HAT-P-11b

The planet, designated HAT P-11b, orbits a sun in the constellation Cygnus some 124 light-years - about a quadrillion kilometres - from Earth. It is roughly four times the width of our home world about the size of Neptune.

A cloud-free atmosphere has allowed scientists to pick out signs of water vapour the smallest "exoplanet" ever to reveal its chemical composition.

"Astronomers have detected water vapour in the atmospheres of larger planets - planets that are closer in size to Jupiter. But you can imagine that eventually we want to be able to detect molecules in the atmospheres of even smaller planets.

Water has obvious implications for life, although HAT P-11b is too close to its star - and therefore too hot - to be habitable.