Artist’s impression of GJ 1214b passing in front of its host red dwarf star. This red dwarf is approximately one-fifth the size of our sun (David A. Aguilar/CfA).

A “super-Earth” has been discovered orbiting a red dwarf star on our cosmic doorstep, only 42 light years away. But here’s the best bit: it’s composed primarily of water, possibly in liquid form.

A “super-Earth” is an exoplanet (or “extra-solar planet”) with a mass of 5-10 Earths. Only one other super-Earth has been confirmed (and a few more candidates have just been announced), the other 400 exoplanets we know of are many times bigger and are often referred to as “hot Jupiters” (many orbit very close to their host stars, getting roasted).

In this case, the exoplanet called GJ 1214b is 6.5 times the mass and 2.7 times wider than our planet, so it is certainly closer to our planet’s characteristics. As a comparison, this world is a celestial body somewhere between the mass of Earth and Uranus.

Although this is exciting news, even more impressive is the fact that astronomers have worked out that its density is low enough to contain large quantities of water. As the exoplanet is so close (in astronomical scales) we should soon be able to analyze its atmosphere to confirm the presence of this water.

David Charbonneau and his team at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. managed to deduce that this super-Earth could be made up of as much as 75 percent water by measuring how much the red dwarf’s light dimmed when GJ 1214b passed in front of it. By knowing how much the light dimmed, the exoplanet’s size could be worked out.

Then, after measuring the exoplanet’s orbital period of 38 hours (i.e. a year on Earth is 365 days, a year on GJ 1214b is a short 1.6 days!), orbital distance and the amount of “wobble” the exoplanet causes on its parent star, they were able to calculate the exoplanet’s mass. Knowing its size and mass led the astronomers to calculate its density.

Knowing the density of this alien world helps us gauge what it is made out of, and in this case GJ 1214b is made up primarily of water.

Although GJ 1214b isn’t thought to be habitable, its surface temperature is around 530 degrees Fahrenheit (280 degrees Celsius). Usually on Earth, water would boil at these temperatures, but the atmospheric conditions on GJ 1214b could maintain hot water oceans in a liquid state.

Naturally, the question of the potential for extraterrestrial life is bubbling, but the thick atmosphere of this super-Earth may be blocking out much of the star’s light from reaching the surface.

Although this particular waterworld might not be suitable for habitation, the researchers who made this discovery think we’re on the right track.

“These planets are like mileposts on the road rather than a destination itself,” said Greg Laughlin of the University of California, Santa Cruz, a coauthor of the paper published in the Dec. 17 edition of the journal Nature. “It’s very exciting to know that we’re getting close.”