A sub-Earth-mass planet orbiting Barnard’s star^★

¹ Instituto de Astrofísica de Canarias, 38205 La Laguna, Tenerife, Spain
² Departamento de Astrofísica, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain
³ Consejo Superior de Investigaciones Científicas, Spain
⁴ Observatoire de Genève, Département d’Astronomie, Université de Genève, Chemin Pegasi 51b, 1290 Versoix, Switzerland
⁵ Instituto de Astrofísica e Ciências do Espaco, CAUP, Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal
⁶ Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal
⁷ Centro de Astrofísica da Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal
⁸ Centro de Astrobiología (CAB), CSIC-INTA, ESAC campus, Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada (Madrid), Spain
⁹ INAF - Osservatorio Astronomico di Trieste, via G. B. Tiepolo 11, 34143 Trieste, Italy
¹⁰ IFPU–Institute for Fundamental Physics of the Universe, via Beirut 2, 34151 Trieste, Italy
¹¹ INAF - Osservatorio Astrofisico di Torino, Strada Osservatorio 20, 10025, Pino Torinese (TO), Italy
¹² ESO - European Southern Observatory, Av. Alonso de Cordova 3107, Vitacura, Santiago, Chile
¹³ Département de Physique, Institut Trottier de Recherche sur les Exoplanètes, Université de Montréal, Montréal, Québec, H3T 1J4, Canada
¹⁴ INAF - Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
¹⁵ Instituto de Astrofísica e Ciências do EspaÇo, Faculdade de Ciências da Universidade de Lisboa, Campo Grande, 1749-016 Lisboa, Portugal
¹⁶ Light Bridges S. L., Avda. Alcalde Ramírez Bethencourt, 17, 35004 Las Palmas de Gran Canaria, Canarias, Spain
¹⁷ Departamento de Física de la Tierra y Astrofísica & IPARCOS-UCM (Instituto de Física de Partículas y del Cosmos de la UCM), Facultad de Ciencias Físicas, Universidad Complutense de Madrid, 28040 Madrid, Spain
¹⁸ Center for Space and Habitability, University of Bern, Gesellschaftsstrasse 6, 3012 Bern, Switzerland
¹⁹ Weltraumforschung und Planetologie, Physikalisches Institut, University of Bern, Gesellschaftsstrasse 6, 3012 Bern, Switzerland
²⁰ Subaru Telescope, National Astronomical Observatory of Japan, 650 N Aohoku Place, Hilo, HI 96720, USA
²¹ Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany

^★★ Corresponding author; e-mail: jonay.gonzalez@iac.es

Received: 29 June 2024
Accepted: 26 August 2024

Abstract

Context. ESPRESSO guaranteed time observations (GTOs) at the 8.2m VLT telescope were performed to look for Earth-like exoplanets in the habitable zone of nearby stars. Barnard’s star is a primary target within the ESPRESSO GTO as it is the second closest neighbour to our Sun after the α Centauri stellar system.

Aims. We present here a large set of 156 ESPRESSO observations of Barnard’s star carried out over four years with the goal of exploring periods of shorter than 50 days, thus including the habitable zone (HZ).

Methods. Our analysis of ESPRESSO data using Gaussian process (GP) to model stellar activity suggests a long-term activity cycle at 3200 d and confirms stellar activity due to rotation at 140 d as the dominant source of radial velocity (RV) variations. These results are in agreement with findings based on publicly available HARPS, HARPS-N, and CARMENES data. ESPRESSO RVs do not support the existence of the previously reported candidate planet at 233 d.

Results. After subtracting the GP model, ESPRESSO RVs reveal several short-period candidate planet signals at periods of 3.15 d, 4.12 d, 2.34 d, and 6.74 d. We confirm the 3.15 d signal as a sub-Earth mass planet, with a semi-amplitude of 55 ± 7 cm s⁻¹, leading to a planet minimum mass m_p sin i of 0.37 ± 0.05 M_⊕, which is about three times the mass of Mars. ESPRESSO RVs suggest the possible existence of a candidate system with four sub-Earth mass planets in circular orbits with semi-amplitudes from 20 to 47 cm s⁻¹, thus corresponding to minimum masses in the range of 0.17–0.32 M_⊕.

Conclusions. The sub-Earth mass planet at 3.1533 ± 0.0006 d is in a close-to circular orbit with a semi-major axis of 0.0229 ± 0.0003 AU, thus located inwards from the HZ of Barnard’s star, with an equilibrium temperature of 400 K. Additional ESPRESSO observations would be required to confirm that the other three candidate signals originate from a compact short-period planet system orbiting Barnard’s star inwards from its HZ.