Astronomers answer key questions about 'hot Jupiters'

A large-scale analysis of archival Hubble observations of 25 hot Jupiters has enabled astronomers to answer key questions about the atmospheres of exoplanets - planets beyond our solar system.

Hot Jupiters are an informal class of exoplanets with short-period orbits and large, inflated gassy atmospheres. Although they are easy to detect, there is no hot Jupiter within our Solar System

Led by researchers based at University College London (UCL), the team used the largest amount of archival data ever examined in a single exoplanet atmosphere survey to analyse the atmospheres of 25 exoplanets. The archival data consisted of 600 hours of Hubble observations and more than 400 hours of observations from NASA's Spitzer Space Telescope

"Hubble enabled the in-depth characterisation of 25 exoplanets, and the amount of information we learnt about their chemistry and formation — thanks to a decade of intense observing campaigns - is incredible," said Quentin Changeat, the lead author of the study.

Below are the key findings:

• Almost all the exoplanets with a thermally inverted atmosphere were extremely hot, with temperatures over 2000 Kelvins.
• Of the exoplanets displaying thermal inversions, almost all of them were found to have H– (negative hydrogen ion), TiO (titanium oxide), VO (vanadium oxide) and FeH (iron hydride) in their atmospheres.
• Conversely, colder hot Jupiters - with temperatures below 2000 Kelvins, and thus without H–, TiO, VO or FeH in their atmospheres - almost never had thermally inverted atmospheres.

The team has proposed a compelling argument for why the presence of all these metallic species might lead to a thermal inversion. It might be that exoplanet atmospheres hot enough to sustain these metallic species tend to be thermally inverted because they then absorb so much stellar light that their upper atmospheres heat up even more.

"As Changeat says: "Many issues such as the origins of the water on Earth, the formation of the Moon, and the different evolutionary histories of Earth and Mars, are still unsolved despite our ability to obtain in-situ measurements. Large exoplanet population studies, such as the one we present here, aim at understanding those general processes," said Changeat.

The findings have been published in the Astrophysical Journal Supplement Series.

1/ Archival Hubble observations of 25 hot Jupiters have been analysed by an international team of astronomers, enabling them to answer five open questions important to our understanding of exoplanet atmospheres.Full story: 🔗 https://t.co/QeigbX3xTfDetails 👇 pic.twitter.com/9AEgsU848a

— HUBBLE (@HUBBLE_space) April 25, 2022