TOI-1231 b: a temperate, Neptune-sized planet transiting the nearby M3 dwarf NLTT 24399

Research output: Contribution to journalArticlepeer-review


  • Jennifer A. Burt
  • Diana Dragomir
  • Paul Mollière
  • Allison Youngblood
  • Antonio García Muñoz
  • John McCann
  • Laura Kreidberg
  • Chelsea X. Huang
  • Karen A. Collins
  • Jason D. Eastman
  • Lyu Abe
  • Jose M. Almenara
  • Ian J. M. Crossfield
  • Carl Ziegler
  • Joseph E. Rodriguez
  • Eric E. Mamajek
  • Keivan G. Stassun
  • Samuel P. Halverson
  • Steven Jr Villanueva
  • R. Paul Butler
  • Sharon Xuesong Wang
  • Richard P. Schwarz
  • George R. Ricker
  • Roland Vanderspek
  • David W. Latham
  • S. Seager
  • Joshua N. Winn
  • Jon M. Jenkins
  • Abdelkrim Agabi
  • Xavier Bonfils
  • David Ciardi
  • Marion Cointepas
  • Jeffrey D. Crane
  • Nicolas Crouzet
  • Fabo Feng
  • Elise Furlan
  • Tristan Guillot
  • Arvind F. Gupta
  • Steve B. Howell
  • Eric L. N. Jensen
  • Nicholas Law
  • Andrew W. Mann
  • Wenceslas Marie-Sainte
  • Rachel A. Matson
  • Elisabeth C. Matthews
  • Djamel Mékarnia
  • Joshua Pepper
  • Nic Scott
  • Stephen A. Shectman
  • Joshua E. Schlieder
  • François-Xavier Schmider
  • Daniel J. Stevens
  • Johanna K. Teske
  • David Charbonneau
  • Zachory K. Berta-Thompson
  • Christopher J. Burke
  • Tansu Daylan
  • Thomas Barclay
  • Bill Wohler
  • C. E. Brasseur

Colleges, School and Institutes


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+0.16 R 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 ± 3.3 M. 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, Ks = 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 Lyα stellar emission away from the geocoronal and interstellar medium absorption features.


Original languageEnglish
Article number87
Number of pages15
JournalThe Astronomical Journal
Issue number3
Early online date3 Aug 2021
Publication statusPublished - Sep 2021


  • astro-ph.EP, Exoplanet astronomy, Transit photometry, Radial velocity, Mini Neptunes