GJ1214b, which has an atmosphere that is at least 50 per cent water by mass, orbits very close to a red dwarf star 40 light-years from Earth. GJ1214b, which has an atmosphere that is at least 50 per cent water by mass, orbits very close to a red dwarf star 40 light-years from Earth. (NASA/ESA/D. Aguilar, Harvard-Smithsonian Center for Astrophysics)

Astronomers have discovered an extremely hot, steamy world with far more water than Earth, which they are calling a completely new type of planet.

GJ1214b is the first "waterworld" ever described by astronomers. The planet was first found in 2009, but astronomers previously assumed that its hazy exterior masked a gaseous core, like other bodies in the solar system.

Now, however, a paper detailing the planet's newly uncovered qualities has been published online and accepted for publication in the Astrophysical Journal.

The planet is located 40 light years from Earth in the constellation Ophiuchus. It has a diameter 2.7 times larger than Earth and a mass that is seven times greater.

Field guide to planets

Rocky planets are relatively small and have a solid surface. In our solar system, such planets include Earth itself, along with Mercury, Venus and Mars.

Gas giants are typically much bigger – at least 10 times bigger – than rocky planets. They have an extremely thick atmosphere of hydrogen and helium and no solid surface to speak of. In our solar system, the gas giants include Jupiter and Saturn.

Ice giants are a type of gas giant with a thinner atmosphere of hydrogen and helium surrounding a molten ice interior that may contain water, ammonia and methane. Examples include Neptune and Uranus.

Hot jupiters are gas giants that orbit extremely close to their star, giving them surface temperatures close to 1,000 C. They don't exist in our solar system, but have been detected around other stars.

Waterworlds are a newly discovered type of planet with atmospheres extremely high in water vapour. High temperatures and pressures turn the water into states of matter that don't exist on Earth, such as hot ice or superfluid water.

It circles a red dwarf star cooler and smaller than the sun, but is just two million kilometres away from its star, completing an orbit every 38 hours. In contrast, Mercury, the nearest planet to the sun in our solar system, orbits an average of 58 million kilometres away and takes 88 days to complete each circuit.

GJ1214b's star-hugging orbit gives it a sizzling surface temperature of 230 C, transforming the water into states that don't exist on Earth.

"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 Zachory Berta, the lead author of the study, in a statement.

The researchers used a camera on the Hubble space telescope to peer through the planet's atmosphere as it crossed in front of its star.

Different chemicals in the atmosphere filter the light in different ways and information about the chemical composition can be teased out by comparing the intensity of different colours of starlight that has passed through the planet's atmosphere to the colour of the starlight that hasn't passed through.

The resulting data, combined with known information about the planet, suggested GJ1214b's atmosphere is at least 50 per cent water by mass.

The researchers think the planet originally formed farther away from the star in an area with lots of water and ice and later moved in to where it is now. Along the way, it would have passed through the "habitable zone," with temperatures similar to Earth's.

GJ1214b was discovered in 2009 by the MEarth project, which uses ground-based robotic telescopes to look for planets around nearby dwarf stars.