  We re-examine the statistical confirmation of small long-period Kepler planet
candidates in light of recent improvements in our understanding of the
occurrence of systematic false alarms in this regime. Using the final Data
Release 25 (DR25) Kepler planet candidate catalog statistics, we find that the
previously confirmed single planet system Kepler-452b no longer achieves a 99%
confidence in the planetary hypothesis and is not considered statistically
validated in agreement with the finding of Mullally et al. (2018). For multiple
planet systems, we find that the planet prior enhancement for belonging to a
multiple planet system is suppressed relative to previous Kepler catalogs, and
we identify the multi-planet system member, Kepler-186f, no longer achieves a
99% confidence in the planetary hypothesis. Because of the numerous confounding
factors in the data analysis process that leads to the detection and
characterization of a signal, it is difficult to determine whether any one
planetary candidate achieves a strict criterion for confirmation relative to
systematic false alarms. For instance, when taking into account a simplified
model of processing variations, the additional single planet systems
Kepler-443b, Kepler-441b, Kepler-1633b, Kepler-1178b, and Kepler-1653b have a
non-negligible probability of falling below a 99% confidence in the planetary
hypothesis. The systematic false alarm hypothesis must be taken into account
when employing statistical validation techniques in order to confirm planet
candidates that approach the detection threshold of a survey. We encourage
those performing transit searches of K2, TESS, and other similar data sets to
quantify their systematic false alarms rates. Alternatively, independent
photometric detection of the transit signal or radial velocity measurements can
eliminate the false alarm hypothesis.
