Densities of small planets around the M dwarfs TOI-4336 A and TOI-4342 with ESPRESSO

Three sub-Neptunes, one super-Earth, and a Neptune-mass candidate

¹ Observatoire de Genève, Département d’Astronomie, Université de Genève, Chemin Pegasi 51, 1290 Versoix, Switzerland
² Institut Trottier de recherche sur les exoplanètes, Département de Physique, Université de Montréal, Montréal, Québec, Canada
³ Institute for Particle Physics and Astrophysics, ETH Zürich, Otto-Stern-Weg 5, 8093 Zürich, Switzerland
⁴ Centro de Astrobiología (CAB), CSIC-INTA, Camino Bajo del Castillo s/n, 28692, Villanueva de la Cañada (Madrid), Spain
⁵ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
⁶ Departamento de Matemática y Física Aplicadas, Universidad Católica de la Santísima Concepción, Alonso de Rivera 2850, Concepción, Chile
⁷ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
⁸ Department of Physics and Astronomy, McMaster University, 1280 Main St W, Hamilton, ON L8S 4M1, Canada
⁹ Center for Astrophysics I Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
¹⁰ Departamento de Física Teórica e Experimental, Universidade Federal do Rio Grande do Norte, Campus Universitário, Natal, RN 59072-970, Brazil
¹¹ Department of Astronomy, Westlake University, Hangzhou 310030, Zhejiang Province, China
¹² Instituto de Astrofísica de Canarias (IAC), Calle Vía Láctea s/n, 38205 La Laguna, Tenerife, Spain
¹³ Departamento de Astrofísica, Universidad de La Laguna (ULL), 38206 La Laguna, Tenerife, Spain
¹⁴ Center for Theoretical Astrophysics and Cosmology, Institute for Computational Science, University of Zürich, Winterthurerstrasse 190, 8057 Zürich, Switzerland
¹⁵ Institute of Space Sciences (ICE, CSIC), Carrer de Can Magrans S/N, Campus UAB, Cerdanyola del Valles 08193, Spain
¹⁶ Institut d’Estudis Espacials de Catalunya (IEEC), 08860 Castelldefels (Barcelona), Spain
¹⁷ Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal

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Received: 28 November 2025
Accepted: 9 February 2026

Abstract

Context. Characterizing the masses, radii, and compositions of small planets orbiting M dwarfs is key to understanding their formation and identifying the best targets for atmospheric follow-up with facilities such as JWST.

Aims. We present the characterization of two planetary systems orbiting the M dwarfs TOI-4336 A (M3.5V) and TOI-4342 (M0V), each hosting two transiting planets previously validated with TESS and ground-based observations.

Methods. We refined the photometry of the TOI-4342 system using TESS and LCOGT data, and characterized the host stars with NIRPS and ESPRESSO spectroscopy. High-precision ESPRESSO radial velocities (RVs) allowed us to constrain the planetary masses and investigate their potential compositions.

Results. The TOI-4336 A system is composed of a sub-Neptune with a period of 16.34 days, a radius of 2.14 ± 0.08 R_⊕, and a mass of 3.33 ± 0.36 M_⊕, along with an inner super-Earth on a 7.59-day orbit with a radius of 1.25 ± 0.07 R_⊕ and a mass of 1.55 ± 0.13 M_⊕. The TOI-4342 system hosts two sub-Neptunes of similar sizes (2.33 ± 0.09 R_⊕ and 2.35 ± 0.09 R_⊕), with periods of 5.54 and 10.69 days. Their masses are measured to be 7.3 ± 1.3 M_⊕ and 4.8 ± 1.4 M_⊕, respectively. The RVs also reveal a planet candidate around TOI-4342, most likely non-transiting, with a period of 47.5 days and a minimum mass of 17.8 ± 3.0 M_⊕.

Conclusions. With precise radii and masses, we derived bulk densities and explored possible compositions. The TOI-4336 A subNeptune and super-Earth have densities of 1.87 ± 0.30 and 4.35 ± 0.79 g cm⁻³, while the two similar-sized sub-Neptunes in TOI-4342 show distinct densities of 3.18 ± 0.67 and 2.01 ± 0.63 g cm⁻³. Using an inference model, we find that TOI-4336 A b, TOI-4342 b, and TOI-4342 c have an atmosphere mass fraction (AMF) of ∼3.7%, ∼1.8%, and ∼2.9%, respectively, while the super-Earth TOI-4336 A c could contain ∼2% of water or have a core-to-mass fraction (CMF) of ∼31%. All four planets are excellent targets for future atmospheric characterization with JWST, and their multi-planet nature makes them especially interesting for comparative planetology. Notably, TOI-4336 A b stands out as one of the best known targets in its size and temperature regime, with a transmission spectroscopy metric (TSM) of 138, comparable to benchmark planets such as K2-18 b and LHS 1140 b. Its inner sibling, TOI-4336 A c, may also be of interest for emission spectroscopy and exploring the “cosmic shoreline”, similarly to the Rocky Worlds DDT JWST program.