Dr. Astudillo-Defru, Nicola

Context. Small planets located at the lower mode of the bimodal radius distribution are generally assumed to be composed of iron and silicates in a proportion similar to that of the Earth. However, recent discoveries are revealing a new group of low-density planets that are inconsistent with that description. Aims. We intend to confirm and characterize the TESS planet candidate TOI-244.01, which orbits the bright (K = 7.97 mag), nearby (d = 22 pc), and early-type (M2.5 V) M-dwarf star GJ 1018 with an orbital period of 7.4 days. Methods. We used Markov chain Monte Carlo methods to model 57 precise radial velocity measurements acquired by the ESPRESSO spectrograph together with TESS photometry and complementary HARPS data. Our model includes a planetary component and Gaussian processes aimed at modeling the correlated stellar and instrumental noise. Results. We find TOI-244 b to be a super-Earth with a radius of Rp = 1.52 ± 0.12 R⊕ and a mass of Mp = 2.68 ± 0.30 M⊕. These values correspond to a density of ρ = 4.2 ± 1.1 g cm−3, which is below what would be expected for an Earth-like composition. We find that atmospheric loss processes may have been efficient to remove a potential primordial hydrogen envelope, but high mean molecular weight volatiles such as water could have been retained. Our internal structure modeling suggests that TOI-244 b has a 479+128 −96 km thick hydrosphere over a 1.17 ± 0.09 R⊕ solid structure composed of a Fe-rich core and a silicate-dominated mantle compatible with that of the Earth. On a population level, we find two tentative trends in the density-metallicity and density-insolation parameter space for the low-density super-Earths, which may hint at their composition. Conclusions. With a 8% precision in radius and 12% precision in mass, TOI-244 b is among the most precisely characterized super-Earths, which, together with the likely presence of an extended hydrosphere, makes it a key target for atmospheric observations

Context. Statistical analyses based on Kepler data show that most of the early-type M dwarfs host multi-planet systems consisting of Earth- to sub-Neptune-sized planets with orbital periods of up to ~250 days, and that at least one such planet is likely located within the habitable zone. M dwarfs are therefore primary targets to search for potentially habitable planets in the solar neighbourhood. Aims. We investigated the presence of planetary companions around the nearby (7.6 pc) and bright (V = 9 mag) early-type M dwarf Gl 514, analysing 540 radial velocities collected over nearly 25 yr with the HIRES, HARPS, and CARMENES spectrographs. Methods. The data are affected by time-correlated signals at the level of 2–3 m s−1 due to stellar activity, which we filtered out, testing three different models based on Gaussian process regression. As a sanity cross-check, we repeated the analyses using HARPS radial velocities extracted with three different algorithms. We used HIRES radial velocities and Hipparcos-Gaia astrometry to put constraints on the presence of long-period companions, and we analysed TESS photometric data. Results. We find strong evidence that Gl 514 hosts a super-Earth on a likely eccentric orbit, residing in the conservative habitable zone for nearly 34% of its orbital period. The planet Gl 514b has minimum mass mb sin ib = 5.2 ± 0.9 M⊕, orbital period Pb = 140.43 ± 0.41 days, and eccentricity eb = 0.45−0.14+0.15. No evidence for transits is found in the TESS light curve. There is no evidence for a longer period companion in the radial velocities and, based on astrometry, we can rule out a ~0.2 MJup planet at a distance of ~3–10 astronomical units, and massive giant planets and brown dwarfs out to several tens of astronomical units. We discuss the possible presence of a second low-mass companion at a shorter distance from the host than Gl 514 b. Conclusions. Gl 514 b represents an interesting science case for studying the habitability of planets on eccentric orbits. We advocate for additional spectroscopic follow-up to get more accurate and precise planetary parameters. Further follow-up is also needed to investigate the presence of additional planetary signals of less than 1 m s−1.