TY - JOUR
T1 - TOI-6478 b
T2 - a cold under-dense Neptune transiting a fully convective M dwarf from the thick disc
AU - Scott, Madison G.
AU - Triaud, Amaury H. M. J.
AU - Barkaoui, Khalid
AU - Sebastian, Daniel
AU - Burgasser, Adam J.
AU - Collins, Karen A.
AU - Dransfield, Georgina
AU - Hellier, Coel
AU - Howell, Steve B.
AU - Piette, Anjali A. A.
AU - Rackham, Benjamin V.
AU - Stassun, Keivan G.
AU - Stockholm, Amalie
AU - Timmermans, Mathilde
AU - Watkins, Cristilyn N.
AU - Fausnaugh, Michael
AU - Fukui, Akihiko
AU - Jenkins, Jon M.
AU - Narita, Norio
AU - Ricker, George
AU - Softich, Emma
AU - Schwarz, Richard P.
AU - Seager, Sara
AU - Shporer, Avi
AU - Theissen, Christopher
AU - Twicken, Joseph D.
AU - Winn, Joshua N.
AU - Watanabe, David
N1 - 19 pages, 17 figures, accepted for publication in MNRAS
PY - 2025/5/1
Y1 - 2025/5/1
N2 - 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 (Teq = 204 K) Neptune-like planet orbiting an M5 star (R⋆ = 0.234 ± 0.012 R⊙, M⋆ = 0.230 ± 0.007 M⊙, Teff = 3230 ± 75 K) which is a member of the Milky Way’s thick disc. We measure a planet radius of Rb = 4.6 ± 0.24 R⊕ on a Pb = 34.005019 ± 0.000025 d orbit. Using radial velocities, we calculate an upper mass limit of Mb ≤ 9.9 M⊕ (Mb ≤ 0.6 MNep), with 3 σ confidence. TOI-6478 b 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-6478 b’s spectroscopic features (in transmission) can be ∼2.5 × as high as the widely studied planet K2-18 b.
AB - 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 (Teq = 204 K) Neptune-like planet orbiting an M5 star (R⋆ = 0.234 ± 0.012 R⊙, M⋆ = 0.230 ± 0.007 M⊙, Teff = 3230 ± 75 K) which is a member of the Milky Way’s thick disc. We measure a planet radius of Rb = 4.6 ± 0.24 R⊕ on a Pb = 34.005019 ± 0.000025 d orbit. Using radial velocities, we calculate an upper mass limit of Mb ≤ 9.9 M⊕ (Mb ≤ 0.6 MNep), with 3 σ confidence. TOI-6478 b 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-6478 b’s spectroscopic features (in transmission) can be ∼2.5 × as high as the widely studied planet K2-18 b.
KW - astro-ph.EP
U2 - 10.48550/arXiv.2504.06848
DO - 10.48550/arXiv.2504.06848
M3 - Article
SN - 0035-8711
JO - Monthly Notices of the Royal Astronomical Society
JF - Monthly Notices of the Royal Astronomical Society
M1 - staf684
ER -