Exoplanets, -moons, -comets

TOI-6478 b: A Cold Under-dense Neptune Transiting A Fully Convective M Dwarf From The Thick Disc

By Keith Cowing
Status Report
astro-ph.EP
April 11, 2025
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TOI-6478 b: A Cold Under-dense Neptune Transiting A Fully Convective M Dwarf From The Thick Disc
Comparison of TOI-6478 b (diamond) to the known exoplanet population. Left: Planet radius vs orbital period for exoplanets with equilibrium temperatures (color bar) < 250 K and radii > 2R⊕. One year is highlighted by the dashed grey line. Of these 25 exoplanets, only 9 are colder than TOI6478 b, but all have orbital periods at least 3× larger. Solar systems planets are shown as reference. Right: Mass-radius diagram with key mass-radius relations highlighted with coloured dashed lines. TOI-6478 b is shown as the pink diamond, placed at the upper-mass limit from the current RVs. Solar system planets (black diamonds) are shown as reference. — astro-ph.EP

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-6478b, a cold (Teq=204K) Neptune-like planet orbiting an M5 star (R⋆=0.234±0.012R⊙, M⋆=0.230±0.007M⊙, Teff=3230±75K) which is a member of the Milky Way’s thick disc. We measure a planet radius of Rb=4.6±0.24R⊕ on a Pb=34.005019±0.000025d orbit. Using radial velocities, we calculate an upper mass limit of Mb≤9.9M⊕ (Mb≤0.6MNep), with 3σ confidence. TOI-6478b is a milestone planet in the study of cold, Neptune-like worlds.

Thanks to its large atmospheric scale height, it is amenable to atmospheric characterisation 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-6478b’s spectroscopic features (in transmission) can be ∼2.5× as high as the widely studied planet K2-18b.

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

Comments: 19 pages, 17 figures, accepted for publication in MNRAS
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2504.06848 [astro-ph.EP] (or arXiv:2504.06848v1 [astro-ph.EP] for this version)

https://doi.org/10.48550/arXiv.2504.06848
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Submission history
From: Madison Scott
[v1] Wed, 9 Apr 2025 13:03:16 UTC (5,778 KB)
https://arxiv.org/abs/2504.06848
Astrobiology,

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻

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