A team of astronomers has analysed the Hubble Space Telescope’s observations of over 25 hot Jupiters to answer five questions that are essential for your understanding of exoplanet atmospheres. Hot Jupiters refers to a class of gaseous exoplanets that are physically similar to Jupiter but are really close to their stars, giving them really high surface temperatures.
Until recently, the field of exoplanet science has long focused on just the detection and characterisation of exoplanets. This new study led by researchers at University College London (UCL) used a large amount of archival data to analyse the atmospheres of 25 exoplanets.
“Our paper marks a turning point for the field: we are now moving from the characterisation of individual exoplanet atmospheres to the characterisation of atmospheric populations,” said Billy Edwards of the UCL, in a press statement.
The team reanalysed a large amount of archival data consisting of 600 hours of Hubble observations and 400 hours of observations from the Spitzer Space Telescope. This data contained eclipses for all 25 exoplanets and transits for 17 of them. An eclipse is when an exoplanet passes behind its star and transits are when a planet passes in front of its star.
“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 Quentin Changeat, lead author of the study, in a press statement.
One of the key discoveries of the study was when the team found that the presence of metal oxides and hydrides in the hottest exoplanet atmospheres was correlated with the atmospheres being thermally inverted. A thermally inverted atmosphere refers to one where it gets hotter the higher you go from the surface of the planet; the exact opposite of how it is on earth.
The team found that almost all exoplanets with a thermally inverted atmosphere were extremely hot (temperatures over 2000 Kelvins) and that metallic oxides like titanium oxide, vanadium oxide and iron hydride are stable in the atmosphere.
According to the European Space Agency, it is challenging to draw inferences from such results because correlation does not necessarily equal causation. But the team was able to propose a fairly compelling argument for why the presence of these compounds could lead to thermal inversion.
These metallic compounds are excellent at absorbing stellar light. The researchers proposed that exoplanets hot enough to sustain these species tend to be thermally inverted because they can absorb so much stellar light that their upper atmospheres heat up even more.