Clouds prevent water detection on hot Jupiters

The study of exoplanets is key to understanding how our own Solar System formed. But a new study suggests that observations of a massive type of exoplanet called hot Jupiters have been hindered by cloudy and hazy atmospheres.

Artist’s impression of a hot Jupiter exoplanet, indicating how clouds could be preventing the detection of water.
Image credit: NASA/JPL-Caltech

Clouds and haze layers could be preventing the detection of water in the atmospheres of hot Jupiters, according to a new study by NASA’s Jet Propulsion Laboratory.

Hot Jupiters are a type of exoplanet so-called because they are the size of Jupiter, but much closer to their host star than Jupiter is to the Sun. This produces massive, scorching planets that can reach temperatures of up to 1,100°C.

Naturally, hot Jupiters cannot hold water on their surface, but it can exist in a vapour form in the planet’s atmosphere. Astronomers have found many hot Jupiters with water in their atmospheres, while others appear to have none. 

"The motivation of our study was to see what these planets would be like if they were grouped together, and to see whether they share any atmospheric properties," says Aishwarya Iyer, a JPL intern who led the study.

The team studied 19 hot Jupiters that had been previously observed by the Hubble Space Telescope, the results of which had shown that there was water vapour in 10, but none in the other nine. However, the studies of the planets had been conducted separately, meaning there was no standardised analysis of all 19.

For this study, the team standardised the data of all 19 exoplanets to create an average overall light spectrum for the group. This was then compared to models of planets with cloud-free atmospheres and those with various thicknesses of cloud. They found that, for almost every planet studied, haze or clouds were blocking on average half the atmosphere.

"Clouds or haze seem to be on almost every planet we studied," Iyer says. "You have to be careful to take clouds or haze into account, or else you could underestimate the amount of water in an exoplanet's atmosphere by a factor of two. In some of these planets, you can see water peeking its head up above the clouds or haze, and there could still be more water below.”

The clouds are unlikely to be made of water, as the planets are too hot, yet their exact nature remains a mystery. Understanding the effect of clouds on the detection of water vapour has implications for understanding how planets form.

"Clouds or haze being on almost all these planets is pretty surprising," says Robert Zellem, co-author of the study. "Did these planets form in their current positions or migrate toward their host stars from farther out? Understanding the abundances of molecules such as water helps us answer those questions," Zellem says.


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