WASP-80b has a dayside within the T-dwarf range
Amaury H.M.J. Triaud, Michael Gillon, David Ehrenreich, Enrique, Herrero, Monika Lendl, David R. Anderson, Andrew Collier Cameron, Laetitia, Delrez, Brice-Olivier Demory, Coel Hellier, Keving Heng, Emmanuel Jehin,, Pierre F.L. Maxted, Don Pollacco, Didier Queloz, Ignasi Ribas

TL;DR
This study characterizes WASP-80b, a warm exoplanet with a dayside spectrum similar to T-dwarfs, revealing insights into its atmospheric properties and potential spectral transition akin to brown dwarfs.
Contribution
It provides the first detection of WASP-80b's dayside flux in infrared, compares its spectrum to T-dwarfs, and reanalyzes its orbital alignment and stellar rotation, offering new understanding of exo-atmospheric transitions.
Findings
WASP-80b's dayside flux detected in Spitzer channels.
Its spectrum resembles a T4 dwarf but is cooler.
Orbital alignment is confirmed with slower stellar rotation.
Abstract
WASP-80b is a missing link in the study of exo-atmospheres. It falls between the warm Neptunes and the hot Jupiters and is amenable for characterisation, thanks to its host star's properties. We observed the planet through transit and during occultation with Warm Spitzer. Combining our mid-infrared transits with optical time series, we find that the planet presents a transmission spectrum indistinguishable from a horizontal line. In emission, WASP-80b is the intrinsically faintest planet whose dayside flux has been detected in both the 3.6 and 4.5 m Spitzer channels. The depths of the occultations reveal that WASP-80b is as bright and as red as a T4 dwarf, but that its temperature is cooler. If planets go through the equivalent of an L-T transition, our results would imply this happens at cooler temperatures than for brown dwarfs. Placing WASP-80b's dayside into a colour-magnitude…
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