We present the confirmation of two new planets transiting the nearby mid-M
dwarf LTT 3780 (TIC 36724087, TOI-732, V=13.07, Ks=8.204, Rs=0.374
R⊙, Ms=0.401 M⊙, d=22 pc). The two planet candidates are
identified in a single TESS sector and are validated with reconnaissance
spectroscopy, ground-based photometric follow-up, and high-resolution imaging.
With measured orbital periods of Pb=0.77 days, Pc=12.25 days and sizes
rp,b=1.33±0.07 R⊕, rp,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 HARPS and HARPS-N, we measure planet masses
of mp,b=2.62−0.46+0.48 M⊕ and mp,c=8.6−1.3+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 from 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.Comment: Accepted to AJ. 8 figures, 6 tables. CSV file of the RV measurements
(i.e. Table 2) are included in the source cod