Characterization of a set of small planets with TESS and CHEOPS and an analysis of photometric performance

TL;DR

This study characterizes nine small exoplanets using TESS and CHEOPS data, compares photometric performance, and discusses implications for understanding the radius valley and planet atmospheres.

Contribution

It provides new validated planet detections, compares the photometric precision of TESS and CHEOPS, and analyzes factors affecting transit measurement uncertainties.

Findings

CHEOPS transits generally yield lower uncertainties than TESS but not always as predicted.

No correlation found between the number of equivalent transits and physical parameters.

Photometric performance varies due to observational factors like in-transit gaps and data detrending.

Abstract

The radius valley carries implications for how the atmospheres of small planets form and evolve, but this feature is visible only with highly precise characterizations of many small planets. We present the characterization of nine planets and one planet candidate with both NASA TESS and ESA CHEOPS observations, which adds to the overall population of planets bordering the radius valley. While four of our planets - TOI 118 b, TOI 455 b, TOI 560 b, and TOI 562 b - have already been published, we vet and validate transit signals as planetary using follow-up observations for five new TESS planets, including TOI 198 b, TOI 244 b, TOI 262 b, TOI 444 b, and TOI 470 b. While a three times increase in primary mirror size should mean that one CHEOPS transit yields an equivalent model uncertainty in transit depth as about nine TESS transits in the case that the star is equally as bright in both bands, we find that our CHEOPS transits typically yield uncertainties equivalent to between two and 12 TESS transits, averaging 5.9 equivalent transits. Therefore, we find that while our fits to CHEOPS transits provide overall lower uncertainties on transit depth and better precision relative to fits to TESS transits, our uncertainties for these fits do not always match expected predictions given photon-limited noise. We find no correlations between number of equivalent transits and any physical parameters, indicating that this behavior is not strictly systematic, but rather might be due to other factors such as in-transit gaps during CHEOPS visits or nonhomogeneous detrending of CHEOPS light curves.