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| Auteurs principaux: | , , , , , , , , , , , , , , , , , , , , , , , , , , , |
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| Format: | Preprint |
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2025
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| Accès en ligne: | https://arxiv.org/abs/2504.06848 |
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| author | Scott, Madison G. Triaud, Amaury H. M. J. Barkaoui, Khalid Sebastian, Daniel Burgasser, Adam J. Collins, Karen A. Dransfield, Georgina Hellier, Coel Howell, Steve B. Piette, Anjali A. A. Rackham, Benjamin V. Stassun, Keivan G. Stockholm, Amalie Timmermans, Mathilde Watkins, Cristilyn N. Fausnaugh, Michael Fukui, Akihiko Jenkins, Jon M. Narita, Norio Ricker, George Softich, Emma Schwarz, Richard P. Seager, Sara Shporer, Avi Theissen, Christopher Twicken, Joseph D. Winn, Joshua N. Watanabe, David |
| author_facet | Scott, Madison G. Triaud, Amaury H. M. J. Barkaoui, Khalid Sebastian, Daniel Burgasser, Adam J. Collins, Karen A. Dransfield, Georgina Hellier, Coel Howell, Steve B. Piette, Anjali A. A. Rackham, Benjamin V. Stassun, Keivan G. Stockholm, Amalie Timmermans, Mathilde Watkins, Cristilyn N. Fausnaugh, Michael Fukui, Akihiko Jenkins, Jon M. Narita, Norio Ricker, George Softich, Emma Schwarz, Richard P. Seager, Sara Shporer, Avi Theissen, Christopher Twicken, Joseph D. Winn, Joshua N. Watanabe, David |
| contents | 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 ($T_{\text{eq}}=204\,$K) Neptune-like planet orbiting an M5 star ($R_\star=0.234\pm0.012\,\text{R}_\odot$, $M_\star=0.230\pm0.007\,\text{M}_\odot$, $T_{\text{eff}}=3230\pm75\,$K) which is a member of the Milky Way's thick disc. We measure a planet radius of $R_b=4.6\pm0.24\,\text{R}_\oplus$ on a $P_b=34.005019\pm0.000025\,$d orbit. Using radial velocities, we calculate an upper mass limit of $M_b\leq9.9\,\text{M}_\oplus$ ($M_b\leq0.6\,\text{M}_{\text{Nep}})$, with $3\,σ$ 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 $\sim2.5\times$ as high as the widely studied planet K2-18b. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2504_06848 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | TOI-6478 b: a cold under-dense Neptune transiting a fully convective M dwarf from the thick disc Scott, Madison G. Triaud, Amaury H. M. J. Barkaoui, Khalid Sebastian, Daniel Burgasser, Adam J. Collins, Karen A. Dransfield, Georgina Hellier, Coel Howell, Steve B. Piette, Anjali A. A. Rackham, Benjamin V. Stassun, Keivan G. Stockholm, Amalie Timmermans, Mathilde Watkins, Cristilyn N. Fausnaugh, Michael Fukui, Akihiko Jenkins, Jon M. Narita, Norio Ricker, George Softich, Emma Schwarz, Richard P. Seager, Sara Shporer, Avi Theissen, Christopher Twicken, Joseph D. Winn, Joshua N. Watanabe, David Earth and Planetary Astrophysics 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 ($T_{\text{eq}}=204\,$K) Neptune-like planet orbiting an M5 star ($R_\star=0.234\pm0.012\,\text{R}_\odot$, $M_\star=0.230\pm0.007\,\text{M}_\odot$, $T_{\text{eff}}=3230\pm75\,$K) which is a member of the Milky Way's thick disc. We measure a planet radius of $R_b=4.6\pm0.24\,\text{R}_\oplus$ on a $P_b=34.005019\pm0.000025\,$d orbit. Using radial velocities, we calculate an upper mass limit of $M_b\leq9.9\,\text{M}_\oplus$ ($M_b\leq0.6\,\text{M}_{\text{Nep}})$, with $3\,σ$ 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 $\sim2.5\times$ as high as the widely studied planet K2-18b. |
| title | TOI-6478 b: a cold under-dense Neptune transiting a fully convective M dwarf from the thick disc |
| topic | Earth and Planetary Astrophysics |
| url | https://arxiv.org/abs/2504.06848 |