Two neighbours of the ultra-short-period Earth-sized planet K2-157 b in the warm Neptunian savanna^★

¹ Centro de Astrobiología, CSIC-INTA, Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain
² Observatoire de l’Université de Genève, 51 chemin Pegasi, 1290 Sauverny, Switzerland
³ Département d’Astronomie, Université de Genève, Chemin Pegasi 51, 1290 Versoix, Switzerland
⁴ CFisUC, Departamento de Física, Universidade de Coimbra, 3004-516 Coimbra, Portugal
⁵ IMCCE, UMR8028 CNRS, Observatoire de Paris, PSL Université, 77 Av. Denfert-Rochereau, 75014 Paris, France
⁶ INAF - Osservatorio Astrofisico di Torin, Via Osservatorio 20, 10025 Pino Torinese, Italy
⁷ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
⁸ Departamento de Fisica e Astronomia, Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal
⁹ European Space Agency (ESA), European Space Astronomy Centre (ESAC), Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain
¹⁰ Université Aix Marseille, CNRS, CNES, LAM, Marseille, France
¹¹ Instituto de Astrofísica e Ciências do Espaço, Universidade de Lisboa, Campo Grande, 1749-016 Lisboa, Portugal
¹² Lund Observatory, Division of Astrophysics, Department of Physics, Lund University, Box 118, 22100 Lund, Sweden
¹³ Observatoire François-Xavier Bagnoud – OFXB, 3961 Saint-Luc, Switzerland
¹⁴ Center for Space and Habitability, University of Bern, Gesellschaftsstrasse 6, 3012 Bern, Switzerland
¹⁵ Physics Institute of University of Bern, Gesellschaftsstrasse 6, 3012 Bern, Switzerland
¹⁶ INAF – Osservatorio Astronomico di Trieste, via G. B. Tiepolo 11, 34143 Trieste, Italy
¹⁷ Instituto de Astrofísica de Canarias, 38205 La Laguna, Tenerife, Spain
¹⁸ Universidad de La Laguna, Dept. Astrofísica, 38206 La Laguna, Tenerife, Spain
¹⁹ European Southern Observatory, Av. Alonso de Cordova 3107, Casilla 19001, Santiago de Chile, Chile
²⁰ Centro de Astrofísica da Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal
²¹ Département d’Astronomie, Université de Genève, Ch. des Maillettes 51, 1290 Versoix, Switzerland
²² Institut de Ciències de l’Espai (ICE, CSIC), Campus UAB, c/ de Can Magrans s/n, 08193 Cerdanyola del Vallès, Barcelona, Spain
²³ Institut d’Estudis Espacials de Catalunya (IEEC), Edifici RDIT, Campus UPC, 08860 Castelldefels (Barcelona), Spain

^★★ Corresponding author: acastro@cab.inta-csic.es

Received: 25 March 2025
Accepted: 25 April 2025

Abstract

Context. The formation and evolution of ultra-short-period (USP) rocky planets is poorly understood. However, it is widely thought that these planets could not have formed at their present-day close-in orbits, but instead migrated inwards through interactions with outer neighbours.

Aims. We aim to confirm and characterise the USP Earth-sized validated planet K2-157 b (P_orb = 8.8 h) and constrain the presence of additional companions in the system through radial velocity (RV) measurements.

Methods. We measured 49 RVs with the ESPRESSO spectrograph and tested different planetary and non-planetary configurations to infer the model that best represents our data set. We derived the orbital and physical properties of the system through a global RV and transit model.

Results. We detected two additional super-Neptune-mass planets located within the warm Neptunian savanna, K2-157 c (P_orb,c = 25.942_−0.044^+0.045d, M_p,c sin i = 30.8 ± 1.9 M_⊕) and K2-157 d (P_orb,d = 66.50_−0.59^+0.71d, M_p,d sin i = 23.3 ± 2.5 M_⊕). The joint analysis constrains the mass of K2-157 b at the 2.7σ level, M_p,b = 1.14_−0.42^+0.41 M_⊕ (< 2.4 M_⊕ at 3σ), which, together with the inferred radius, R_p,b = 0.935 ± 0.090 R_⊕, make the planet compatible with a rocky composition with a likely (68% confidence) higher iron-to-silicate mass fraction than Earth. K2 data discard non-grazing transit configurations for K2-157 c (i_c < 88.4° at 3σ), and ESPRESSO data constrain the eccentricities of K2-157 c and K2-157 d to e_c < 0.2 and e_d < 0.5 at 3σ. Our dynamical analysis indicates that the system is stable for eccentricities up to e_c, e_d ~ 0.3 and mutual inclinations up to ~60°. At a population level, we find that the trend that the closest USP planets tend to orbit late-type stars does not hold when scaling the orbital separation to the Roche limit, which suggests that the orbital distribution of the closest planets across spectral types is primarily determined by tidal disruption.

Conclusions. The orbital architecture of K2-157 is unusual in the known exoplanet plethora, with only one similar case reported to date: 55 Cnc. The USP planets of these systems, being accompanied by massive, long-period, relatively spaced, and possibly misaligned neighbours, could have migrated inwards through eccentricity-based mechanisms triggered by secular interactions.