Are clouds blocking our view of water on exoplanets?

Hot Jupiters, exoplanets around the same size as Jupiter that orbit very closely to their stars, often have cloud or haze layers in their atmospheres, which may prevent telescopes from detecting atmospheric water.

Clouds might be hindering our ability to detect atmospheric water on exoplanets, the far away worlds where we one day we might find signs of life.

Recent discoveries of so-called “hot Jupiters” – planets with masses similar to that of Jupiter, but which are much closer to their parent star than Jupiter is to the sun – show some with water and some without.

But a new study published in The Astrophysical Journal suggests that all might have atmospheric water, but we just can’t see it because of the clouds.

Hot Jupiters reach temperatures as high as 1,100 °C, which means any water would be in the form of vapour.

And when scientists at NASA’s Jet Propulsion Laboratory in Pasadena focused on a collection of hot Jupiters studied by NASA’s Hubble Space Telescope, they found that the atmospheres of about half were blocked by clouds or haze.

While scientists do not know yet what forms the clouds and haze, but they are unlikely to be made of water like Earthly clouds, as the planets are too hot.

“Clouds or haze seem to be on almost every planet we studied,” says Aishwarya Iyer of California State University, Northridge, who led the study.

“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.”

The aim of the study was to determine if the hot Jupiter category of planets shared any atmospheric properties.

To do so, scientists looked at a set of 19 hot Jupiters previously observed by Hubble Space Telescope. 

The telescope’s Wide Field Camera 3 had detected water vapour in the atmospheres of 10 of these planets, and no water on the other nine. But a closer look often revealed signs of it.

“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,” Iyer said.

The research could help with strategies for space observatories, such as NASA’s James Webb Space Telescope.

Exoplanets with thick cloud cover blocking the detection of water and other substances may be less desirable targets for more extensive study.

But the results could also help us discover how planets form, scientists say.

“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,” says co-author Robert Zellem.

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