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 _ { \odot } , M _ { s } =0.401 M _ { \odot } , 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 P _ { b } = 0.77 days , P _ { c } = 12.25 days and sizes r _ { p,b } = 1.33 \pm 0.07 R _ { \oplus } , r _ { p,c } = 2.30 \pm 0.16 R _ { \oplus } , 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 55 precise radial-velocity measurements from HARPS and HARPS-N , we measure planet masses of m _ { p,b } = 3.12 \pm 0.51 M _ { \oplus } and m _ { p,c } = 8.5 \pm 1.6 M _ { \oplus } , 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 physical and orbital planet parameters , combined with the brightness and small size of LTT 3780 , render both of the known LTT 3780 planets as accessible targets for atmospheric characterization of planets within the same planetary system and spanning the radius valley .