Nasa says distant exoplanet could have rare water ocean and possible hint of life

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Scientists at Nasa have announced the existence of a possible rare water ocean on a giant exoplanet scores of light years away and also a chemical hint of a sign of potential life.

Related: The story of light: Nasa discoveries reimagined – in pictures

The “intriguing” discovery was made by the space agency’s James Webb telescope, peering 120 light years from Earth in the constellation Leo, building on earlier studies of the region using Webb’s predecessors, Hubble and Kepler.

Researchers have named the exoplanet K2-18 b, an unremarkable moniker for something with such potential significance. Almost nine times the mass of Earth, it is, Nasa, says: “a Hycean exoplanet, one which has the potential to possess a hydrogen-rich atmosphere and a water ocean-covered surface”.

The space agency said that its observations of the chemical makeup of the planet’s atmosphere suggested the possibility of an ocean world. “The abundance of methane and carbon dioxide, and shortage of ammonia, support the hypothesis that there may be a water ocean underneath a hydrogen-rich atmosphere in K2-18 b,” it said.

But the agency also hinted at even more remarkable possibility in the potential finding of a molecule called dimethyl sulfide (DMS), which on Earth is only produced by life.

“The bulk of the DMS in Earth’s atmosphere is emitted from phytoplankton in marine environments,” the Nasa press release said.

The presence of DMS, however, is still to be confirmed, and requires further investigation. “Upcoming Webb observations should be able to confirm if DMS is indeed present in the atmosphere of K2-18 b at significant levels,” said Nikku Madhusudhan, a University of Cambridge astronomer and lead author of the Nasa research.

It would not be the first time Nasa has found indications of water on other planets. Water vapor was previously discovered on the smaller exoplanet, HAT-P-11b, roughly the size of Neptune in the constellation Cygnus, also 120 light years away.

But scientists are excited by the revelation, even though they caution it does not necessarily mean the planet could support life.

“Our findings underscore the importance of considering diverse habitable environments in the search for life elsewhere,” said Madhusudhan.

“Traditionally, the search for life on exoplanets has focused primarily on smaller rocky planets, but the larger Hycean worlds are significantly more conducive to atmospheric observations.”

Orbiting the cool dwarf star K2-18, the exoplanet, 2.6 times the radius of Earth, lies in what Nasa calls the habitable zone, a region around a star where planets with liquid water may be present.

Its interior probably contains a large mantle of high-pressure ice, similar to Neptune, Nasa says, but probably has a thinner hydrogen-rich atmosphere and an ocean surface. Hycean worlds are predicted to have oceans of water, but on K2-18 b it is also possible the ocean is too hot to be habitable.

The planet’s existence was first discovered by Nasa’s K2 mission in 2015, but Webb’s improved technology over the earlier telescopes allowed for a more detailed analysis, and revelation it could be an ocean world. Scientists were able to study a tiny fraction of the star’s light as it passed through the exoplanet’s atmosphere.

“This result was only possible because of the extended wavelength range and unprecedented sensitivity of Webb, which enabled robust detection of spectral features with just two transits,” Madhusudhan said.

“For comparison, one transit observation with Webb provided comparable precision to eight observations with Hubble conducted over a few years and in a relatively narrow wavelength range.”

Nasa celebrated the first anniversary of operation of the Webb space telescope in July by releasing an “unprecedented” closeup image of the nearest star-forming region to Earth.

In little more than a year, it has allowed humans to look closer towards the origins of the universe than ever before, and produced high-resolution pictures of far-distant worlds and the mysterious structures that surround them.

They include the “rare and fleeting” phase of a star on the cusp of death; early galaxies formed just 350m years after the big bang; and evidence of “universe breaker galaxies” far larger than scientists thought possible, with the potential to upend current theories of cosmology.