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

The radius valley carries implications for how the atmospheres of small planets form and evolve, but
this feature is only visible with highly-precise characterizations of many small planets. We present the
characterization of 9 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 five
of our planets - TOI 118 b, TOI 262 b, TOI 455 b, TOI 560 b, and TOI 562 b - have already been
published, we vet and validate transit signals as planetary using followup observations for four new
TESS planets, including TOI 198 b, TOI 244 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 under ideal conditions, we find that our CHEOPS transits
typically yield uncertainties equivalent to between 2 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
non-homogeneous detrending of CHEOPS light curves.