Astronomers Can Now Detect Water on Exoplanets That Are Light Years Away

HAT P-11b is only four times bigger than Earth, and it’s decked out in water vapor.

Sep 24 2014, 5:20pm

Water is the magic ingredient for life as we know it, and detecting it on Earthlike exoplanets is the best strategy for rooting out habitable worlds. Well, researchers at the University of Maryland have just detected water vapor in the atmosphere of an exoplanet 124 light years away.

That's huge for a couple reasons. For one, it's always good news for those searching for extraterrestrial life when they find water on another planet (in this case, the exoplanet HAT P-11b). For two, we now have the means of detecting water vapor that's light years away.

The planet is four times times bigger than Earth and boasts about 26 times its mass, making it roughly the size of Neptune. That might seem big, but HAT P-11b is actually the smallest exoplanet astronomers have ever found to be hosting water vapor. In fact, it's the smallest world for which the atmospheric conditions have been discovered at all, making it a key step towards finding water on even smaller terrestrial exoplanets resembling Earth.

"The atmosphere [of HAT P-11b] is mostly molecular hydrogen, with an admixture of heavier molecules including water vapor," Drake Deming, co-author of a paper describing the discovery in Nature told me, adding that only water vapor was directly detected.

"The presence of molecular hydrogen is strongly inferred," he explained. "Without hydrogen, the atmosphere would be too 'compact' to detect anything, [because] hydrogen reduces the mean molecular weight and allows the atmosphere to puff upward to the point where it blocks enough starlight for a detection." 

The planet's atmosphere may also contain methane, but Deming's team needs more data to substantiate its presence.

In an accompanying article, astronomer Eliza Kempton of Grinnell College noted that the finding is "paving the way towards the search for water on smaller Earth-like exoplanets." 

This detailed snapshot of HAT P-11b's composition was captured as the exoplanet transited across its host star. The team imaged the solar system, located 124 light years away in the constellation Cygnus, using both the Hubble Space Telescope and the Spitzer Space Telescope, and compared it to data collected by the Kepler Space Telescope.


The Spitzer Space Telescope. Image: NASA/JPL-Caltech.

When exoplanets transit, they can appear to be magnified by starlight interacting with atmospheric elements. The apparent size change can be used to make a transmission spectrum of the planet's atmospheric makeup. 

"At wavelengths where water vapor does not absorb, the planet's atmosphere is more transparent," Deming said.

"But at the water vapor wavelength, the atmosphere blocks light. In transit the atmosphere is like a fuzzy ring around the planet," he continued. "At the water vapor wavelength, that fuzzy ring becomes opaque—making the planet look effectively larger." 

And that's how you peg a planet as a water world, even if water is not particularly abundant on it. Here's what that looks like (with a little extra explanation on how, exactly, it works):

Image: Nature

This natural magnification technique has been used by many astronomers to calculate the makeup of much larger exoplanets: Jupiter-sized and above. But Deming and his colleagues have proven that "hot Neptunes" like HAT P-11b can also spill their secrets during transits.

That's useful not only for rooting out candidates for habitable worlds—which at a roasting 1,120 degrees Fahrenheit, HAT P-11b is most assuredly not—but also for understanding how the formation of our own solar system stacks up compared against that of other systems.

"Our result is broadly consistent with the core-accretion model of planetary formation," said Deming. "That model predicts that less massive planets will gravitationally attract and hold less hydrogen gas."

"Although HAT-P-11b's atmosphere is mostly hydrogen, the dilution of water by molecular hydrogen is not as pronounced as it would be for a larger and more massive planet," he continued. "To make further progress in understanding planet formation, we will have to observe more planets, and less massive ones especially."