We present follow-up observations of the K2-133 multi-planet system. Previously, we announced that K2-133 contained three super-Earths orbiting an M1.5V host star - with tentative evidence of a fourth outer-planet orbiting at the edge of the temperate zone. Here we report on the validation of the presence of the fourth planet, determining a radius of 1.73−0.13+0.14 R⊕. The four planets span the radius gap of the exoplanet population, meaning further follow-up would be worthwhile to obtain masses and test theories of the origin of the gap. In particular, the trend of increasing planetary radius with decreasing incident flux in the K2-133 system supports the claim that the gap is caused by photo-evaporation of exoplanet atmospheres. Finally, we note that K2-133 e orbits on the edge of the stars temperate zone, and that our radius measurement allows for the possibility that this is a rocky world. Additional mass measurements are required to confirm or refute this scenario.
@article{arxiv.1905.05206,
title = {Validation of a Temperate Fourth Planet in the K2-133 Multi-planet System},
author = {R. Wells and K. Poppenhaeger and C. A. Watson},
journal= {arXiv preprint arXiv:1905.05206},
year = {2019}
}