Water discovered in atmosphere of potentially habitable planet

An artist's impression of K2-18b, the only known exoplanet believed to have liquid water essential for life. EPA

Water has been discovered for the first time in the atmosphere of an exoplanet with Earth-like temperatures that could support life as we know it, scientists revealed on Wednesday.

Eight times the mass of Earth and twice as big, the planet named K2-18b orbits in its star's "habitable zone" – a distance neither too far or too close, where water can exist in liquid form, the journal Nature Astronomy reports.

"This planet is the best candidate we have outside our solar system" in the search for signs of life, co-author Prof Giovanna Tinetti, an astronomer at University College London, told AFP.

"We cannot assume that it has oceans on the surface but it is a real possibility."

Of the more than 4,000 exoplanets detected so far, this is the first known to combine a rocky surface and an atmosphere containing water.

Most exoplanets with atmospheres are giant balls of gas and the handful of rocky planets for which data is available seem to have no atmosphere at all.

Even if they did, most Earth-like planets are too far from their stars to have liquid water, or so close that any water has evaporated.

Discovered in 2015, K2-18b is one of hundreds of "super-Earths", planets with less than 10 times the mass of ours, seen by Nasa's Kepler spacecraft.

Future space missions are expected to detect hundreds more in the coming decades.

"Finding water in a potentially habitable world other than Earth is incredibly exciting," said lead author Dr Angelos Tsiaras, from the same university.

"K2-18b is not 'Earth 2.0'. However, it brings us closer to answering the fundamental question: 'Is the Earth unique?'"

Working with spectroscopic data captured in 2016 and 2017 by the Hubble Space Telescope, Dr Tsiaras and his team used open-source algorithms to analyse the starlight filtered through K2-18b's atmosphere.

They found the unmistakable signature of water vapour. Exactly how much water remains uncertain but computer modelling suggested concentrations between 0.1 and 50 per cent.

By comparison, the percentage of water vapour in Earth's atmosphere varies between 0.2 per cent above the poles, and up to 4 per cent in the tropics.

There was also evidence of hydrogen and helium. Nitrogen and methane may also be present but they are undetectable with current technology, the study said.

Further research will be able to determine the extent of cloud coverage and the percentage of water in the atmosphere.

Water is crucial in the search for life, in part because it carries oxygen.

"Life as we know is based on water," Dr Tinetti said.

K2-18b orbits a red dwarf star about 110 light years or a million billion kilometres distant, in the Leo constellation of the Milky Way, and is probably bombarded with more destructive radiation than Earth.

"It is likely that this is the first of many discoveries of potentially habitable planets," said UCL astronomer Dr Ingo Waldmann, also a co-author of the report.

"This is not only because super-Earths like K2-18b are the most common planets in our galaxy but also because red dwarfs – stars smaller than our Sun – are the most common stars."

A new generation of space-based star gazing instruments led by the James Webb Space Telescope and the European Space Agency's Ariel mission will be able to describe exoplanet atmospheres in far greater detail.

Ariel, scheduled for a 2028 launch, will look at about 1,000 planets, a large enough sample to seek patterns and identify outliers.

"Over 4,000 exoplanets have been detected but we don't know much about their composition and nature," Dr Tinetti said.

"By observing a large sample of planets, we hope to reveal secrets about their chemistry, formation and evolution."