A giant waterworld that is wet to its core has been spotted in orbit around a dim but not too distant star, improving the odds that habitable planets may exist in our cosmic neighborhood. The planet is nearly three times as large as Earth and made almost entirely of water, forming a global ocean more than 15,000km deep.
Astronomers detected the alien world as it passed in front of its sun, a red dwarf star 40 light years away in a constellation called Ophiuchus, after the Greek for "snake holder".The discovery, made with a network of amateur telescopes, is being hailed as a major step forward in the search for planets beyond our solar system that are hospitable to life as we know it.
Measurements suggest the planet is shrouded in a thick atmosphere of hydrogen and helium that blocks visible light from its sun, plunging the watery surface into permanent darkness. The weight of the atmosphere keeps the water liquid despite it being a searing 120C to 282C.
Writing in the journal Nature, David Charbonneau at the Harvard-Smithsonian Centre for Astrophysics describes how his team used a suite of eight amateur-sized telescopes to spot the planet as it moved across the face of its star, which is less than 0.5% as bright as our own sun.
The telescopes picked up a slight dimming in light from the star as the waterworld, named GJ1214b, passed in front of it every 1.6 days. The planet has a radius 2.7 times as large as the Earth's and orbits at a distance of only two million kilometres from its star. Our own planet circles the sun at an average distance of around 150 million kilometres.
"It would be very difficult to imagine life as we know it on the surface. It's hot and dark and there are probably no rocky surfaces like we have on Earth," said Charbonneau.
Charbonneau heads the MEarth project, which trains telescopes on a class of star called M-dwarfs or red dwarfs, which are much cooler and dimmer than our own sun. Planets orbiting close to these can lie in what astronomers call the "Goldilocks zone", where temperatures are neither too hot nor too cold for water to flow and life to flourish.