TOI-3884 b: A rare 6-RE planet that transits a low-mass star with a giant and likely polar spot
Bouchy, F.; Bonfils, X.; Ciardi, D. R.; Collins, K. A.; Seager, S.; Almenara, J. M.; Delfosse, X.; Díaz, R. F.; Forveille, T.; Latham, D. W.; Ricker, G.; Schwarz, R. P.; Murgas, F.; Astudillo-Defru, N.; Charbonneau, D.; Kunimoto, M.; Vezie, M.; Kidwell, R. C.; Lissauer, J. J.; Cointepas, M.; Lund, M. B.; Watanabe, D.
France, Chile, United States, Switzerland, Argentina, Spain
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-RE planet. We validate the planet with seeing-limited photometry, high-resolution imaging, and radial velocities. TOI-3884 b, with a radius of 6.00 ± 0.18 RE, 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.