Waterworld Planet is More Earth-like Than any Discovered Before
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.
“We’ve found this planet in the first few months of MEarth being in operation, so we are either extremely lucky or these kinds of planets are very common,” Charbonneau said.
“In time, we expect to find planets that are further away from their parent stars and so are likely to have surface temperatures much closer to those found on Earth,” he added.
The latest planet is only a stone’s throw away in astronomical terms, meaning scientists will be able to turn the Hubble Space Telescope towards it and analyse its atmosphere, potentially revealing signs of life. Charbonneau’s team has already requested time on the space telescope.
“Using the Hubble, we can look at the atmosphere and say not only whether it’s habitable, but whether it’s inhabited,” Charbonneau told the Guardian. “If we find oxygen in the atmosphere things will get really interesting, because on Earth all the oxygen in the atmosphere comes from life.”
After spotting GJ1214b in orbit, the astronomers measured tiny movements in the parent star as the planet circled around it. From these wobbles they calculated the mass of the planet to be 6.6 times as great as the Earth’s. The most likely composition of the planet is 75% water, with 22% silicon and 3% iron forming a solid core, the scientists report.
In an accompanying article, Geoffrey Marcy, an astronomer at the University of California in Berkeley, said the extrasolar planet or “exoplanet” provides “the most watertight evidence so far for a planet that is something like our own Earth outside our solar system”.
Zachory Berta, a co-author on the latest paper, said: “Despite its hot temperature, this appears to be a waterworld. It is much smaller, cooler and more Earth-like than any other known exoplanet.” Some of the planet’s water is expected to be in an exotic form called ice VII, a crystalline form of water that exists under immense pressures.
Astronomers have discovered more than 400 planets beyond our solar system in the past twenty years. Two dedicated space missions, the French space agency’s Corot telescope and Nasa’s Kepler telescope have been launched to look for Earth-sized rocky planets in stars’ Goldilocks zone that could be hospitable to life.