Kepler Transit Depths Contaminated by a Phantom Star

TL;DR

Ground-based observations revealed that the Kepler-445c transit depth was overestimated due to a phantom star contaminating the crowding metric, significantly impacting the inferred planetary properties and highlighting the need for careful data analysis.

Contribution

This study identifies and corrects a systematic error caused by a phantom star in Kepler data, demonstrating its effect on transit depth and planetary characterization.

Findings

Kepler-445c's transit depth is 50% shallower than Kepler data suggested.

The phantom star caused a 17-sigma discrepancy in transit parameters.

Stellar crowding metrics can be contaminated by non-existent stars, affecting exoplanet studies.

Abstract

We present ground-based observations from the Discovery Channel Telescope (DCT) of three transits of Kepler-445c---a supposed super-Earth exoplanet with properties resembling GJ 1214b---and demonstrate that the transit depth is approximately 50 percent shallower than the depth previously inferred from Kepler Spacecraft data. The resulting decrease in planetary radius significantly alters the interpretation of the exoplanet's bulk composition. Despite the faintness of the M4 dwarf host star, our ground-based photometry clearly recovers each transit and achieves repeatable 1-sigma precision of approximately 0.2 percent (2 millimags). The transit parameters estimated from the DCT data are discrepant with those inferred from the Kepler data to at least 17-sigma confidence. This inconsistency is due to a subtle miscalculation of the stellar crowding metric during the Kepler pre-search data conditioning (PDC). The crowding metric, or CROWDSAP, is contaminated by a non-existent "phantom star" originating in the USNO-B1 catalog and inherited by the Kepler Input Catalog (KIC). Phantom stars in the KIC are likely rare, but they have the potential to affect statistical studies of Kepler targets that use the PDC transit depths for a large number of exoplanets where individual follow-up observation of each is not possible. The miscalculation of Kepler-445c's transit depth emphasizes the importance of stellar crowding in the Kepler data, and provides a cautionary tale for the analysis of data from the Transiting Exoplanet Survey Satellite (TESS), which will have even larger pixels than Kepler.