ExEP: TOI-1231 b: A temperate sub-Neptune transiting the nearby M3 dwarf NLTT 24399

We report the discovery of a transiting, temperate, sub-Neptune exoplanet orbiting TOI-1231(NLTT 24399, L 248-27, 2MASS J10265947-5228099), a nearby M3V star (d= 27.6 pc). The planet was detected using photometric data from theTransiting Exoplanet Survey Satellite and followed up with observations from the Las Cumbres Observatory and the Antarctica Search for Transiting Ex-oPlanets program. Combining the photometric data sets, we find that the newly discovered planet has a radius of 3.68+0.20−0.19R⊕, and an orbital period of 24.246 days. Radial velocity measurements using the Planet Finder Spectrograph on the Magellan Clay telescope confirm the existence of the planet and lead to a mass measurement of 15.1+3.1−3.2M⊕. With an equilibrium temperature of just336K and a Transmission Spectroscopy Metric of 102, TOI-1231 b is one of the coolest small planets accessible for atmospheric studies thus far, and its host star’s bright NIR brightness (J=8.88, Ks=8.07)make it an exciting target for HST and JWST. Future atmospheric observations would enable the first comparative planetology efforts in the 250-350 K temperature regime via comparisons with K2-18 b.Furthermore, TOI-1231’s high systemic radial velocity (70.5 km s−1) may allow for the detection of low-velocity hydrogen atoms escaping the planet by Doppler shifting the H I Ly-alpha stellar emission away from the geocoronal and ISM absorption features.