In a study published in this week's Nature, researchers report evidence that HAT P-11b's atmosphere contains water vapor. "We're talking about water at over 1,000 degrees, so this isn't a case for life on the planet," said Jonathan Fraine, a PhD candidate in astronomy at the University of Maryland and first author of the study. "This isn't a liquid ocean or breathable air. In fact, the planet is a gaseous object — there's no surface that liquid water could sit on."
But generally speaking, water is something that astronomers look for when they're trying to find life in space — so it's important that they know how to find it. When a planet travels in front of its star, it absorbs some of the light — and looks bigger. Astronomers calculate the changes in a planet's appeared size to see how much of the star's radiation is being absorbed, which can in turn be used to determine which molecules in the planet's atmosphere are blocking the light.
Astronomers have had better luck doing that with much larger planets (ones like our Jupiter), but smaller planets like HAT P-11b (which is about the same size as Neptune) have proved elusive.
Other objects of this size have all had clouds high in their atmosphere, which block out most of the light. "We can look at those and find some cool information, but not what we're looking for," Fraine said. "This is the first time we've seen a clear atmosphere on a Neptune-sized planet."
In fact, Neptune itself has too much cloud cover to observe with a space telescope. Because it's nearby, scientists were able to send the space probe Voyager 2 to take a peek for us. But outside of our solar system, astronomers are woefully dependent on a clear cosmic forecast.
"We need to find, like, 20 more of these [planets] before we're happy," Fraine said. Upcoming programs plan to use the Hubble Space Telescope to investigate about 50 similar exoplanets. "The great thing is that now we know that clouds aren't blocking all of the light on every single one of these planets," Fraine said.
Astronomers like Fraine and his colleagues hope that understanding exoplanet atmospheres will help us understand how planets form. "We want to figure out how unique or normal we are, as a planet," Fraine said. And as scientists are able to puzzle out smaller and smaller exoplanets, we'll get closer and closer to ones like our own.
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