Classifying exoplanet atmospheres to open new field of study

Researchers have identified and classified populations of similar exoplanet atmospheres in order to improve our knowledge of planetary formation.

An international team of researchers examined data for 25 exoplanets and discovered a variety of links among the properties of the atmospheres, including the thermal profiles and chemical abundances in them. This marks the first time that exoplanet atmospheres have been studied as populations, rather than individually. These findings will contribute to establishing a generalised theory of planet formation, which will improve our understanding of all planets, including Earth.

Currently, there are approximately 3,000 confirmed exoplanets, which are planets orbiting stars other than the Sun. As they reside far away from Earth, it is difficult to examine them in detail. Thus, determining the characteristics of even one exoplanet has been a noteworthy accomplishment.

Analysing Hot Jupiters to classify exoplanet atmospheres

In this research, astronomers utilised archival data for 25 Hot Jupiters, which are gas giant planets that orbit close to their host stars, to investigate exoplanet atmospheres. The data included 600 hours of observations from the Hubble Space Telescope and more than 400 hours of observations from the Spitzer Space Telescope.

One of the characteristics investigated by scientists was the presence or absence of a ‘thermal inversion.’ Planetary atmospheres trap heat, and as a result, the temperature increases as you probe deeper into the atmosphere. However, some planets exhibit a thermal inversion where an upper layer of the atmosphere is warmer than the layer beneath it.

Thermal inversion

On Earth, the presence of ozone in the atmosphere causes a thermal inversion. The team discovered that almost all of the hot Jupiters with a thermal inversion also displayed evidence for hydrogen anion (H) and metallic species, such as titanium oxide (TiO), vanadium oxide (VO), or iron hydride (FeH).

Conversely, exoplanets without these chemicals almost never had thermal inversions. It is difficult to draw conclusions based on correlation alone. However, since these metallic species are efficient absorbers of stellar light, one theory holds that when these chemicals are present in the upper atmosphere, they absorb light from the host star and cause the temperature to increase.

Gas giant formation

“The theory of gas giant formation proposed by my students, and I predicted diversity in the composition of Hot Jupiter atmospheres and helped to motivate this systematic survey of atmospheric characteristics,” explained Masahiro Ikoma at the National Astronomical Observatory of Japan, a co-investigator in this study.

This new study, identifying populations of similar exoplanet atmospheres, will help in refining the theoretical models, and bring us closer to a comprehensive understanding of planetary formation. In the coming decade, new data from next-generation space telescopes, including the James Webb Space Telescope, Twinkle, and Ariel, will provide data for thousands of exoplanets, both enabling and necessitating new categories for classifying exoplanets beyond the methods explored in this research.

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