Kavli Affiliate: Avi Shporer
| First 5 Authors: Madison G. Scott, Amaury H. M. J. Triaud, Khalid Barkaoui, Daniel Sebastian, Adam J. Burgasser
| Summary:
Growing numbers of exoplanet detections continue to reveal the diverse nature
of planetary systems. Planet formation around late-type M dwarfs is of
particular interest. These systems provide practical laboratories to measure
exoplanet occurrence rates for M dwarfs, thus testing how the outcomes of
planet formation scale with host mass, and how they compare to Sun-like stars.
Here, we report the discovery of TOI-6478b, a cold ($T_{text{eq}}=204,$K)
Neptune-like planet orbiting an M5 star
($R_star=0.234pm0.012,text{R}_odot$,
$M_star=0.230pm0.007,text{M}_odot$, $T_{text{eff}}=3230pm75,$K) which
is a member of the Milky Way’s thick disc. We measure a planet radius of
$R_b=4.6pm0.24,text{R}_oplus$ on a $P_b=34.005019pm0.000025,$d orbit.
Using radial velocities, we calculate an upper mass limit of
$M_bleq9.9,text{M}_oplus$ ($M_bleq0.6,text{M}_{text{Nep}})$, with
$3,sigma$ confidence. TOI-6478b is a milestone planet in the study of cold,
Neptune-like worlds. Thanks to its large atmospheric scale height, it is
amenable to atmospheric characterisation with facilities such as JWST, and will
provide an excellent probe of atmospheric chemistry in this cold regime. It is
one of very few transiting exoplanets that orbit beyond their system’s ice-line
whose atmospheric chemical composition can be measured. Based on our current
understanding of this planet, we estimate TOI-6478b’s spectroscopic features
(in transmission) can be $sim2.5times$ as high as the widely studied planet
K2-18b.
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