Characterization of the K2-38 planetary system

Unraveling one of the densest planets known to date^★,★★

¹ Instituto de Astrofísica de Canarias, 38205 La Laguna, Tenerife, Spain
e-mail: btoledo@iac.es
² Universidad de La Laguna, Departamento de Astrofísica, 38206 La Laguna, Tenerife, Spain
³ Observatoire Astronomique de l’Université de Genève, 51 chemin des Maillettes, 1290 Versoix, Switzerland
⁴ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
⁵ Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal
⁶ INAF Osservatorio Astrofisico di Torino, Via Osservatorio 20, 10025 Pino Torinese, Italy
⁷ Centro de Astrobiología (CSIC-INTA), Carretera de Ajalvir km 4, 28850 Torrejón de Ardoz, Madrid, Spain
⁸ Consejo Superior de Investigaciones Científicas, 28006 Madrid, Spain
⁹ INAF Osservatorio Astronomico di Trieste, Via G. Tiepolo 11, 34143 Trieste, Italy
¹⁰ Centro de Astrobiología (CAB, CSIC-INTA), Dpto. de Astrofísica, ESAC campus 28692 Villanueva de la Cañada, Madrid, Spain
¹¹ Scuola Normale Superiore, Piazza dei Cavalieri, 7, 56126 Pisa, Italy
¹² European Southern Observatory, Alonso de Córdova 3107, Vitacura Casilla 19001, Santiago 19, Chile
¹³ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
¹⁴ INAF Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
¹⁵ Instituto de Astrofísica e Ciências do Espaço, Universidade de Lisboa, Edifício C8, 1749-016 Lisboa, Portugal
¹⁶ Departamento de Física da Faculdade de Ciências da Universidade de Lisboa, Edifício C8, 1749-016 Lisboa, Portugal
¹⁷ Physikalisches Institut & Center for Space and Habitability, Universität Bern, Gesellschaftsstrasse 6, 3012 Bern, Switzerland
¹⁸ INAF Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate, Italy
¹⁹ INAF Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
²⁰ Centro de Astrofísica da Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal
²¹ Institute for Fundamental Physics of the Universe, Via Beirut 2, 34151 Miramare, Trieste, Italy
²² Fundación Galileo Galilei, INAF, Rambla José Ana Fern2ndez Pérez 7, 38712 Breña Baja, Spain

Received: 17 April 2020
Accepted: 26 June 2020

Abstract

Context. An accurate characterization of the known exoplanet population is key to understanding the origin and evolution of planetary systems. Determining true planetary masses through the radial velocity (RV) method is expected to experience a great improvement thanks to the availability of ultra-stable echelle spectrographs.

Aims. We took advantage of the extreme precision of the new-generation echelle spectrograph ESPRESSO to characterize the transiting planetary system orbiting the G2V star K2-38 located at 194 pc from the Sun with V ~ 11.4. This system is particularly interesting because it could contain the densest planet detected to date.

Methods. We carried out a photometric analysis of the available K2 photometric light curve of this star to measure the radius of its two known planets, K2-38b and K2-38c, with P_b = 4.01593 ± 0.00050 d and P_c = 10.56103 ± 0.00090 d, respectively. Using 43 ESPRESSO high-precision RV measurements taken over the course of 8 months along with the 14 previously published HIRES RV measurements, we modeled the orbits of the two planets through a Markov chain Monte Carlo analysis, significantly improving their mass measurements.

Results. Using ESPRESSO spectra, we derived the stellar parameters, T_eff = 5731 ± 66, log g = 4.38 ± 0.11 dex, and [Fe/H] = 0.26 ± 0.05 dex, and thus the mass and radius of K2-38, M_⋆ = 1.03_−0.02^+0.04 M_⊕ and R_⋆ = 1.06_−0.06^+0.09 R_⊕. We determine new values for the planetary properties of both planets. We characterize K2-38b as a super-Earth with R_P = 1.54 ± 0.14 R_⊕ and M_p = 7.3_−1.0^+1.1 M_⊕, and K2-38c as a sub-Neptune with R_P = 2.29 ± 0.26 R_⊕ and M_p = 8.3_−1.3^+1.3 M_⊕. Combining the radius and mass measurements, we derived a mean density of ρ_p = 11.0_−2.8^+4.1 g cm⁻³ for K2-38b and ρ_p = 3.8_−1.1^+1.8 g cm⁻³ for K2-38c, confirming K2-38b as one of the densest planets known to date.

Conclusions. The best description for the composition of K2-38b comes from an iron-rich Mercury-like model, while K2-38c is better described by a rocky-model with H2 envelope. The maximum collision stripping boundary shows how giant impacts could be the cause for the high density of K2-38b. The irradiation received by each planet places them on opposite sides of the radius valley. We find evidence of a long-period signal in the RV time-series whose origin could be linked to a 0.25–3 M_J planet or stellar activity.