Astronomers have found evidence of water vapor in the atmosphere of a Neptune-size planet 122 light-years from Earth—the first time water has been identified on a planet that small, despite several attempts.

The discovery offers new hope in the search for planets that could sustain life, astronomers report Wednesday in the journal Nature.

"It's a fantastic observation," says Knicole Colon, an astronomer at Lehigh University in Bethlehem, Pennsylvania, and a National Geographic Young Explorer, who was not involved in the research. "It opens up a whole new regime in our attempt to understand how exoplanets form and what they're made of."

The detection of water will prove crucial in the search for life in the universe, since water is considered one of the essential ingredients for life as we know it. It's highly unlikely that this world, known as HAT-P-11b, could support life. Located about 122 light-years away in the constellation Cygnus, the planet is about five times as big as Earth and probably has a deep gaseous atmosphere like Neptune. (See "The Hunt for Life Beyond Earth.")

Beyond that, HAT-P-11b orbits its parent star at a distance of less than five million miles (eight million kilometers), compared with Earth's 93-million-mile distance (150 million kilometers) from the sun. Its "year" is less than five days long, and its surface temperature is above 1,100°F (600°C).

Nevertheless, says Colon, detecting water even on this hellish world is important. "Eventually," she says, "we'll be able to study smaller, Earthlike planets, but if we can pin down the properties of larger planets and perfect our observing techniques, we'll be ready."

Peering Through the Clouds

The discovery provides a much-needed sigh of relief for astronomers. "For the past two or three years, we've felt a growing concern that our efforts to study the composition of exoplanet atmospheres might be stymied," says MIT astronomer Zachory Berta-Thompson, who was not involved in the new research.

Astronomers already had a way to look for water vapor on exoplanets: They watch as a planet moves in front of its star, then carefully monitor the starlight that streams through the planet's atmosphere. In principle, atoms and molecules, including H2O, should alter the starlight, imprinting a telltale signature that can be read by powerful telescopes.

This has been done successfully for larger, Jupiter-size exoplanets. But until now, said Jonathan Fraine of the University of Maryland, College Park, lead author of the Nature report, "the light has always been blocked by haze or clouds when we tried to look at smaller planets." That's especially frustrating because planets are more numerous in smaller sizes, and because planets closer in size to Earth are considered more likely to be habitable—and maybe even inhabited.

Using the Hubble and Spitzer Space Telescopes in tandem, the new study finally did manage to pick up water's signature in the starlight passing through HAT-P-11b's atmosphere, simply because the planet has clear skies. If other exoplanets this size have similarly clear skies, there's no reason astronomers couldn't detect water or other molecules there as well. In theory, says Fraine, it should also work for even smaller planets.

One reason this planet may have clear skies while others of similar size are thickly clouded over, says Fraine, might have to do with temperature. Hot as it is, HAT-P-11b is cooler than the similar-size exoplanets whose atmospheres have been probed before. "Maybe it's like Earth and Venus, which have a similar chemical composition and size, but one ended up with an incredibly cloudy carbon-dioxide-rich atmosphere and the other relatively clear and nitrogen-rich," he says.

That could be the explanation, or it could be something else—but whatever it ultimately turns out to be, says Laura Kreidberg of the University of Chicago, "it speaks to the extraordinary diversity of exoplanets."

Astronomers will have many more planets to explore, with results still rolling in from NASA's Kepler planet-hunting mission and another huge haul that could come from the planned Transiting Exoplanet Survey Satellite (TESS) mission.