A temperate Earth-sized planet with tidal heating transiting an M6 star

Temperate Earth-sized exoplanets around late-M dwarfs offer a rare opportunity to explore under which conditions planets can develop hospitable climate conditions. The small stellar radius amplifies the atmospheric transit signature, making even compact secondary atmospheres dominated by N<SUB>2</SUB> or CO<SUB>2</SUB> amenable to characterization with existing instrumentation<SUP>1</SUP>. Yet, despite large planet search efforts<SUP>2</SUP>, detection of low-temperature Earth-sized planets around late-M dwarfs has remained rare and the TRAPPIST-1 system, a resonance chain of rocky planets with seemingly identical compositions, has not yet shown any evidence of volatiles in the system<SUP>3</SUP>. Here we report the discovery of a temperate Earth-sized planet orbiting the cool M6 dwarf LP 791-18. The newly discovered planet, LP 791-18d, has a radius of 1.03 ± 0.04 R<SUB>⊕</SUB> and an equilibrium temperature of 300-400 K, with the permanent night side plausibly allowing for water condensation. LP 791-18d is part of a coplanar system<SUP>4</SUP> and provides a so-far unique opportunity to investigate a temperate exo-Earth in a system with a sub-Neptune that retained its gas or volatile envelope. On the basis of observations of transit timing variations, we find a mass of 7.1 ± 0.7 M<SUB>⊕</SUB> for the sub-Neptune LP 791-18c and a mass of 0.9<SUB>−0.4</SUB><SUP>+0.5</SUP>M<SUB>⊕</SUB> for the exo-Earth LP 791-18d. The gravitational interaction with the sub-Neptune prevents the complete circularization of LP 791-18d's orbit, resulting in continued tidal heating of LP 791-18d's interior and probably strong volcanic activity at the surface<SUP>5,6</SUP>.