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 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 Transiting Exoplanet Survey Satellite and followed up with observations Las Cumbres Observatory Antarctica Search for ExoPlanets program. Combining sets, we find that newly discovered has radius 3.65$^{+0.16}_{-0.15}$ R$_{\oplus}$, an orbital period 24.246 days. Radial velocity measurements obtained Planet Finder Spectrograph on Magellan Clay telescope confirm existence lead to mass measurement 15.5$\pm$3.3 M$_{\oplus}$. With equilibrium temperature just 330K b is one coolest small planets accessible atmospheric studies thus far, its host star's bright NIR brightness (J=8.88, K$_{s}$=8.07) make it exciting target HST JWST. Future would enable first comparative planetology efforts in 250-350 K regime via comparisons K2-18 b. Furthermore, TOI-1231's high systemic radial (70.5 k\ms) may allow detection low-velocity hydrogen atoms escaping by Doppler shifting H I Ly-alpha stellar emission away geocoronal ISM absorption features.