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

Kavli Affiliate: Christopher J. Burke

| First 5 Authors: Jennifer A. Burt, Diana Dragomir, Paul Mollière, Allison Youngblood, Antonio García Muñoz

| Summary:

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$^{+0.16}_{-0.15}$ R$_{oplus}$, and an
orbital period of 24.246 days. Radial velocity measurements obtained with the
Planet Finder Spectrograph on the Magellan Clay telescope confirm the existence
of the planet and lead to a mass measurement of 15.5$pm$3.3 M$_{oplus}$. With
an equilibrium temperature of just 330K TOI-1231 b is one of the coolest small
planets accessible for atmospheric studies thus far, and its host star’s bright
NIR brightness (J=8.88, K$_{s}$=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 kms) 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.

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