Letter to the Editor

TOI-3884 b: A rare 6-R_E planet that transits a low-mass star with a giant and likely polar spot

¹ Univ. Grenoble Alpes, CNRS, IPAG, 414 rue de la Piscine, 38400 St-Martin d’Hères, France
e-mail: Jose-Manuel.Almenara-Villa@univ-grenoble-alpes.fr
² Departamento de Matemática y Física Aplicadas, Universidad Católica de la Santísima Concepción, Alonso de Rivera 2850, Concepción, Chile
³ NASA Exoplanet Science Institute, Caltech/IPAC, 1200 E California Blvd, Pasadena, CA, 91125 USA
⁴ Center for Astrophysics, Harvard & Smithsonian, 60 Garden Street, Cambridge, MA, 02138 USA
⁵ Observatoire de Genève, Département d’Astronomie, Université de Genève, Chemin Pegasi 51, 1290 Versoix, Switzerland
⁶ International Center for Advanced Studies (ICAS) and ICIFI (CONICET), ECyT-UNSAM, Campus Miguelete, 25 de Mayo y Francia, 1650 Buenos Aires, Argentina
⁷ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD, 21218 USA
⁸ Department of Physics and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA, 02139 USA
⁹ NASA Ames Research Center, Moffett Field, CA, 94035 USA
¹⁰ Instituto de Astrofísica de Canarias (IAC), Vía Láctea, s/n, 38200 La Laguna, Tenerife, Spain
¹¹ Dept. Astrofísica, Universidad de La Laguna (ULL), Avenida Astrofísico Francisco Sánchez, s/n, 38206 La Laguna, Tenerife, Spain
¹² Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA, 02139 USA
¹³ Department of Aeronautics and Astronautics, MIT, 77 Massachusetts Avenue, Cambridge, MA, 02139 USA
¹⁴ Planetary Discoveries in Fredericksburg, 22 Woodford Dr, Fredericksburg, VA, 22405 USA

Received: 22 August 2022
Accepted: 12 October 2022

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.