TOI-3862 b: A dense super-Neptune deep in the hot Neptune desert

¹ INAF – Osservatorio Astrofisico di Torino, Via Osservatorio 20, 10025 Pino Torinese, Italy
² Instituto de Astrofísica de Canarias (IAC), 38205 La Laguna, Tenerife, Spain
³ Astronomical Observatory, University of Geneva, Chemin Pegasi 51b, 1290 Versoix, Switzerland
⁴ Departamento de Astrofísica, Universidad de La Laguna (ULL), 38206 La Laguna, Tenerife, Spain
⁵ Institute of Astronomy, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University, Grudziądzka 5, 87-100 Toruń, Poland
⁶ Departamento de Astronomía, Universidad de Chile, Casilla 36-D, 7591245 Santiago, Chile
⁷ Centro de Astrofísica y Tecnologías Afines (CATA), Casilla 36-D, 7591245 Santiago, Chile
⁸ Núcleo de Astronomía, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Av. Ejército 441 Santiago, Chile
⁹ INAF – Osservatorio Astrofisico di Catania, Via S. Sofia 78, 95123 Catania, Italy
¹⁰ Department of Physics & Astronomy, Vanderbilt University, Nashville, TN, USA
¹¹ Dipartimento di Fisica, Universitá degli Studi di Torino, via Pietro Giuria 1, 10125 Torino, Italy
¹² NASA Exoplanet Science Institute, IPAC, California Institute of Technology, Pasadena, CA 91125, USA
¹³ Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
¹⁴ Thüringer Landessternwarte Sternwarte 5 07778 Tautenburg, Germany
¹⁵ Astrophysics Group, Keele University, Staffs ST5 5BG, UK
¹⁶ NASA Ames Research Center, Moffett Field, CA 94035, USA
¹⁷ Centro de Astrobiología (CAB, CSIC-INTA), Depto. de Astrofísica, ESAC campus, 28692 Villanueva de la Cañada (Madrid), Spain
¹⁸ Astrobiology Center, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
¹⁹ National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
²⁰ Department of Astronomy, The Graduate University for Advanced Studies (SOKENDAI), 2-21-1 Osawa, Mitaka, Tokyo, Japan
²¹ The University of Texas at Austin, 2515 Speedway, Stop C1402, Austin, TX 78712-1206, USA
²² Chalmers University of Technology, Department of Space, Earth and Environment, Onsala Space Observatory, 439 92 Onsala, Sweden
²³ Astronomy Department and Van Vleck Observatory, Wesleyan University, Middletown, CT 06459, USA
²⁴ Sternberg Astronomical Institute, Lomonosov Moscow State University, 119992 Universitetskii prospekt 13, Moscow, Russia
²⁵ Physics Department, Austin College, Sherman, TX 75090, USA
²⁶ Department of Physics and Astronomy, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA
²⁷ Observatori Astronòmic Albanyà, Camí de Bassegoda S/N, Albanyà, 17733, Girona, Spain
²⁸ Komaba Institute for Science, The University of Tokyo, 3-8-1 Komaba, Meguro, Tokyo 153-8902, Japan
²⁹ Department of Multi-Disciplinary Sciences, Graduate School of Arts and Sciences, The University of Tokyo, 3-8-1 Komaba, Meguro, Tokyo 153-8902, Japan
³⁰ Graduate School of Social Data Science, Hitotsubashi University, 2-1 Naka, Kunitachi, Tokyo 186-8601, Japan
³¹ Department of Physical Sciences, Ritsumeikan University, Kusatsu, Shiga 525-8577, Japan
³² Stellar Astrophysics Centre, Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, 8000 Aarhus C, Denmark
³³ Sabadell Astronomical Society, 08206 Sabadell, Barcelona, Spain
³⁴ Europlanet Society, Department of Planetary Atmospheres of the Royal Belgian Institute for Space Aeronomy, 1180 Brussels, Belgium
³⁵ Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
³⁶ Villa ’39 Observatory, Landers, CA 92285, USA
³⁷ Waffelow Creek Observatory, 10780 FM 1878, Nacogdoches, TX 75961, USA
³⁸ INAF – Osservatorio Astronomico di Palermo, Piazza del Parlamento, 1, 90134 Palermo, Italy
³⁹ Department of Physics and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
⁴⁰ Bozeman, MT 59718, USA

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Received: 10 September 2025
Accepted: 12 January 2026

Abstract

Context. The structure and evolution of close-in exoplanets are shaped by atmospheric loss and migration processes, which give rise to key population features such as the hot Neptune desert, ridge, and savanna – regions of the period-radius space whose boundaries offer critical insights into planetary formation and survival.

Aims. As part of the KESPRINT collaboration, we selected the TESS transiting planet candidate TOI-3862.01 for radial velocity follow-up to confirm its planetary nature and characterize its mass and bulk properties. This planet candidate is of particular interest due to its position in the middle of the hot Neptune desert, making it a valuable probe for testing theories of planet migration and atmospheric loss.

Methods. We confirmed the planetary nature and determined the mass of TOI-3862.01 (hereinafter TOI-3862 b) by performing a joint fit with both transit and radial velocity data, precisely characterizing the bulk properties of this planet.

Results. TOI-3862b is a super-Neptune on a 1.56-day orbit around a Sun-like star with an effective temperature of 5300±50 K. It has a mass of 53.7_−2.9^+2.8 M_⊕ and a radius of 5.53 ± 0.18 R_⊕, corresponding to a density of 1.7±0.2 g/cm³. This places it among the rare population of hot and dense super-Neptune desert planets.

Conclusions. TOI-3862b, residing deep in the hot Neptune desert, represents a rare occurrence in an otherwise sparsely populated region, offering a valuable opportunity to probe the processes that may allow planets to survive in such environments.