Paris: In a technical feat, astronomers said Wednesday they had detected water vapour in the atmosphere of a planet the size of Neptune orbiting a star some 130 light years from Earth.
It is the smallest and coldest exoplanet -- a world outside our Solar System -- for which these vital signs have been measured.
Water vapour, along with copious amounts of hydrogen, was found in the atmosphere of HAT-P-11b, which whizzes around its star in a 4.9-day, egg-shaped orbit.
HAT-P-11b is about four times the size of Earth and is among the smallest exoplanets discovered.
Atmospheres are telltales of a planet's formation and a guide to what is happening on its surface.
But until now, scientists had only been able to scrutinise the atmospheres of large exoplanets -- scorching-hot "gas giants" similar to, or even bigger than, our own Jupiter.
Neptune, the outermost acknowledged planet of our Solar System, is around a third of the size of Jupiter.
Astronomers use a method called transmission spectroscopy, with telescopes studying a planet's atmosphere as it passes in front of its host star -- relying on the fact that different gases absorb light at different wavelengths.
Four Neptune-sized planets have been studied before, but roiling clouds or a dusty haze skewed the view for spectroscopy.
HAT-P-11b, though, was luckily free of the problem.
Its atmosphere seems similar to those of the giant planets in our own Solar System -- mostly hydrogen with trace amounts of heavier atoms like oxygen in the form of water vapour, said Eliza Kempton at Grinnell College in Iowa in a comment on the study published in the journal Nature.
The study research was conducted using NASA's Hubble and Spitzer space telescopes.
The discovery that not all smaller exoplanets have clouds is good news, said Kempton.
It raises expectations for NASA's new James Webb space telescope to be launched in 2018 with a larger mirror than Hubble's.