TOI-3884 b: A rare 6-<i>R</i><sub>E</sub>planet that transits a low-mass star with a giant and likely polar spot
J. M. Almenara, X. Bonfıls, T. Forveille, N. Astudillo-Defru, David R. Ciardi, Richard P. Schwarz, Karen A. Collins, Marion Cointepas, Michael B. Lund, F. Bouchy, D. Charbonneau, R. F. Díaz, X. Delfosse, R. C. Kidwell, M. Kunimoto, David W. Latham, J. J. Lissauer, F. Murgas, G. Ricker, Sara Seager, M. Vezie, David Watanabe
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 E 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 R E , 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.