Planets occur most frequently around cool dwarfs , but only a handful of specific examples are known to orbit the latest-type M stars .
Using TESS photometry , we report the discovery of two planets transiting the low-mass star called LP 791-18 ( identified by TESS as TOI 736 ) .
This star has spectral type M6V , effective temperature 2960 K , and radius 0.17 R _ { \odot } , making it the third-coolest star known to host planets .
The two planets straddle the radius gap seen for smaller exoplanets ; they include a 1.1 R _ { \oplus } planet on a 0.95 day orbit and a 2.3 R _ { \oplus } planet on a 5 day orbit .
Because the host star is small the decrease in light during these planets ’ transits is fairly large ( 0.4 % and 1.7 % ) .
This has allowed us to detect both planets ’ transits from ground-based photometry , refining their radii and orbital ephemerides .
In the future , radial velocity observations and transmission spectroscopy can both probe these planets ’ bulk interior and atmospheric compositions , and additional photometric monitoring would be sensitive to even smaller transiting planets .