# Validation of a Temperate Fourth Planet in the K2-133 Multi-planet System

## TL;DR

This paper validates the existence of a fourth temperate planet in the K2-133 system, providing insights into planetary formation and atmospheric evolution, and highlighting the potential for rocky composition in the outer planet.

## Contribution

The study confirms a fourth planet in the K2-133 system and discusses its implications for planetary population gaps and atmospheric photo-evaporation theories.

## Key findings

- The fourth planet has a radius of approximately 1.73 Earth radii.
- The four planets span the radius gap, supporting atmospheric loss theories.
- K2-133 e may be a rocky world on the edge of the temperate zone.

## Abstract

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$_{\oplus}$. 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.