WASP-31b is a low-density (puffy) "hot Jupiter" exoplanet with a mass about 0.48 times that of Jupiter and a radius about 1.55 times that of Jupiter.[2][1] The planetary atmosphere has indeed the largest scale height, equal to 1150km, among exoplanets with measurable atmospheres as at 2021.[5]
The exoplanet orbits WASP-31, its host star, every 3.4 days.[2]
Comparison of "hot Jupiter" exoplanets, including WASP-31b (top row; 3rd from left) (artist concept)
In 2012, it was found from the Rossiter–McLaughlin effect that WASP-31b is orbiting the parent star in a prograde direction, with the WASP-31 star rotational axis inclined to the planetary orbit by 2.8±3.1 degrees.[6] The spectroscopic study in 2014 revealed that WASP-31b has a dense cloud deck overlaid by a hazy atmosphere.[7] WASP-31b was also reported to have significant amounts of potassium in its upper atmosphere, but the detection of potassium was refuted in 2015.[8] The potassium detection discrepancy was resolved in 2020 with the improved cloud deck model,[9] with the best fit being a very small amount of water over clouds and no potassium at all.[10]
Reanalysis of planetary spectroscopic data in 2020 has revealed the presence of chromium monohydride besides water.[11]