A compact multi-planet system transiting HIP 29442 (TOI-469) discovered by TESS and ESPRESSO

Radial velocities lead to the detection of transits with low signal-to-noise ratio^★

¹ INAF – Osservatorio Astrofisico di Torino, Via Osservatorio 20, 10025 Pino Torinese, Italy
e-mail: mario.damasso@inaf.it
² Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
³ Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal
⁴ Centro de Astrobiología (CAB,CSIC-INTA), Dep. de Astrofísica, ESAC campus, Villanueva de la Cañada, 28692 Madrid, Spain
⁵ Département d’Astronomie, Université de Genève, Chemin Pegasi 51, 1290 Versoix, Switzerland
⁶ Center for Space and Habitability, University of Bern, Gesellschaftsstrasse 6, 3012 Bern, Switzerland
⁷ Physics Institute of University of Bern, Gesellschaftsstrasse 6, 3012 Bern, Switzerland
⁸ Centro de Astrobiología (CSIC-INTA), Carretera de Ajalvir km 4, 28850 Torrejón de Ardoz, Madrid, Spain
⁹ School of Physics, H.H. Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol BS8 1TL, UK
¹⁰ Astrophysics Group, Department of Physics, Imperial College London, Prince Consort Rd, London, SW7 2AZ, UK
¹¹ Institute of Geochemistry and Petrology, ETH Zürich, Clausiusstrasse 25, 8092 Zürich, Switzerland
¹² Department of Physics, Engineering and Astronomy, Stephen F. Austin State University, 1936 North St, Nacogdoches, TX 75962, USA
¹³ NASA Exoplanet Science Institute-Caltech/IPAC, 1200 East California Blvd., Pasadena, CA 91125, USA
¹⁴ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
¹⁵ Instituto de Astrofisica de Canarias, Via Lactea, 38200 La Laguna, Tenerife, Spain
¹⁶ Universidad de La Laguna, Departamento de Astrofísica, Avda. Astrofisico Francisco Sánchez s/n, 38206 La Laguna, Tenerife, Spain
¹⁷ INAF – Osservatorio Astronomico di Trieste, Via Tiepolo 11, 34143 Trieste, Italy
¹⁸ Institute for Fundamental Physics of the Universe, IFPU, Via Beirut 2, 34151 Grignano, Trieste, Italy
¹⁹ Consejo Superior de Investigaciones Científicas, C. de Serrano 117, 28006 Madrid, Spain
²⁰ INAF – Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
²¹ Cerro Tololo Inter-American Observatory/NSF’s NOIRLab, Casilla 603, La Serena, Chile
²² Department of Astronomy and Astrophysics, University of California, 1156 High Street, Santa Cruz, CA 95064, USA
²³ Department of Physics and Astronomy, The University of North Carolina at Chapel Hill, 120 E. Cameron Ave., Phillips Hall CB3255, Chapel Hill, NC 27599, USA
²⁴ ESO, European Southern Observatory, Alonso de Cordova 3107, Vitacura, Santiago, Chile
²⁵ Centro de Astrofísica da Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal
²⁶ Instituto de Astrofísica e Ciências do Espaço, Faculdade de Ciências da Universidade de Lisboa, Campo Grande, PT1749-016 Lisboa, Portugal
²⁷ INAF – Osservatorio Astronomico di Brera, Via Bianchi 46, 23807 Merate, Italy
²⁸ Exoplanets and Stellar Astrophysics Laboratory, NASA Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771, USA

Received: 20 June 2023
Accepted: 12 August 2023

Abstract

Context. One of the goals of the Echelle Spectrograph for Rocky Exoplanets and Stable Spectroscopic Observations (ESPRESSO) Guaranteed Time Observations (GTO) consortium is the precise characterisation of a selected sample of planetary systems discovered by TESS. One such target is the K0V star HIP 29442 (TOI-469), already known to host a validated sub-Neptune companion TOI-469.01, which we followed-up with ESPRESSO.

Aims. We aim to verify the planetary nature of TOI-469.01 by obtaining precise mass, radius, and ephemeris, and constraining its bulk physical structure and composition.

Methods. Following a Bayesian approach, we modelled radial velocity and photometric time series to measure the dynamical mass, radius, and ephemeris, and to characterise the internal structure and composition of TOI-469.01.

Results. We confirmed the planetary nature of TOI-469.01 (now renamed HIP 29442 b), and thanks to the ESPRESSO radial velocities we discovered two additional close-in companions. Through an in-depth analysis of the TESS light curve, we could also detect their low signal-to-noise transit signals. We characterised the additional companions, and conclude that HIP 29442 is a compact multi-planet system. The three planets have orbital periods P_orb,b = 13.63083 ± 0.00003, P_orb,c = 3.53796 ± 0.00003, and P_orb,d = 6.42975_−0.00010^+0.00009 days, and we measured their masses with high precision: m_p,b = 9.6 ± 0.8 M_⊕, m_p,c = 4.5 ± 0.3 M_⊕, and m_p,d = 5.1 ± 0.4 M_⊕. We measured radii and bulk densities of all the planets (the 3σ confidence intervals are shown in parentheses): R_p,b = 3.48_{−0.08(−0.28)}^+0.07(+0.19) R_⊕ and ρ_p,b = 1.3 ± 0.2(0.3)g cm⁻³; R_p,c = 1.58_{−0.11(−0.34)}^+0.10(+0.30) R_⊕ and ρ_p,c = 6.3_{−1.3(−2.7)}^+1.7(+6.0)g cm⁻³; R_p,d = 1.37 ± 0.11_(−0.43)^(+0.32) R_⊕ and ρ_p,d = 11.0_{−2.4(−6.3)}^+3.4(+21.0)g cm⁻³. Due to noisy light curves, we used the more conservative 3σ confidence intervals for the radii as input to the interior structure modelling. We find that HIP 29442 b appears as a typical sub-Neptune, likely surrounded by a gas layer of pure H-He with amass of 0.27_−0.17^+0.24 M_⊕ and a thickness of 1.4 ± 0.5 R_⊕. For the innermost companions HIP 29442 c and HIP 29442 d, the model supports an Earth-like composition.

Conclusions. The compact multi-planet system orbiting HIP 29442 offers the opportunity to study simultaneously planets straddling the gap in the observed radius distribution of close-in small-size exoplanets. High-precision photometric follow-up is required to obtain more accurate and precise radius measurements, especially for planets c and d. This, together with our determined high-precision masses, will provide the accurate and precise bulk structure of the planets, and enable an accurate investigation of the system’s evolution.