Homogeneous search for spot transits in Kepler and TESS photometry of K – M-type main-sequence stars

Department of Physics, P.O. Box 64 00014 University of Helsinki, Finland

^⋆ Corresponding author: andras.haris-kiss@helsinki.fi

Received: 16 October 2024
Accepted: 15 September 2025

Abstract

Late-type stars are known to host numerous exoplanets, and their photometric variability, primarily caused by rotational modulation, provides a unique opportunity to study starspots. As exoplanets transit in front of their host stars, they may occult darker, spotted regions on the stellar surfaces. The monitoring of starspots from planetary transits, known as transit mapping, offers a possibility to detect small dark regions on magnetically active, late-type stars. These spots may be so small that they would be undetectable to other methods used to reconstruct stellar magnetic activity. We describe a Bayesian analysis framework on the transit light curves of planets orbiting K- and M-type main-sequence stars in search for spot occultation event candidates. We present a systematic analysis of high-precision, high-cadence light curves from Kepler and TESS to detect and characterise starspots during exoplanetary transits. According to our tests, the set of criteria applied in the analysis is robust and not prone to false positives. Our sample comprises K and M dwarfs hosting transiting exoplanets observed by the Kepler or TESS space telescopes at a high cadence, totalling 99 planets meeting our selection criteria. After analysing 3273 transit light curves from 99 planets, we find 102 candidates for starspot occultation events by six planets. We report new spot occultation candidates for the K dwarfs HD 189733 and TOI-1268. The identified dark regions have a lower limit for radii between 1.6 degrees and 29.5 degrees and contrasts up to 0.69. We estimate a spot detection frequency of 3.7% and 4.2% for K and M dwarfs by TESS, and 37.5% for K dwarfs by Kepler.