Astronomers at the Harvard Smithsonian Center for Astrophysics have confirmed the existence of a new class of planet - a waterworld in a solar system outside Earth's.
The exoplanet GJ 1214b, 40 light-years away, is a "Super Earth" - bigger than our planet, but smaller than gas giants such as Jupiter.
It has a thick, steamy atmosphere and observations using the Hubble telescope now seem to confirm that a large fraction of its mass is water.
The BBC reports the The planet was discovered in 2009 orbiting a comparatively cool red-dwarf star at a distance of two million km - meaning temperatures on GJ 1214b probably reach above 200C.
It is about 2.7 times the Earth's diameter.
The study has been accepted for publication by the Astrophysical Journal.