Kavli Affiliate: David Charbonneau
| First 5 Authors: Mallory Harris, Diana Dragomir, Ismael Mireles, Karen A. Collins, Solène Ulmer-Moll
| Summary:
We report the discovery of two TESS sub-Neptunes orbiting the early M dwarf
TOI-904 (TIC 261257684). Both exoplanets, TOI-904 b and c, were initially
observed in TESS Sector 12 with twin sizes of ${2.426}_{-0.157}^{+0.163}$ and
${2.167}_{-0.118}^{+0.130}$ R$_oplus$, respectively. Through observations in
five additional sectors in the TESS primary mission and the first and second
extended missions, the orbital periods of the planets were measured to be
10.887$pm$0.001 and 83.999$pm$0.001 days, respectively. Reconnaissance radial
velocity measurements (taken with EULER/CORALIE and SMARTS/CHIRON) and
high-resolution speckle imaging with adaptive optics (obtained from SOAR/HRCAM
and Gemini South/ZORRO) show no evidence of an eclipsing binary or a nearby
companion, which, together with the low false-positive probabilities calculated
with the statistical validation software TRICERATOPS, establishes the planetary
nature of these candidates. The outer planet, TOI-904 c, is the longest-period
M dwarf exoplanet found by TESS, with an estimated equilibrium temperature of
217 K. As the three other validated planets with comparable host stars and
orbital periods were observed by Kepler around much dimmer stars (J mag > 12),
TOI-904 c, orbiting a brighter star (J mag = 9.6), is the coldest M dwarf
planet easily accessible for atmospheric follow-up. Future mass measurements
and transmission spectroscopy of the similar-sized planets in this system could
determine whether they are also similar in density and composition, suggesting
a common formation pathway, or whether they have distinct origins.
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