Exeter astronomers find carbon dioxide and water vapour in atmosphere of temperate world 41 light years away

A rocky planet orbiting a dim red star 41 light years from Earth has a thick atmosphere containing carbon dioxide and water vapour, astronomers at the University of Exeter said on Wednesday, in what they describe as the strongest evidence yet that a temperate world around a red dwarf can hold on to its air.

The planet, LHS 5213 c, is about 1.3 times the width of Earth and completes an orbit every 11 days. Its star is a quarter of the mass of the sun and emits frequent ultraviolet flares that theorists had long assumed would strip any atmosphere from a planet so close. The team combined 14 transits observed with the James Webb space telescope between 2024 and this spring to build up a spectrum of starlight filtered through the planet’s atmosphere.

The spectrum shows a strong absorption feature at 4.3 micrometres attributed to carbon dioxide and a weaker, but statistically significant, signal from water vapour. Models that best fit the data give a surface pressure of between two and ten times Earth’s and a surface temperature of about 35C, though the researchers stress that the cloud layer is poorly constrained.

“Five years ago the consensus was that these planets would be bare rocks,” said Prof Nadia Blackwood, who led the study, published in Nature Astronomy. “What we have found is a world with a thick, carbon-dioxide-rich atmosphere, some water, and a surface that is warm but not hostile. Whether it is anything like habitable is a question we cannot answer yet. Whether it has an atmosphere, we can.”

Red dwarfs make up about three-quarters of the stars in the galaxy, and most rocky planets found so far orbit them. If such planets can keep atmospheres, the number of potentially habitable worlds rises enormously. The result follows several frustrating non-detections, including on the TRAPPIST-1 system, where Webb found no atmosphere on the two innermost planets.

Independent astronomers urged caution. Dr Tomasz Wierzbicki of the University of Edinburgh, who was not involved, said the carbon dioxide feature was convincing but the water detection was “at the edge of what the instrument can do”. “Fourteen transits is a lot of telescope time, and it is a very well done piece of work,” he said. “I would want to see the water signal again before I put it in a textbook.”

The Exeter group has been allocated a further 40 hours of Webb time next year to search for methane, which in combination with carbon dioxide would be difficult to explain without biology. LHS 5213 c is too faint to be studied by any ground-based telescope, but will be among the first targets of the European Extremely Large Telescope in Chile when it begins operations at the end of the decade.

Britain contributes about 16% of the European Space Agency’s science budget and built the mid-infrared instrument on Webb that was used for the measurement, at the UK Astronomy Technology Centre in Edinburgh and the Rutherford Appleton Laboratory in Oxfordshire.