Discovered in 2009, HAT-P-11b is a so-called exo-Neptune located in the constellation of Cygnus, about 122 light-years away.

The planet, also known as Kepler 3b, is four times the radius of Earth and about 26 times Earth’s mass.

It orbits very close to its host star, the 10th magnitude K-type star HAT-P-11, and therefore is incredibly hot, with temperatures around 605 degrees Celsius.

HAT-P-11b probably has a rocky core, wrapped in a thick, gaseous envelope of about 90 percent hydrogen.

Its atmosphere is cloudless at high altitude, but as the team led by Jonathan Fraine of the University of Maryland’s Department of Astronomy found, it contains the signature of water vapor.

Mr Fraine and his colleagues observed HAT P-11b using NASA’s Hubble Space Telescope and Spitzer Space Telescope between July 2011 and December 2012.

“We set out to look at the atmosphere of HAT-P-11b without knowing if its weather would be cloudy or not. 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,” explained Dr Nikku Madhusudhan of the University of Cambridge’s Institute of Astronomy, a team member and a co-author of the paper published in the journal Nature.

The astronomers then compared the data to observations by NASA’s Kepler Space Telescope.

The results from all three telescopes demonstrate that the exoplanet is blanketed in water vapor, hydrogen gas, and other yet-to-be-identified molecules.

So HAT-P-11b is not only the smallest exoplanet to have water vapor found in its atmosphere but is also the smallest exoplanet for which molecules of any kind have been directly detected using spectroscopy.

The scientists now plan to examine more exo-Neptunes, and hope to apply the same method to smaller super-Earths.