A Pair of TESS Planets Spanning the Radius Valley around the Nearby Mid-M Dwarf LTT 3780

Abstract

We present the confirmation of two new planets transiting the nearby mid-M dwarf LTT 3780 (TIC 36724087, TOI-732, V=13.07, K s =8.204, R s =0.374 R e ,M s =0.401 M e , d=22 pc). The two planet candidates are identified in a single Transiting Exoplanet Survey Satellite sector and validated with reconnaissance spectroscopy, ground-based photometric follow-up, and high-resolution imaging. With measured orbital periods of P b =0.77, P c =12.25 days and sizes r p,b =1.33±0.07,r p,c =2.30±0.16 R ⊕ , the two planets span the radius valley in period-radius space around low-mass stars, thus making the system a laboratory to test competing theories of the emergence of the radius valley in that stellar mass regime. By combining 63 precise radial velocity measurements from the High Accuracy Radial velocity Planet Searcher (HARPS) and HARPS-N, we measure planet masses of =-+ m 2.62 p b , 0.46 0.48 and =-+ m 8.6 p c , 1.3 1.6 M ⊕ , which indicates that LTT 3780b has a bulk composition consistent with being Earth-like, while LTT 3780c likely hosts an extended H/He envelope. We show that the recovered planetary masses are consistent with predictions from both photoevaporation and core-powered mass-loss models. The brightness and small size of LTT 3780, along with the measured planetary parameters, render LTT 3780b and c as accessible targets for atmospheric characterization of planets within the same planetary system and spanning the radius valley.

Two Bright M Dwarfs Hosting Ultra-Short-Period Super-Earths with Earth-like Compositions*

“…However, the number of "well-characterized" USP planets with precisely measured masses and radii is still limited to date. In particular, only two USP planets around M dwarfs (LTT 3780 and GJ 1252) have precise mass measurements (Cloutier et al 2020;Nowak et al 2020;Shporer et al 2020). Radial velocity (RV) follow-up observations are important for USP planets not only in terms of confirmation of the candidates but also for constraining the bulk compositions of the planets, which shed some light on the origin and evolution of USP planets.…”

TOI 540 b: A Planet Smaller than Earth Orbiting a Nearby Rapidly Rotating Low-mass Star

“…These results were based on numerous RV studies that had obtained masses for small transiting planets around G and K dwarfs such as Kepler-10 (Batalha et al 2011), Kepler-78 (Howard et al 2013;Pepe et al 2013), and Kepler-93 . Recent RV measurements have established rocky bulk composition for many small planets orbiting M dwarfs as well, including L 98-59 (Cloutier et al 2019), LHS 1140 (Ment et al 2019), LTT 3780 (Cloutier et al 2020a), and TRAPPIST-1 (Grimm et al 2018). Consistent with the interpretation of two distinct planet populations, Fulton et al (2017) noticed a deficit of planets with radii 1.5-2 Å R (a radius valley).…”

TOI-1634 b: An Ultra-short-period Keystone Planet Sitting inside the M-dwarf Radius Valley

“…These four GP hyperparameters are appended to the set of model parameters, thus resulting in a total of 20 model parameters. Our GP implementation methodology is standard and has been outlined in detail in previous work (Cloutier et al 2019a(Cloutier et al , 2020a. We have no prior constraints on the GP hyperparameters from a training set because no available activity-sensitive time series shows evidence for stellar activity.…”