Where’s the water? Telescopes team up to solve hot Jupiter mystery


As astronomers discover more and more gas giants around other stars, they’ve increasingly wondered: Where’s the water? These distant worlds, known as hot Jupiters, have appeared to be strangely dry – a mystery that seemed to contradict our ideas of planet formation.

But now, scientists using the Hubble and Spitzer space telescopes think they may have solved the mystery: The water has been blocked by thick clouds in these gassy planets’ atmospheres.

The findings, published by the journal Nature, could shed new light on how planets form and evolve, and could help scientists better understand these distant worlds’ atmospheres.


While gas giants might be relatively easy to spot due to their size, their interiors remain largely shrouded in layers of mystery. We don’t even know all that much about our own Jupiter, said study coauthor Jonathan Fortney, a planetary astrophysicist at UC Santa Cruz. The last time NASA sent a probe plunging into the planet’s thick atmosphere was the Galileo 20 years ago, he pointed out.

Researchers have long assumed that gas giant planets would have plenty of water within them – after all, oxygen, after hydrogen and helium, is the most abundant element in the universe. And at planetary temperatures, much of that oxygen is locked in water molecules – so gas giants, forming out of the disc of debris around a newborn star, should have incorporated plenty of the stuff into their bodies.

But for a planet like Jupiter, that’s hard to tell, because the water is actually hidden deep within the planet’s many-layered atmosphere. (While the Galileo probe descended deep through the atmosphere, sending back some water data, it succumbed to the high temperatures and pressures before it could get a full reading, Fortney said.)

But hot Jupiters were thought to skirt this problem – their atmospheres, heated by the sun, are very well mixed, so the water vapor would be distributed throughout the atmosphere, not hidden below. And so, ironically, it should be easier to detect water in these distant gas giants than in our own.

“One of the promises of hot Jupiters is that … lots of different molecules should be easily visible in their atmospheres,” Fortney said. “So we’d like to be able to just look at the planet, take a spectrum of the atmosphere and instantly tell how much water vapor that is.”

Here’s the thing, though: Scientists have taken the spectra of a few such planets, and the results have had many unexpected dry spots.


“It was kind of all over the board,” Fortney said. “Some planets looked like they had the expected amount of water and some looked really depleted in water, maybe by a factor of 100. And that was really confusing.”

Was the water truly missing? If so, it could force scientists to reevaluate a few ideas about planetary formation. But perhaps the water was simply hiding out of sight.

To determine if this was the case, the researchers surveyed 10 different hot Jupiters in visible and near-infrared wavelengths with Hubble and longer infrared wavelengths using Spitzer. They found that some planets looked bigger when they were observed in visible light and yet smaller when seen in infrared wavelengths. It seemed that the visible wavelengths were bouncing off of the planets’ atmospheres, blocked from penetrating further by thick clouds of rock dust, while the longer infrared wavelengths could penetrate deeper into the atmosphere.

This discrepancy, the researchers concluded, must be caused by clouds. The wider the difference between visible and infrared, the more clouds there were. In contrast, those hot Jupiters whose visible and infrared profiles largely matched would have clear, cloudless skies.

Those with thicker cloud layers were also the ones that appeared to be drier, while the clearer-skied planets showed plenty of water – lending further support to the idea that the clouds were simply obscuring our view of an atmosphere that was actually plenty wet.

“I think it’s really interesting that we’re able to make such a clean story,” Fortney said. “A lot of the time — most of the time — planets are messy.”

The next steps, the scientist said, would be to use the data set to measure with greater precision the abundance of water in these planets’ atmospheres, and then measure the oxygen abundance in the parent stars, to start determining whether there’s a direct connection between the chemical makeup of a star and its planets. (People think there must be, given that planets form essentially out of the same stuff as their parent star, but more data is needed to establish and define that relationship.)

If that link is made, it could help planet hunters looking for life-friendly worlds to narrow down the search by first looking for stars with a relative abundance of the right chemical ingredients.

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