A global team of astronomers has examined the Hubble Space Telescope’s archival observations of 25 hot planets, enabling them to answer five unanswered questions about exoplanet atmospheres. Among other things, the researchers determined that the presence of metal oxides and hydrates in the hottest atmospheres of the outer planets must be related to thermal inversions.
The field of exoplanet science has long shifted its focus from discovery only to characterization, although characterization remains a major challenge. To date, most research on characterization has focused on modeling or studying one or more exoplanets. This new research study, led by researchers at University College London (UCL), uses the most archival data ever examined in a single atmosphere survey of an exoplanet to assess the atmospheres of 25 exoplanets. Most of the data comes from observations made with the NASA/ESA Hubble Space Telescope.
Lead author Quentin Changet explains: “Hubble has enabled the in-depth characterization of 25 exoplanets, and the amount of information we have learned about their chemistry and formation – thanks to a decade of intense observational campaigns – is extraordinary.”
The scientific team is trying to find answers to five open-ended questions about the exoplanet’s atmosphere – an ambitious goal they have achieved. Their questions explore what H- and some minerals can tell us about the chemistry and rotation of exoplanets, and about planet formation. They chose to investigate a variety of hot Jupiters, with the aim of identifying trends in population samples that might provide insight into exoplanet atmospheres in general.
To examine a sample of 25 exoplanets, the team reanalyzed a large amount of archival data, consisting of 600 hours of Hubble observations, plus more than 400 hours of observations from the Spitzer Space Telescope. Their data contains eclipses of all 25 exoplanets, and transits of 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. Eclipse and transit data can provide important information about the exoplanet’s atmosphere.
Study co-leader Billy Edwards of UCL and the United Nations Commission on Atomic Energy and Energy Alternatives (CEA), said: “Our paper represents a turning point in the field: We are now moving from characterizing the atmospheres of individual exoplanets to characterizing atmospheric populations.”
The large-scale survey paid off, as the team was able to identify several clear trends and correlations between the formation of the exoplanet’s atmosphere and the observed behavior. Some of their main findings relate to the presence or absence of thermal inversions in the envelopes of exoplanet samples. They found that almost all exoplanets with inverted atmospheres are thermally very hot, with temperatures over 2,000 K. Importantly, this is hot enough that the metal species TiO (titanium oxide), VO (vanadium oxide), and FeH (iron hydride) are stable. in the atmosphere. Among exoplanets that display thermal inversion, almost all planets are found to have H-, TiO, VO, or FeH in their atmospheres.
It is always difficult to draw conclusions from these findings because correlation does not always equal causation. Nevertheless, the team was able to put forward a compelling argument as to why the presence of H-, TiO, VO or FeH could cause a thermal reversal – that is, all of these metal species are very effective absorbers of starlight. The exoplanet’s atmospheres may be hot enough to sustain this type of heat reversal because they then absorb so much of the star’s light that their upper atmospheres heat up even more. In contrast, the team also found that cooler Jupiter (with temperatures below 2,000 K, and thus without H-, TiO, VO, or FeH in its atmosphere) never had a thermally inverted atmosphere.
An important aspect of the study was that the team was able to use a large exoplanet sample and a very large amount of data to determine trends, which could be used to predict behavior on other exoplanets. This is very useful, because it provides insight into how planets form, and also because it allows other astronomers to plan more effectively for future observations. On the other hand, if this paper studies an exoplanet in great detail, while this is the value, it is very difficult to predict trends from it. A better understanding of this group of exoplanets could also bring us closer to solving open mysteries about our solar system. As Changeat puts it: “Many issues such as the origin of water on Earth, the composition of the Moon, and the different evolutionary histories of Earth and Mars, remain unresolved even though we have the ability to get measurements on the spot. Studies of large populations of exoplanets, such as those we present here, are aimed at understanding these general processes.
News summary:
- Hubble Space Telescope observations are used to answer important questions about exoplanets
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