Revisiting the multi-planetary system of the nearby star HD 20794

Confirmation of a low-mass planet in the habitable zone of a nearby G-dwarf^★,★★

¹ Light Bridges S.L., Observatorio del Teide, Carretera del Observatorio, s/n Guimar, 38500 Tenerife, Canarias, Spain
² Instituto de Astrofísica de Canarias, 38205 La Laguna, Tenerife, Spain
³ Departamento de Astrofísica, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain
⁴ Consejo Superior de Investigaciones Científicas, Madrid, Spain
⁵ Observatoire de Genève, Département d’Astronomie, Université de Genève, Chemin Pegasi 51b, 1290 Versoix, Switzerland
⁶ Aix Marseille Univ, CNRS, CNES, Institut Origines, LAM, Marseille, France
⁷ Department of Physics, University of Oxford, Oxford OX13RH, UK
⁸ Instituto de Astrofísica e Ciências do Espaço, CAUP, Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal
⁹ Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal
¹⁰ Centro de Astrofísica da Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal
¹¹ Centro de Astrobiología (CAB), CSIC-INTA, ESAC campus, Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada (Madrid), Spain
¹² INAF - Osservatorio Astronomico di Trieste, via G. B. Tiepolo 11, 34143 Trieste, Italy
¹³ IFPU–Institute for Fundamental Physics of the Universe, via Beirut 2, 34151 Trieste, Italy
¹⁴ INAF - Osservatorio Astrofisico di Torino, Strada Osservatorio 20, 10025 Pino Torinese (TO), Italy
¹⁵ ESO – European Southern Observatory, Av. Alonso de Cordova 3107, Vitacura, Santiago, Chile
¹⁶ INAF - Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
¹⁷ Instituto de Astrofísica e Ciências do Espaço, Faculdade de Ciências da Universidade de Lisboa, 1749-016 Lisboa, Portugal
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 ETH Zurich, Institute for Particle Physics & Astrophysics, Wolfgang-Pauli-Str. 27, 8093 Zurich, Switzerland
²¹ ETH Zurich, Department of Earth Sciences, Sonneggstrasse 5, 8092 Zurich, Switzerland
²² Subaru Telescope, National Astronomical Observatory of Japan, 650 N Aohoku Place, Hilo, HI 96720, USA
²³ Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany

^★★★ Corresponding author; nicola.nari@lightbridges.es

Received: 2 August 2024
Accepted: 15 October 2024

Abstract

Context. Close-by Earth analogs and super-Earths are of primary importance because they will be preferential targets for the next generation of direct imaging instruments. Bright and close-by G-to-M type stars are preferential targets in radial velocity surveys to find Earth analogs. Their brightness allows us to achieve the best precision on RV measurements and search for signals with amplitudes of less than 1 m s⁻¹.

Aims. We present an analysis of the RV data of the star HD 20794, a target whose planetary system has been extensively debated in the literature. The broad time span of the observations makes it possible to find planets with signal semi-amplitudes below 1 m s⁻¹ in the habitable zone.

Methods. We analyzed RV datasets spanning more than 20 years. We monitored the system with ESPRESSO. We joined ESPRESSO data with the HARPS data, including archival data and new measurements from a recent program. We applied the post-processing pipeline YARARA to HARPS data to correct systematics, improve the quality of RV measurements, and mitigate the impact of stellar activity.

Results. We confirm the presence of three planets, with periods of 18.3142 ± 0.0022 d, 89.68 ± 0.10 d, and 647.6_−2.7^+2.5 d, along with masses of 2.15 ± 0.17 M⊕, 2.98 ± 0.29 M⊕, and 5.82 ± 0.57 M⊕ respectively. For the outer planet, we find an eccentricity of 0.45_−0.11^+0.10, whereas the inner planets are compatible with circular orbits. The latter is likely to be a rocky planet in the habitable zone of HD 20794. From the analysis of activity indicators, we find evidence of a magnetic cycle with a period of ~3000 d, along with evidence pointing to a rotation period of ~39 d.

Conclusions. We have determined the presence of a system of three planets orbiting the solar-type star HD 20794. This star is bright (V=4.34 mag) and close (d = 6.04 pc), and HD 20794 d resides in the stellar habitable zone, making this system a high-priority target for future atmospheric characterization with direct imaging facilities.