signals are small, challenging optical measurements but feasible in near-infrared.

Abstract

Young planets offer a direct view of the formation and evolution processes that produced the diverse population of mature exoplanet systems known today. The repurposed Kepler mission K2 is providing the first sample of young transiting planets by observing populations of stars in nearby, young clusters or stellar associations. We report the detection and confirmation of two planets transiting K2-264, an M2.5 dwarf in the 650 Myr old Praesepe open cluster. Using our notch-filter search method on the K2 lightcurve, we identify planets with periods of 5.84 d and 19.66 d. This is currently the second known multi-transit system in open clusters younger than 1 Gyr. The inner planet has a radius of 2.27$_{-0.16}^{+0.20}$ R$_\oplus$ and the outer planet has a radius of 2.77$_{-0.18}^{+0.20}$ R$_\oplus$. Both planets are likely mini-Neptunes. These planets are expected to produce radial velocity signals of 3.4 and 2.7 m/s respectively, which is smaller than the expected stellar variability in the optical ($\simeq$30 m/s), making mass measurements unlikely in the optical, but possible with future near-infrared spectrographs. We use an injection-recovery test to place robust limits on additional planets in the system, and find that planets larger than 2 R$_\oplus$ with periods of 1-20 d are unlikely.