TOI-3884 b: A rare 6-R$_{\oplus}$ planet that transits a low-mass star with a giant and likely polar spot

TL;DR

This paper reports the discovery and validation of TOI-3884 b, a rare 6-Earth-radius planet transiting a low-mass M4 dwarf star, with unique features including a polar spot and potential for atmospheric study.

Contribution

It presents the first sub-Saturn-sized planet transiting a mid-M dwarf, confirmed through multiple observations, and highlights its suitability for atmospheric characterization.

Findings

First sub-Saturn planet transiting a mid-M dwarf

Host star has a large polar spot affecting transit signals

Planet is a prime target for JWST atmospheric studies

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.