For the first time ever, astronomers managed to discover evidence of water in the atmosphere of a relatively small exoplanet. Named HAT-P-11b, the planet is located 120 light years from Earth in the constellation of Cygnus. The planet is very different from ours though, with four times Earth's diameter it's the size of Earth and the planet orbits so close to its star that surface temperatures are estimated to reach over 600°C. Like Neptune, the planet is a ball of gas and observations suggest it has an atmosphere made up of mostly hydrogen with around 25 percent water vapour.
Until now, researchers have been frustrated in their efforts to study the atmospheres of planets much smaller than Jupiter because their skies were thick with clouds. The problem was so persistent that astronomers had begun to think that all warm, small planets formed with substantial cloud cover.
But writing in the journal Nature, scientists in the US describe how they found a Neptune-sized planet with cloud-free skies, enabling them to make detailed measurements of a small planet’s atmosphere for the first time.