TOI-1231 b: A Temperate, Neptune-Sized Planet Transiting the Nearby M3 Dwarf NLTT 24399
Creators: name: Burt, Jennifer A.
name: Dragomir, Diana
name: Mollière, Paul
name: Youngblood, Allison
name: Muñoz, Antonio García
name: McCann, John
name: Kreidberg, Laura
name: Huang, Chelsea X.
name: Collins, Karen A.
name: Eastman, Jason D.
name: Abe, Lyu
name: Almenara, Jose M.
name: Crossfield, Ian J. M.
name: Ziegler, Carl
name: Rodriguez, Joseph E.
name: Mamajek, Eric E.
name: Stassun, Keivan G.
name: Halverson, Samuel P.
name: Villanueva, Steven Jr.
name: Butler, R. Paul
name: Wang, Sharon Xuesong
name: Schwarz, Richard P.
name: Ricker, George R.
name: Vanderspek, Roland
name: Latham, David W.
name: Seager, S.
name: Winn, Joshua N.
name: Jenkins, Jon M.
name: Agabi, Abdelkrim
name: Bonfils, Xavier
name: Ciardi, David
name: Cointepas, Marion
name: Crane, Jeffrey D.
name: Crouzet, Nicolas
name: Dransfield, Georgina
name: Feng, Fabo
name: Furlan, Elise
name: Guillot, Tristan
name: Gupta, Arvind F.
name: Howell, Steve B.
name: Jensen, Eric L. N.
name: Law, Nicholas
name: Mann, Andrew W.
name: Marie-Sainte, Wenceslas
name: Matson, Rachel A.
name: Matthews, Elisabeth C.
name: Mékarnia, Djamel
name: Pepper, Joshua
name: Scott, Nic
name: Shectman, Stephen A.
name: Schlieder, Joshua E.
name: Schmider, François-Xavier
name: Stevens, Daniel J.
name: Teske, Johanna K.
name: Triaud, Amaury H. M. J.
name: Charbonneau, David
name: Berta-Thompson, Zachory K.
name: Burke, Christopher J.
name: Daylan, Tansu
name: Barclay, Thomas
name: Wohler, Bill
name: Brasseur, C. E.
Publisher: name: arXiv
Year: 2021
Subjects: subject: Earth and Planetary Astrophysics (astro-ph.EP)
subject: FOS: Physical sciences
subject: FOS: Physical sciences

description: 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 k\ms) 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.
description: 20 pages, 9 figures. Accepted for publication in The Astronomical Journal