The young planetary system K2-25: constraints on companions and starspots

TL;DR

This study investigates the dynamical history of the young exoplanet K2-25b in the Hyades cluster by analyzing transit data for signs of additional companions and starspot effects, finding no significant perturbing planets.

Contribution

The paper provides detailed transit analysis of K2-25b, constraining the presence of nearby planetary companions and examining starspot effects in a young stellar system.

Findings

No significant transit timing variations detected.

No companions > 1 Earth mass within 2:1 period ratio.

No companions > 5 Earth masses within 7:2 period ratio.

Abstract

The abundance of planets with orbital periods of a few to tens of days suggests that exoplanets experience complex dynamical histories. Planets in young stellar clusters or associations have well-constrained ages and therefore provide an opportunity to explore the dynamical evolution of exoplanets. K2-25b is a Neptune-sized planet in an eccentric, 3.48 day orbit around an M4.5 dwarf star in the Hyades cluster (650 Myr). In order to investigate its non-zero eccentricity and tight orbit, we analyze transit timing variations (TTVs) which could reveal clues to the migration processes that may have acted on the planet. We obtain 12 non-consecutive transits using the MEarth Observatories and long-term photometric monitoring, which we combine with 10 transits from the Spitzer Space Telescope and 20 transits from K2. Tables of MEarth photometry accompany this work. We fit each transit lightcurve independently. We first investigate whether inhomogeneities on the stellar surface (such as spots or plages) are differentially affecting our transit observations. The measured transit depth does not vary significantly between transits, though we see some deviations from the fiducial transit model. We then looked for TTVs as evidence of a non-transiting perturber in the system. We find no evidence for > 1 $M_\oplus$ companions within a 2:1 period ratio, or for > 5 $M_\oplus$ planets within a 7:2 period ratio.