What's more, researchers believe that it once lingered in the so-called 'Goldilocks Zone' near its star, before wandering closer in and heating up.
It means it could have played host to alien life millions of years ago...
Astronomers at America's Harvard-Smithsonian Center for Astrophysics (CfA) used the Hubble Space Telescope to probe the planet - discovered in 2009 and codenamed GJ1214b - and proved that it is a waterworld enshrouded by a thick, steamy atmosphere.
Wet, wet, wet: An artist's conception of GJ1214b, which is a watery super-Earth orbiting a red dwarf star 40 light-years away
‘GJ1214b is like no planet we know of,’ said astronomer Zachory Berta. ‘A huge fraction of its mass is made up of water.’
Theorists expect that GJ1214b formed farther out from its star, where water ice was plentiful, and migrated inward early in the system's history.
In the process, it would have passed through the star's habitable zone. How long it lingered there is unknown.
Two years ago CfA astronomers using the European Southern Observatory’s 3.6m telescope in Chile discovered possible evidence of water on the planet – now this has been confirmed by Hubble.
This super-Earth is about 2.7 times Earth's diameter and weighs almost seven times as much.
It orbits a red-dwarf star in the constellation Ophiuchus every 38 hours at a distance of 1.3million miles - 70 times closer than the Earth is to the Sun - giving it an estimated temperature of 450F (232C).
Berta and his co-authors used Hubble's infrared spectrometer to study GJ1214b when it crossed in front of its host star.
During such a transit, the star's light is filtered through the planet's atmosphere, giving clues to the mix of gases.
Hubble: The advanced space telescope used its infrared instruments to probe the planet
They found the spectrum of GJ1214b to be featureless over a wide range of wavelengths, or colours.
The atmospheric model most consistent with the Hubble data is a dense atmosphere of water vapour.
‘The Hubble measurements really tip the balance in favour of a steamy atmosphere,’ said Berta.
In fact, after calculating the planet’s mass, size and density, the researchers concluded that GJ1214b has much more water than Earth, and much less rock.
As a result, the internal structure of GJ1214b would be very different than our world.
‘The high temperatures and high pressures would form exotic materials like “hot ice” or “superfluid water” - substances that are completely alien to our everyday experience,’ said Berta.
Astronomer Dr Heather Couper was very excited by the team's results and hopes that follow-up tests for signs of life will be carried out.
She told MailOnline: 'There's overwhelming evidence that there's water on the planet. It shows that water could be common in the universe. It's a most most exciting thing.
'It's essential to do follow-up observations for these planets to see if they're emitting radio waves. I hope we home in on that.'
Mark Birkinshaw, the William P Coldrick Professor of Cosmology and Astrophysics at the University of Bristol, meanwhile, believes that life may once have been present on GJ1214b - and still could be.
He said: 'The problem with GJ1214b is that the water is supposed to be close to boiling at the moment. So if it migrated inwards from further out, where it might have picked up the water, then it might once have held life, but that life is likely now to be extinct.
'That is, unless it's managed the trick of moving deep into the dark parts of the oceans, and is behaving like the tube worms that are found deep in our oceans, near black smoker vents.