An unusually low-density super-Earth transiting the bright early-type M-dwarf GJ 1018 (TOI-244)^★,★★

¹ Centro de Astrobiología, CSIC-INTA, ESAC campus, 28692 Villanueva de la Cañada, Madrid, Spain
e-mail: acastro@cab.inta-csic.es
² Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
³ Departamento de Fisica e Astronomia, Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal
⁴ Observatoire de l’Université de Genève, 51 chemin Pegasi, 1290 Sauverny, Switzerland
⁵ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
⁶ Aix-Marseille Univ., CNRS, CNES, LAM, 38 rue Frédéric Joliot-Curie, 13388 Marseille, France ; Division Technique INSU, CS20330, 83507 La Seyne-sur-Mer cedex, France
⁷ Department of Astronomy and Astrophysics, University of California, 1156 High St, Santa Cruz, CA 95064, USA
⁸ Centro de Astrobiología, CSIC-INTA, Carretera de Ajalvir km 4, 28850 Torrejón de Ardoz, Madrid, Spain
⁹ Physikalisches Institut, University of Bern, Gesellsschaftstrasse 6, 3012 Bern, Switzerland
¹⁰ Center for Space and Habitability, University of Bern, Gesellsschaftstrasse 6, 3012 Bern, Switzerland
¹¹ INAF - Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate (LC), Italy
¹² Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544, USA
¹³ Instituto de Astrofísica de Canarias, C. Vía Láctea 1, 38205 La Laguna, Santa Cruz de Tenerife, Spain
¹⁴ Universidad de La Laguna, Dept. Astrofísica, 38206 La Laguna, Tenerife, Spain
¹⁵ NASA Ames Research Center, Building N-254 Moffett Field, CA 94035, USA
¹⁶ INAF – Osservatorio Astrofisico di Torin, Via Osservatorio 20, 10025 Pino Torinese, Italy
¹⁷ Departamento de Matemática y Física Aplicadas, Universidad Católica de la Santísima Concepción, Alonso de Rivera 2850, Concepción, Chile
¹⁸ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
¹⁹ INAF – Osservatorio Astronomico di Trieste, via G. B. Tiepolo 11, 34143 Trieste, Italy
²⁰ European Southern Observatory, Av. Alonso de Cordova 3107, Casilla 19001, Santiago de Chile, Chile
²¹ Centro de Astrofísica da Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal
²² Instituto de Astrofísica e Ciências do Espaço, Faculdade de Ciências da Universidade de Lisboa, Campo Grande, 1749-016 Lisboa, Portugal
²³ Département d’Astronomie, Université de Genève, Ch. des Maillettes 51, 1290 Versoix, Switzerland
²⁴ Department of Physics and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
²⁵ Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
²⁶ Department of Aeronautics and Astronautics, MIT, 77 Massachusetts Avenue, Cambridge, MA 02139, USA

Received: 30 March 2023
Accepted: 2 May 2023

Abstract

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 R_p = 1.52 ± 0.12 R_⊕ and a mass of M_p = 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 479₋₉₆⁺¹²⁸ 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.