Hot Jupiters are giant gas exoplanets - 'alien' planets only found outside our own solar system. They have a short (10 day or less) orbital period around their parent stars.
Scientists, including from Queen Mary University of London, used observational data of 25 hot Jupiters – the largest amount of archival data ever examined in a single exoplanet atmosphere survey.
The research explains why the atmospheres of many hot Jupiters are thermally inverted, meaning their atmosphere gets hotter with altitude - and shows their temperatures can soar by a massive 1000 Kelvins (726 Celsius) from their night to day sides.
The scientists believe their research is important for understanding key processes that remain a mystery even in our own solar system.
The research, led by UCL and involving Queen Mary, has been published in The Astrophysical Journal Supplement Series. The team used more than 1000 hours of archival data from both the NASA/ESA Hubble and Spitzer Space Telescopes, employing high performance computers at the DiRAC HPC facility to analyse the data.
While previous studies have tended to focus on one or a few exoplanets at a time, this huge sample size means the researchers were able to identify some clear trends in their atmospheric properties and behaviours and answer previously unresolved questions.
Jack Skinner, PhD Researcher in the Astronomy Unit at Queen Mary University of London, said: “Using such a large amount of Hubble data has enabled many exoplanet atmospheres to be characterised in great detail – marking an important turning point from studying individual exoplanet atmospheres to studying large atmospheric populations. This is only the beginning as the next generation of space telescopes, specifically tailored to atmospheric characterisation, come online and provide us with a wealth of new observational data.”
The data contained eclipses for all 25 exoplanets, and transits for 17 of them. An eclipse occurs when an exoplanet passes behind its star as seen from Earth, and a transit occurs when a planet passes in front of its star. Data from both can provide crucial information about an exoplanet’s atmosphere.
The team found that all the planets in their sample that had a thermally inverted atmosphere were extremely hot, with temperatures over 2000 Kelvins (about 1727 Celsius or 3140 Fahrenheit). At this temperature the metal species such as titanium oxide, vanadium oxide and iron hydride are stable in an atmosphere.
It could be that these metallic species, which are efficient absorbers of visible light, absorb the light from their nearest star at the highest altitudes - causing their upper atmospheres to get hotter. The hot Jupiters in the sample with temperatures less than 2000 Kelvins almost never had thermally inverted atmospheres.
The team also discovered that temperatures of hot Jupiters plunge enormously from day to night (a 1000K difference on average), and that molecules were breaking apart in the hottest planets.
Other findings in the study may hold clues to how these exoplanets were formed. Some of them had less water than expected, while others had more metals than predicted - meaning these planets likely formed in a different way to what was previously thought.
The researchers believe large exoplanet surveys such as this can help us to understand the general processes of planet formation - solving unresolved questions about the evolution of our own solar system.
Jack Skinner said: “Large scale studies like this are key to addressing complex questions in planetary science. The recently launched James Webb Space Telescope and upcoming missions, such as Twinkle and Ariel, will allow many such large-scale population studies to be performed in detail never seen before.”
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