Abstract
Abstract 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 ⊙, making it the third-coolest star known to host planets. The two planets straddle the radius gap seen for smaller exoplanets; they include a 1.1R ⊕ planet on a 0.95 day orbit and a 2.3R ⊕ 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.
Highlights
Cool, low-mass stars — M dwarfs — are more numerous and host more short-period planets per star than the more massive stars that host most of the known planets (Bonfils et al 2013; Dressing & Charbonneau 2015; Mulders et al 2015)
This star has spectral type M6V, effective temperature 2960 K, and radius 0.17R, making it the third-coolest star known to host planets
We model all three Las Cumbres Observatory (LCO) light curves with BATMAN (Kreidberg 2015) keeping all parameters — except for Rp/R∗ and the mid-transit time — fixed to the values derived from the TESS light curve (Table 2)
Summary
Low-mass stars — M dwarfs — are more numerous and host more short-period planets per star than the more massive stars that host most of the known planets (Bonfils et al 2013; Dressing & Charbonneau 2015; Mulders et al 2015) Whether they are seen to transit, inferred from radial velocity spectroscopy, or detected via gravitational microlensing, exoplanets tend to be easier to characterize when they orbit M dwarfs instead of larger, hotter, more massive stars. These red dwarfs are popular targets for exoplanet surveys of all types. Like TRAPPIST-1 (Gillon et al 2017), GJ 1214 (Charbonneau et al 2009), Proxima Centauri (Anglada-Escude et al 2016), and other similar systems, it represents another rare laboratory to study exoplanets around the very smallest stars
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