HIP 41378 observed by CHEOPS: Where is planet d?

TL;DR

This study investigates the orbital period of HIP 41378 d, a long-period exoplanet, using CHEOPS observations, revealing potential transit timing variations and emphasizing the importance of space-based long-duration monitoring.

Contribution

The paper presents the first CHEOPS observation targeting HIP 41378 d and explores alternative orbital periods, highlighting the role of space telescopes in studying long-period planets with TTVs.

Findings

No transit detected during CHEOPS observation at predicted time.

Large transit timing variations (>22.4 hours) could explain non-detection.

Alternative orbital periods of 101.22 d and 371.14 d are plausible.

Abstract

HIP 41378 d is a long-period planet that has only been observed to transit twice, three years apart, with K2. According to stability considerations and a partial detection of the Rossiter-McLaughlin effect, $P_\mathrm{d} = 278.36$ d has been determined to be the most likely orbital period. We targeted HIP 41378 d with CHEOPS at the predicted transit timing based on $P_\mathrm{d}= 278.36$ d, but the observations show no transit. We find that large ($>22.4$ hours) transit timing variations (TTVs) could explain this non-detection during the CHEOPS observation window. We also investigated the possibility of an incorrect orbital solution, which would have major implications for our knowledge of this system. If $P_\mathrm{d} \neq 278.36$ d, the periods that minimize the eccentricity would be $101.22$ d and $371.14$ d. The shortest orbital period will be tested by TESS, which will observe HIP 41378 in Sector 88 starting in January 2025. Our study shows the importance of a mission like CHEOPS, which today is the only mission able to make long observations (i.e., from space) to track the ephemeris of long-period planets possibly affected by large TTVs.