Astronomers have detected water vapour in the atmosphere of a Neptune-sized planet that orbits a star far beyond our solar system.

Observations of the small planet revealed that its atmosphere was mostly hydrogen with around 25% made up from water vapour, reports The Guardian.

Up to now, researchers could not study the atmospheres of planets smaller than Jupiter because their skies were thick with clouds.

The planet, named HAT-P-11b, is about four times the diameter of Earth and its surface temperature reaches at least 600 degree Celsius. The planet does not have a rocky surface and is thought to be lifeless.

“Scientists from the University of Maryland used Hubble’s wide field camera to analyse light from HAT-P-11b’s host star through the planet’s atmosphere” mentions The Guardian. What they found was that light with a wavelength of 1.4 micrometres was absorbed, matching the absorption spectrum of water molecules.

“Although this planet is not classically habitable, it reveals to us that when we find Earth 2.0, we will be able to use this technique, transmission spectroscopy, to understand its atmosphere and determine the quality of life available on its shores,” said Jonathan Fraine, a graduate student and first author on the study.