A large rocky planet twice the size of the Earth has been discovered orbiting inside its star’s habitable zone and could have the right conditions to sustain alien life.

The exoplanet, called K2-18b, is 124 light-years away appears to be covered in water, according to researchers from Cambridge University.

While they can’t tell whether there is life on the planet now, by the end of the decade scientists think new telescopes will be able to spot the gases made by alien species.

Computer models suggest an ocean world – with liquid water below the atmosphere at pressures and temperatures similar to those found in our seas.

Last autumn two different teams of astronomers detected water vapour encircling the exoplanet and new analysis hints at a life-supporting ecosystem.

The Cambridge team confirmed the atmosphere is hydrogen-rich with a significant amount of water vapour but levels of other chemicals such as methane and ammonia were lower than expected.

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If the lower levels of methane and ammonia are due to biological processes then it is likely that the planet contains active life of one form or another.

Lead author Dr Nikku Madhusudhan said water vapour has been detected in the atmosphere of a number of exoplanets studied so far.

According to Dr. Nikku Madhusudhan from Cambridge’s Institute of Astronomy, who led the new research, ‘Even if the planet is in the habitable zone, that doesn’t necessarily mean there are habitable conditions on the surface,’.

‘To establish the prospects for habitability, it’s important to obtain a unified understanding of the interior and atmospheric conditions on the planet – in particular, whether liquid water can exist beneath the atmosphere.’

The researchers determined the composition and structure of both the atmosphere and interior for the first time using existing observations.

They used detailed numerical and statistical models and took into account its radius and mass. These are 2.6 and 8.6 times greater than Earth’s, respectively.

K2-18b is in a planet category known as a ‘super Earth’ – and has a temperature cool enough to have liquid water, between 32F and 104F – similar to Earth.

This is likely to cause it to have a significant hydrogen ‘envelope’ surrounding a layer of high-pressure water – with an inner core of rock and iron.

If the hydrogen envelope is too thick, the temperature and pressure at the surface of the water layer beneath would be far too great to support life.

The paper published in the Astrophysical Journal Letters.

Co-author Matthew Nixon said: ‘We wanted to know the thickness of the hydrogen envelope – how deep the hydrogen goes. While this is a question with multiple solutions, we’ve shown you don’t need much hydrogen to explain all the observations together.’

The researchers found the maximum extent of the hydrogen envelope allowed by the data is around six percent of the planet’s mass.

In particular, a ‘number of scenarios allow for an ocean world,’ said the researchers. The study will also enable the search for habitable conditions and signs of life on planets outside the solar system that are much bigger than Earth.

First discovered in 2015, the exoplanet is 124 light-years away, is 2.6 times the radius, and 8.6 times the mass of Earth. Based on its size, K2-18b could be a Super-Earth or a mini-Neptune and it takes 33 days to complete one orbit around its sun. More information about the exoplanet will be obtained next year with the launch of the upcoming James Webb Space Telescope by NASA.