TOI-1231 b: A Temperate, Neptune-sized Planet Transiting the Nearby M3 Dwarf NLTT 24399

We report the discovery of a transiting, temperate, Neptune-sized exoplanet orbiting the nearby (d = 27.5 pc), M3V star TOI-1231 (NLTT 24399, L 248-27, 2MASS J10265947-5228099). The planet was detected using photometric data from the Transiting Exoplanet Survey Satellite and followed up with observations from the Las Cumbres Observatory and the Antarctica Search for Transiting ExoPlanets program. Combining the photometric data sets, we find that the newly discovered planet has a radius of 3.65-0.15+0.16R circle plus M (circle plus). With an equilibrium temperature of just 330 K, TOI-1231 b is one of the coolest small planets accessible for atmospheric studies thus far, and its host star's bright near-infrared brightness (J = 8.88, K (s) = 8.07) makes it an exciting target for the Hubble Space Telescope and the James Webb Space Telescope. 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 interstellar medium absorption features.

Automated summary

The discovery of a temperate, Neptune-sized exoplanet orbiting the M3V star TOI-1231 has been reported in this study. The newly discovered planet, with a radius of 3.65R circle plus M (circle plus) and an equilibrium temperature of 330K, presents a cool and exciting target for atmospheric studies. Additionally, future observations may allow for comparative planetology efforts and investigation of low-velocity hydrogen atoms escaping the planet.