Context. Transiting super-Earths orbiting bright stars in short orbital
periods are interesting targets for the study of planetary atmospheres.
Aims: While selecting super-Earths suitable for further
characterisation from the ground from a list of confirmed and validated
exoplanets detected by K2, we found some suspicious cases that led to us
reassess the nature of the detected transiting signal. Methods:
We performed a photometric analysis of the K2 light curves and centroid
motions of the photometric barycenters. Results: Our study shows
that the validated planets K2-78b, K2-82b, and K2-92b are not planets,
but background eclipsing binaries. The eclipsing binaries are inside the
Kepler photometric aperture, but outside the ground-based
high-resolution images that were used for validation.
Conclusions: We advise extreme care in the validation of candidate
planets that are discovered by space missions. It is important that all
the assumptions in the validation process are carefully checked. An
independent confirmation is mandatory in order to avoid wasting valuable
resources on further characterisation of non-existent targets.