K2-137 b: an Earth-sized planet in a 4.3-hour orbit around an M-dwarf

TL;DR

This paper reports the discovery of K2-137 b, an Earth-sized planet with an ultra-short 4.3-hour orbit around an M-dwarf, confirmed through multiple observational methods and characterized by its small size and high iron content.

Contribution

The study presents the first confirmed ultra-short-period Earth-sized planet orbiting an M-dwarf with detailed validation and characterization.

Findings

K2-137 b has a radius of 0.89 Earth radii.

The planet orbits its star every 4.3 hours.

It has a high iron mass fraction (>0.45).

Abstract

We report the discovery from K2 of a transiting terrestrial planet in an ultra-short-period orbit around an M3-dwarf. K2-137 b completes an orbit in only 4.3 hours, the second-shortest orbital period of any known planet, just 4 minutes longer than that of KOI 1843.03, which also orbits an M-dwarf. Using a combination of archival images, AO imaging, RV measurements, and light curve modelling, we show that no plausible eclipsing binary scenario can explain the K2 light curve, and thus confirm the planetary nature of the system. The planet, whose radius we determine to be 0.89 +/- 0.09 Earth radii, and which must have a iron mass fraction greater than 0.45, orbits a star of mass 0.463 +/- 0.052 Msol and radius 0.442 +/- 0.044 Rsol.