Kavli Affiliate: Sara Seager
| First 5 Authors: Samuel Gill, Daniel Bayliss, Solène Ulmer-Moll, Peter J. Wheatley, Rafael Brahm
| Summary:
Discovering transiting exoplanets with relatively long orbital periods ($>$10
days) is crucial to facilitate the study of cool exoplanet atmospheres ($T_{rm
eq} < 700 K$) and to understand exoplanet formation and inward migration
further out than typical transiting exoplanets. In order to discover these
longer period transiting exoplanets, long-term photometric and radial velocity
campaigns are required. We report the discovery of TOI-2447 b ($=$ NGTS-29b), a
Saturn-mass transiting exoplanet orbiting a bright (T=10.0) Solar-type star
(T$_{rm eff}$=5730 K). TOI-2447 b was identified as a transiting exoplanet
candidate from a single transit event of 1.3% depth and 7.29 h duration in
$TESS$ Sector 31 and a prior transit event from 2017 in NGTS data. Four further
transit events were observed with NGTS photometry which revealed an orbital
period of P=69.34 days. The transit events establish a radius for TOI-2447 b of
$0.865 pm 0.010rm R_{rm J}$, while radial velocity measurements give a mass
of $0.386 pm 0.025 rm M_{rm J}$. The equilibrium temperature of the planet
is $414$ K, making it much cooler than the majority of $TESS$ planet
discoveries. We also detect a transit signal in NGTS data not caused by
TOI-2447 b, along with transit timing variations and evidence for a $sim$150
day signal in radial velocity measurements. It is likely that the system hosts
additional planets, but further photometry and radial velocity campaigns will
be needed to determine their parameters with confidence. TOI-2447 b/NGTS-29b
joins a small but growing population of cool giants that will provide crucial
insights into giant planet composition and formation mechanisms.
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