The CARMENES search for exoplanets around M dwarfs. Two planets on opposite sides of the radius gap transiting the nearby M dwarf LTT 3780

Abstract

We present the discovery and characterisation of two transiting planets observed by the Transiting Exoplanet Survey Satellite (TESS) orbiting the nearby (d[SUB]⋆[/SUB] ≈ 22 pc), bright (J ≈ 9 mag) M3.5 dwarf LTT 3780 (TOI-732). We confirm both planets and their association with LTT 3780 via ground-based photometry and determine their masses using precise radial velocities measured with the CARMENES spectrograph. Precise stellar parameters determined from CARMENES high-resolution spectra confirm that LTT 3780 is a mid-M dwarf with an effective temperature of T[SUB]eff[/SUB] = 3360 ± 51 K, a surface gravity of log g[SUB]⋆[/SUB] = 4.81 ± 0.04 (cgs), and an iron abundance of [Fe/H] = 0.09 ± 0.16 dex, with an inferred mass of M[SUB]⋆[/SUB] = 0.379 ± 0.016M[SUB]☉[/SUB] and a radius of R[SUB]⋆[/SUB] = 0.382 ± 0.012R[SUB]☉[/SUB]. The ultra-short-period planet LTT 3780 b (P[SUB]b[/SUB] = 0.77 d) with a radius of 1.35[SUB]-0.06[/SUB][SUP]+0.06[/SUP] R[SUB]⊕[/SUB], a mass of 2.34[SUP]-0.23[/SUP][SUB]+0.24[/SUB] M[SUB]⊕[/SUB], and a bulk density of 5.24[SUB]-0.81[/SUB][SUP]+0.94[/SUP] g cm[SUP]-3[/SUP] joins the population of Earth-size planets with rocky, terrestrial composition. The outer planet, LTT 3780 c, with an orbital period of 12.25 d, radius of 2.42[SUB]-0.10[/SUB][SUP]+0.10[/SUP] R[SUB]⊕[/SUB], mass of 6.29[SUB]-0.61[/SUB][SUP]+0.63[/SUP] M[SUB]⊕[/SUB], and mean density of 2.45[SUB]-0.37[/SUB][SUP]+0.44[/SUP] g cm[SUP]-3[/SUP] belongs to the population of dense sub-Neptunes. With the two planets located on opposite sides of the radius gap, this planetary system is anexcellent target for testing planetary formation, evolution, and atmospheric models. In particular, LTT 3780 c is an ideal object for atmospheric studies with the James Webb Space Telescope (JWST)