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 _⋆ ≈ 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 _eff = 3360 ± 51 K, a surface gravity of log g _⋆ = 4.81 ± 0.04 (cgs), and an iron abundance of [Fe/H] = 0.09 ± 0.16 dex, with an inferred mass of M _⋆ = 0.379 ± 0.016 M _⊙ and a radius of R _⋆ = 0.382 ± 0.012 R _⊙. The ultra-short-period planet LTT 3780 b ( P _b = 0.77 d) with a radius of 1.35 _−0.06 ^+0.06 R _⊕, a mass of 2.34 ^−0.23 _+0.24 M _⊕, and a bulk density of 5.24 _−0.81 ^+0.94 g cm ⁻³ 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 _−0.10 ^+0.10 R _⊕, mass of 6.29 _−0.61 ^+0.63 M _⊕, and mean density of 2.45 _−0.37 ^+0.44 g cm ⁻³ 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).