When it comes to planetary science, astronomers are always looking for water. Even in extreme worlds, finding water tells us a lot about what’s going on. In fact, water has been detected on several "hot Jupiters", puffy giants orbiting very close to their stars, but not all of them.

Scientists at NASA's Jet Propulsion Laboratory wanted to understand why this was the case, and also see what the different types of hot Jupiters have in common. Interestingly, they found that clouds or haze might be stopping water from being detected on worlds that seem otherwise relatively dry.

"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," said Aishwarya Iyer, lead author and master's degree candidate at California State University, Northridge, in a statement.

In the study, published by the Astrophysical Journal, the researchers used NASA’s Hubble Space Telescope to observe 19 hot Jupiters. They profiled the light coming from the planets, and then compared those readings to a model of cloud-free atmospheres as well as models with different types of clouds.

"Clouds or haze seem to be on almost every planet we studied," said Iyer. "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."

Hot Jupiters can reach a temperature of over 1,100°C (2,000°F), so the clouds are likely not made of water themselves. The researchers determined that on average half of a planet’s atmosphere is blocked by either haze or clouds.

Although a previous study suggested that clouds were hiding the water on hot Jupiters, this new research is the first one to quantify how much an atmosphere is shielded. Understanding molecules and the atmosphere is fundamental to finding out the history of these planets.

"Did these planets form in their current positions or migrate toward their host stars from farther out?" said Robert Zellem, co-author of the study, in the statement. "Understanding the abundances of molecules such as water helps us answer those questions."

This work could be very useful when the James Webb Space Telescope comes online in 2018. One of the science goals of the new space observatory is to study exoplanets directly, so targets with fewer clouds could get preferential treatment.