TOI-6478 b: a cold underdense Neptune transiting a fully convective M dwarf from the thick disc

ABSTRACT Growing numbers of exoplanet detections continue to reveal the diverse nature of planetary systems. Planet formation around late-type M dwarfs is of particular interest. These systems provide practical laboratories to measure exoplanet occurrence rates for M dwarfs, thus testing how the outcomes of planet formation scale with host mass, and how they compare to Sun-like stars. Here, we report the discovery of TOI-6478 b, a cold ($T_{\text{eq}}=204\,$ K) Neptune-like planet orbiting an M5 star ($R_\star =0.234\pm 0.012\, \text{R}_\odot$, $M_\star =0.230\pm 0.007\, \text{M}_\odot$, $T_{\text{eff}}=3230\pm 75\,$ K) that is a member of the Milky Way’s thick disc. We measure a planet radius of $R_b=4.6\pm 0.24\, \text{R}_{\oplus }$ on a $P_b=34.005019\pm 0.000025\,$ d orbit. Using radial velocities, we calculate an upper mass limit of $M_b\le 9.9\, \text{M}_{\oplus }$ ($M_b\le 0.6\, \text{M}_{\text{Nep}})$, with $3\, \sigma$ confidence. TOI-6478 b is a milestone planet in the study of cold Neptune-like worlds. Due to its large atmospheric scale height, it is amenable to atmospheric characterization with facilities such as JWST, and will provide an excellent probe of atmospheric chemistry in this cold regime. It is one of very few transiting exoplanets that orbit beyond their system’s ice-line whose atmospheric chemical composition can be measured. Based on our current understanding of this planet, we estimate TOI-6478 b’s spectroscopic features (in transmission) can be $\sim 2.5\times$ as high as the widely studied planet K2-18 b.

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Publication Details

Journal
Monthly Notices of the Royal Astronomical Society
Published
2025-05-01
DOI
https://doi.org/10.1093/mnras/staf684
Citations
4
Primary Topic
Stellar, planetary, and galactic studies
Type
article
Field-Weighted Citation Impact
4.62

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article

TOI-6478 b: a cold underdense Neptune transiting a fully convective M dwarf from the thick disc

Benjamin V. Rackham, Jon M. Jenkins, Daniel Sebastian, Akihiko Fukui et al.
4 citations
Monthly Notices of the Royal Astronomical Society
Stellar, planetary, and galactic studies
4.62
article

TOI-6478 b: a cold underdense Neptune transiting a fully convective M dwarf from the thick disc

Benjamin V. Rackham, Jon M. Jenkins, Daniel Sebastian, Akihiko Fukui, Joseph D. Twicken, Georgina Dransfield, Adam J. Burgasser, Karen A. Collins, Steve B. Howell, A. H. M. J. Triaud, Sara Seager, Richard P. Schwarz, Mathilde Timmermans, Khalid Barkaoui, Michael Fausnaugh, G. Ricker, Christopher A. Theissen, David Watanabe, C. Hellier, Amalie Stokholm, Emma Softich, Norio Narita, Anjali A. A. Piette, Keivan G. Stassun, Avi Shporer, Cristilyn N. Watkins, Joshua N. Winn, M. G. Scott
article en
4 citations

Abstract

ABSTRACT Growing numbers of exoplanet detections continue to reveal the diverse nature of planetary systems. Planet formation around late-type M dwarfs is of particular interest. These systems provide practical laboratories to measure exoplanet occurrence rates for M dwarfs, thus testing how the outcomes of planet formation scale with host mass, and how they compare to Sun-like stars. Here, we report the discovery of TOI-6478 b, a cold ($T_{\text{eq}}=204\,$ K) Neptune-like planet orbiting an M5 star ($R_\star =0.234\pm 0.012\, \text{R}_\odot$, $M_\star =0.230\pm 0.007\, \text{M}_\odot$, $T_{\text{eff}}=3230\pm 75\,$ K) that is a member of the Milky Way’s thick disc. We measure a planet radius of $R_b=4.6\pm 0.24\, \text{R}_{\oplus }$ on a $P_b=34.005019\pm 0.000025\,$ d orbit. Using radial velocities, we calculate an upper mass limit of $M_b\le 9.9\, \text{M}_{\oplus }$ ($M_b\le 0.6\, \text{M}_{\text{Nep}})$, with $3\, \sigma$ confidence. TOI-6478 b is a milestone planet in the study of cold Neptune-like worlds. Due to its large atmospheric scale height, it is amenable to atmospheric characterization with facilities such as JWST, and will provide an excellent probe of atmospheric chemistry in this cold regime. It is one of very few transiting exoplanets that orbit beyond their system’s ice-line whose atmospheric chemical composition can be measured. Based on our current understanding of this planet, we estimate TOI-6478 b’s spectroscopic features (in transmission) can be $\sim 2.5\times$ as high as the widely studied planet K2-18 b.

Monthly Notices of the Royal Astronomical SocietyVol. 540(2)
Texas Tech University (US), Ames Research Center (US), Planetary Science Institute (US), University of Liège (BE), Universidad de La Laguna (ES), Vanderbilt University (US), Princeton University (US), Northeast Catholic College (US), University of California San Diego (US), University of Oxford (GB), IIT@MIT (US), Astrobiology Center (JP), Université Bourgogne Franche-Comté (FR), Center for Astrophysics Harvard & Smithsonian (US), Instituto de Astrofísica de Canarias (ES), Tokyo Metropolitan Komaba High School (JP), Keele University (GB), Massachusetts Institute of Technology (US), University of Maryland, College Park (US), The University of Tokyo (JP), University of Birmingham (GB), Search for Extraterrestrial Intelligence (US)
National Science Foundation, National Aeronautics and Space Administration, California Institute of Technology, Space Telescope Science Institute, Fonds De La Recherche Scientifique - FNRS, Ministry of Science, ICT and Future Planning, Nuclear Safety and Security Commission, Korea Astronomy and Space Science Institute, Ministério da Ciência, Tecnologia, Inovações e Comunicações, Agencia Nacional de Investigación y Desarrollo, Ames Research Center, Science Mission Directorate, Japan Society for the Promotion of Science, Precursory Research for Embryonic Science and Technology
Openalex Percentile: Top 6%
Stellar, planetary, and galactic studies
4.62
DOIOpen Access PDFOpenAlex
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