New research out this week has helped to clear up one of the biggest mysteries of planetary astronomy: why do so many large “hot Jupiter” gas giant planets have very little water in their atmospheres?


According to the new study using data from NASA’s Hubble and Spitzer Space Telescopes, it turns out the answer is that the water was actually there, just hiding beneath the clouds.

Water Hiding Below Clouds On Hot Jupiter Planets [Study]

The data from the hot Jupiter study were published in the December 14th issue of the academic journal Nature.

Finding water on hot Jupiters

Hot Jupiters are gas giant planets even bigger than our Jupiter orbiting their home stars closer than Mercury orbits the sun. This makes the hot Jupiter planets both sizzlingly hot and difficult to observe. The glare of starlight is a major issue for telescopes. Because of this problem, Hubble has only studied a few hot Jupiters previously. These studies have curiously found that these planets contained notably less water than would be predicted by planetary atmospheric models.

The international research team decided to approach the issue  by making the largest-ever spectroscopic catalog of exoplanet atmospheres. One key feature of the study is that the planets follow orbits oriented so the planet passes in front of their parent star relative to the Earth. During this “transit”, some starlight travels through the planet’s outer atmosphere. “The atmosphere leaves its unique fingerprint on the starlight, which we can study when the light reaches us,” co-author Hannah Wakeford, of NASA’s Goddard Space Flight Center notes.

By combining data from two different telescopes, the team was able to observe a broad spectrum of light including wavelengths ranging from the optical to infrared. Whether a planet in the sample was cloudy or clear was determined by the difference in planetary radius as measured between visible and infrared wavelengths. Of note, a cloudy planet will seem larger in visible light than at infrared wavelengths, which can travel farther into the atmosphere. Using this comparison permitted the researchers to ascertain a correlation between hazy/cloudy atmospheres and weak detection of water.

Keep in mind that relatively little is known about exoplanetary atmospheres today. The successor to the ground-breaking Hubble, the James Webb Space Telescope, will provide a valuable new infrared tool to  study of exoplanets and their atmospheres.

Statements from hot Jupiter study authors

“I’m really excited to finally see the data from this wide group of planets together, as this is the first time we’ve had sufficient wavelength coverage to compare multiple features from one planet to another,” commented David Sing of the University of Exeter, the lead author of the study. “We found the planetary atmospheres to be much more diverse than we expected.”

Co-author Jonathan Fortney of the University of California, Santa Cruz explains the significance of the results of the study: “Our results suggest it’s simply clouds hiding the water from prying eyes, and therefore rule out dry hot Jupiters. The alternative theory to this is that planets form in an environment deprived of water, but this would require us to completely rethink our current theories of how planets are born.”