The compact multi-planet system GJ 9827 revisited with ESPRESSO^★,★★

¹ Instituto de Astrofísica de Canarias, c/ Vía Láctea s/n, 38205 La Laguna, Tenerife, Spain
e-mail: vpassegger@hs.uni-hamburg.de
² Departamento de Astrofísica, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain
³ Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany
⁴ Department of Physics, and Trottier Institute for Research on Exoplanets, Université de Montréal, Montréal H3T 1J4, Canada
⁵ Observatoire astronomique de l’Université de Genève, Chemin Pegasi 51b, 1290 Versoix, Switzerland
⁶ Instituto de Astrofísica e Ciências do Espaço, CAUP, Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal
⁷ Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal
⁸ 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
⁹ INAF – Osservatorio Astronomico di Trieste, Via Tiepolo 11, 34143 Trieste, Italy
¹⁰ Département d’astronomie de l’Université de Genève, Chemin Pegasi 51, 1290 Versoix, Switzerland
¹¹ Consejo Superior de Investigaciones Científicas (CSIC), 28006 Madrid, Spain
¹² Physikalisches Institut & Center for Space and Habitability, Universität Bern, Gesellschaftsstrasse 6, 3012 Bern, Switzerland
¹³ Centro de Astrobiología, CSIC-INTA, Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain
¹⁴ European Southern Observatory, Av. Alonso de Cordova 3107, Vitacura, Santiago de Chile, Chile
¹⁵ Centro de Astrofísica da Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal
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, Campo Grande, 1749-016 Lisboa, Portugal
¹⁸ INAF – Osservatorio Astronomico di Brera, Via Bianchi 46, 23807 Merate, Italy
¹⁹ Observatoire François-Xavier Bagnoud – OFXB, 3961 Saint-Luc, Switzerland
²⁰ INAF – Osservatorio Astrofisico di Torino, Strada Osservatorio 20, 10025 Pino Torinese (TO), Italy
²¹ Light Bridges S.L., Avda. Alcalde Ramírez Bethencourt 17, 35004 Las Palmas de Gran Canaria, Canarias, Spain

Received: 13 November 2023
Accepted: 4 January 2024

Abstract

GJ 9827 is a bright, nearby K7V star orbited by two super-Earths and one mini-Neptune on close-in orbits. The system was first discovered using K2 data and then further characterized by other spectroscopic and photometric instruments. Previous literature studies provide several mass measurements for the three planets, however, with large variations and uncertainties. To better constrain the planetary masses, we added high-precision radial velocity measurements from ESPRESSO to published datasets from HARPS, HARPS-N, and HIRES and we performed a Gaussian process analysis combining radial velocity and photometric datasets from K2 and TESS. This method allowed us to model the stellar activity signal and derive precise planetary parameters. We determined planetary masses of M_b = 4.28_−0.33^+0.35 M_⊕, M_c = 1.86_−0.39^+0.37 M_⊕, and M_d = 3.02_−0.57^+0.58 M_⊕, and orbital periods of 1.208974 ± 0.000001 days for planet b, 3.648103_−0.000010^+0.000013 days for planet c, and 6.201812 ± 0.000009 days for planet d. We compared our results to literature values and found that our derived uncertainties for the planetary mass, period, and radial velocity amplitude are smaller than the previously determined uncertainties. We modeled the interior composition of the three planets using the machine-learning-based tool ExoMDN and conclude that GJ 9827 b and c have an Earth-like composition, whereas GJ 9827 d has an hydrogen envelope, which, together with its density, places it in the mini-Neptune regime.