TOI-3884 b: A rare 6-R$_{\oplus}$ planet that transits a low-mass star with a giant and likely polar spot
Authors: J. M. Almenara, X. Bonfils, T. Forveille, N. Astudillo-Defru, D. R. Ciardi, R. P. Schwarz, K. A. Collins, M. Cointepas, M. B. Lund, F. Bouchy, D. Charbonneau, R. F. Díaz, X. Delfosse, R. C. Kidwell, M. Kunimoto, D. W. Latham, J. J. Lissauer, F. Murgas, G. Ricker, S. Seager, M. Vezie, D. Watanabe
AI-generated Key Points
- TESS mission identifies unique exoplanet, TOI-3884 b, orbiting mid-M dwarf star with 4.5 day periodicity
- Follow-up observations by ExTrA and LCOGT confirm presence of planet with chromatic transit
- Host star has large polar spot being transited by sub-Saturn planet with radius of $6.00 \pm 0.18$ R$_{\oplus}$
- First sub-Saturn planet discovered transiting mid-M dwarf
- Transit data analyzed to model starspot using MAP model and juliet to normalize TESS, ExTrA, and LCOGT transits
- Brightness and small size of TOI-3884 make it ideal target for atmospheric characterization using ground-based telescopes and space-based observatories like JWST
- Large transmission spectroscopy metrics for its size make it one of top targets for atmospheric characterization among all known planets
- Discovery provides valuable insights into planetary formation around low-mass stars and highlights how advanced observational techniques can help us better understand our universe's diversity
Abstract: The Transiting Exoplanet Survey Satellite mission identified a deep and asymmetric transit-like signal with a periodicity of 4.5 days orbiting the M4 dwarf star TOI-3884. The signal has been confirmed by follow-up observations collected by the ExTrA facility and Las Cumbres Observatory Global Telescope, which reveal that the transit is chromatic. The light curves are well modelled by a host star having a large polar spot transited by a 6-R$_{\oplus}$ planet. We validate the planet with seeing-limited photometry, high-resolution imaging, and radial velocities. TOI-3884 b, with a radius of $6.00 \pm 0.18$ R$_{\oplus}$, is the first sub-Saturn planet transiting a mid-M dwarf. Owing to the host star's brightness and small size, it has one of the largest transmission spectroscopy metrics for this planet size and becomes a top target for atmospheric characterisation with the James Webb Space Telescope and ground-based telescopes.
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