An astronaut attempting to visit recently discovered planet GJ1214b would land in hot water -- literally, US scientists say.

Researchers at the Harvard-Smithsonian Center for Astrophysics said they have identified an entirely new kind of planet, dominated not by rock, gas or other common materials, but water.

The planet is "a waterworld enshrouded by a thick, steamy atmosphere," they said in a statement, after scrutinizing the planet with NASA's Hubble Space Telescope.

"GJ1214b is like no planet we know of," astronomer Zachary Berta said. "A huge fraction of its mass is made up of water."

GJ1214b was discovered in 2009 by the ground-based MEarth Project. Described as a "super-Earth," it is about 2.7 times Earth's diameter and weighs almost 7 times as much.

Further studies in 2010 led to scientists suspecting that the planet, where the temperature is some 450 degrees Fahrenheit (232 Celsius), was largely covered in water. This was confirmed by Berta and his co-authors using Hubble to study the planet when it crossed in front of its host star.

The light of the star, filtered through the planet's atmosphere, gave clues to the mix of gasses, backing up the water vapor theory.

"The Hubble measurements really tip the balance in favor of a steamy atmosphere," Berta said.

Further measurements and estimates led scientists to conclude that the planet has much more water than Earth and much less rock. That, together with high temperatures and pressure, likely produce some highly exotic results, including "hot ice," scientists say.

Our solar system contains three basic planet types: rocky, like Earth; gas giants like Jupiter or Saturn; and ice giants like Uranus.