A CHEOPS Search for Massive, Long-Period Companions to the Warm Jupiter K2-139 b

TL;DR

This study refines the orbital parameters of the warm Jupiter K2-139 b using CHEOPS and K2 data, enabling precise future transit predictions and constraining the presence of nearby massive companions.

Contribution

It provides improved ephemeris accuracy for K2-139 b and rules out the existence of massive companions within certain orbital periods, demonstrating the effectiveness of CHEOPS observations.

Findings

Transit ephemeris precision improved to less than 10 minutes for the next decade

No evidence of additional massive planets or brown dwarfs within 150 days or one year periods

Stellar spot configuration changed, with no spot-crossing events detected in CHEOPS data

Abstract

K2-139 b is a warm Jupiter with an orbital period of 28.4 d, but only three transits of this system have previously been observed, in the long-cadence mode of K2, limiting the precision with which the orbital period can be determined, and future transits predicted. We report photometric observations of four transits of K2-139 b with ESA's CHaracterising ExOPlanet Satellite (CHEOPS), conducted with the goal of measuring the orbital obliquity via spot-crossing events. We jointly fit these CHEOPS data alongside the three previously-published transits from the K2 mission, considerably increasing the precision of the ephemeris of K2-139 b. The transit times for this system can now be predicted for the next decade with a $1 \sigma$ precision less than 10 minutes, compared to over one hour previously, allowing the efficient scheduling of observations with Ariel. We detect no significant deviation from a linear ephemeris, allowing us to exclude the presence of a massive outer planet orbiting with a period less than 150 d, or a brown dwarf with a period less than one year. We also determine the scaled semi-major axis, the impact parameter, and the stellar limb-darkening with improved precision. This is driven by the shorter cadence of the CHEOPS observations compared to that of K2, and validates the sub-exposure technique used for analysing long-cadence photometry. Finally, we note that the stellar spot configuration has changed from the epoch of the K2 observations; unlike the K2 transits, we detect no evidence of spot-crossing events in the CHEOPS data.