In a study published on April 25, 2022 in the Astrophysical Journal Supplement Series, astronomers analyzed archival spectroscopic and photometric data of 25 hot Jupiters, obtained with the NASA/ESA Hubble Space Telescope and NASA’s Spitzer Space Telescopes.
Hot Jupiters are gas giants that orbit close to their star, typically in less than 10 days.
While there are none in our Solar System, they are a commonly observed type of exoplanet.
“Our paper marks a turning point for the field: we are now moving from the characterization of individual exoplanet atmospheres to the characterization of atmospheric populations,” said Dr. Billy Edwards, an astronomer in the Department of Physics and Astronomy at University College London, CNRS, and the Université Paris-Saclay.
In the research, Dr. Edwards and colleagues used data from 600 hours of observations from Hubble, and 400 hours from Spitzer.
They combined two techniques: studying information from transits (where the planet passes in front of its star) and eclipses (when the planet passes behind its star).
They were able to determine trends and resolve questions about hot Jupiters that smaller studies have been unable to conclusively answer over many years.
Among the findings, the authors discovered that the night and day sides of hot Jupiters are very different, with temperatures plunging by hundreds of degrees centigrade from day to night (on average the researchers found a 1,000 K difference).
They also found that many hot Jupiters had thermally inverted atmospheres, also known as stratospheres — that is, their upper atmospheres has temperatures that increase with altitude.
This appeared to be caused by the presence of metallic elements (titanium oxide, vanadium oxide and iron hydride) which absorbed the star’s light and thus heated up the atmosphere. This is a similar phenomenon as occurs on Earth via the ozone layer.
The researchers also confirmed that molecules were breaking apart in the hottest planets.
They found some planets had less water than expected, suggesting they formed in a different way to the more water-abundant planets.
They also detected more metals than predicted by models, meaning that those planets likely formed differently from what was previously thought.
“Many issues such as the origins of 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,” said Dr. Quentin Changeat, an astronomer in the Department of Physics and Astronomy at University College London.
“Large exoplanet population studies, such as the one we present here, aim at understanding those general processes.”
Q. Changeat et al. 2022. Five Key Exoplanet Questions Answered via the Analysis of 25 Hot-Jupiter Atmospheres in Eclipse. ApJS 260, 3; doi: 10.3847/1538-4365/ac5cc2
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