Astronomers Discover Water Vapor on Small, Neptune-Sized Exoplanet with Clear Skies

Astronomers have officially spotted the smallest exoplanet to have ever been found with water vapor. Using the NASA Hubble Space Telescope, the Kepler Space Telescope and the Spitzer Space Telescope, scientists have found that the planet, HAT-P-11b, has clear skies and an abundance of steamy water vapor.

"When astronomers go observing at night with telescopes, they say 'clear skies' to mean good luck," said Jonathan Fraine, lead author of the new study, in a news release. "In this case, we found clear skies on a distant planet. That's lucky for us because it means clouds didn't block our view of water molecules.

HAT-P-11b is what is known as an exo-Neptune, which is a Neptune-sized planet that orbits a star outside our solar system. It's located about 120 light-years away in the constellation of Cygnus. Unlike Neptune, though, this planet is relatively close to its star and makes one lap every five days. A rocky, warm world, it's thought to have a mantle made of fluid and ice, and a thick gaseous atmosphere.

"We set out to look at the atmosphere of HAT-P-11b without knowing if its weather would be cloudy or not," said Nikku Madhusudhan, one of the researchers. "By using transmission spectroscopy, we could use Hubble to detect water vapor in the planet. This told us that the planet didn't have thick clouds blocking the view and is a very hopeful sign that we can find and analyze more cloudless, smaller planets in the future. It is groundbreaking!"

The small planet is blanketed in water vapor, hydrogen gas and other yet-to-be-identified molecules. Currently, researchers are working on models in order to explain the planet's makeup and origins. In the future, they hope to examine more exo-Neptunes and apply similar methods of examination to smaller super-Earths.

"We are working our way down the line, from hot Jupiters to exo-Neptunes," said Drake Deming, co-author of the new study. "We want to expand our knowledge to a diverse range of exoplanets."

The findings are published in the journal Nature.