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Clear skies and water vapour found on an exoplanet

Jenny Winder, News Writer
Sep 25, 2014, 16:40 UTC

Sen—Data from three of NASA's space telescopes, Hubble, Spitzer and Kepler, have discovered clear skies and water vapour on a gaseous exoplanet that is about the size of Neptune, making it the smallest planet from which molecules of any kind have been detected. 

"This discovery is a significant milepost on the road to eventually analyzing the atmospheric composition of smaller, rocky planets more like Earth," said John Grunsfeld, assistant administrator of NASA's Science Mission Directorate. "Such achievements are only possible today with the combined capabilities of these unique and powerful observatories."

Finding clear skies on a Neptune-size planet is a good sign that smaller planets might have similarly good visibility. Clouds in a planet's atmosphere can block the view to underlying molecules that reveal information about the planet's composition and history.

The planet, HAT-P-11b, is categorized as an exo-Neptune, a Neptune-sized planet that orbits the star HAT-P-11. It is located 120 light-years away in the constellation Cygnus. This planet orbits closer to its star than does our Neptune to our Sun, making one lap roughly every five days. It is a warm world thought to have a rocky core and gaseous atmosphere. Not much else was known about the composition of the planet, or other exo-Neptunes like it, until now.

Larger Jupiter-like planets are easier to see because of their impressive girth and relatively inflated atmospheres. Researchers already have detected water vapour in the atmospheres of those planets. The handful of smaller planets observed previously had proved more difficult to probe, partly because they all appeared to be cloudy.

The new study used Hubble's Wide Field Camera 3 and a technique called transmission spectroscopy, in which a planet is observed as it crosses in front of its parent star. Starlight filters through the rim of the planet's atmosphere. If molecules like water vapour are present, they absorb some of the starlight, leaving distinct signatures in the light that reaches our telescopes.

Hubble was able to detect water vapour in HAT-P-11b. But the team had to show that cool starspots on the parent star were not the real sources of water vapour. 

Kepler had been observing one patch of sky for years, and HAT-P-11b happens to lie in the field. Those visible-light data were combined with Spitzer observations taken at infrared wavelengths. By comparing these observations, the astronomers figured out that the starspots were too hot to have steam. This discovery indicates the planet did not have clouds blocking the view, a hopeful sign that more cloudless planets can be located and analyzed in the future.

The results from all three telescopes demonstrate that HAT-P-11b is blanketed in water vapour, hydrogen gas and likely other unidentified molecules. Theorists will be drawing up new models to explain the planet's makeup and origins.

The astronomers hope to apply the same method to super-Earths, massive, rocky cousins to our home world with up to 10 times the mass. 

"The work we are doing now is important for future studies of super-Earths and even smaller planets, because we want to be able to pick out in advance the planets with clear atmospheres that will let us detect molecules," said study co-author Heather Knutson of the California Institute of Technology.

This video shows a zoom from a ground-based image of the region surrounding HAT-P-11, to a close up of the star taken with the NASA/ESA Hubble Space Telescope. Credit: HubbleESA