spaceSpace and PhysicsspaceAstronomy

Water Vapour Spotted By JWST – But From An Earth-Sized Planet Or Its Star?

The signal is exciting but its origin is not fully confirmed.


Dr. Alfredo Carpineti


Dr. Alfredo Carpineti

Senior Staff Writer & Space Correspondent

Alfredo (he/him) has a PhD in Astrophysics on galaxy evolution and a Master's in Quantum Fields and Fundamental Forces.

Senior Staff Writer & Space Correspondent

This artist concept represents the rocky exoplanet GJ 486 b, which orbits a red dwarf star that is only 26 light-years away in the constellation Virgo

H-2-Oh, maybe... Image Credit: NASA, ESA, CSA, Joseph Olmsted (STScI)

Water vapor has been seen in the atmospheres of exoplanets before, but they were massive gas giants. Now, researchers report that detection but in a completely different system, GJ 486, where a rocky super-Earth orbits a red dwarf star. But the team is unsure if the signal comes from the star or the planet.

The observations conducted by JWST use the transit method. The planet is 26 light-years from Earth and it regularly passes in front of its star; when this happens, some of the starlight is blocked by the planet. If an atmosphere is present, some of the light is filtered through that, and the gases present leave a signature. One such signature has been found and it indicates the presence of water.


This could be the first evidence of water in the atmosphere of a rocky planet, but the team is being cautious because the models suggest that the water signal might come from the star instead. Water vapor can be present in stars, especially in star spots that are cooler than the rest of the stellar surface.  

“We see a signal, and it’s almost certainly due to water. But we can't tell yet if that water is part of the planet's atmosphere, meaning the planet has an atmosphere, or if we’re just seeing a water signature coming from the star,” lead author Sarah Moran of the University of Arizona in Tucson, said in a statement.

“Water vapor in an atmosphere on a hot rocky planet would represent a major breakthrough for exoplanet science. But we must be careful and make sure that the star is not the culprit,” added Kevin Stevenson of the Johns Hopkins University Applied Physics Laboratory, the principal investigator on the program.

This graphic shows the transmission spectrum obtained by JWST observations of rocky exoplanet GJ 486 b. The science team’s analysis shows hints of water vapor; however, computer models show that the signal could be from a water-rich planetary atmosphere (indicated by the blue line) or from starspots from the red dwarf host star (indicated by the yellow line). The two models diverge noticeably at shorter infrared wavelengths, indicating that additional observations with other Webb instruments will be needed to constrain the source of the water signal.
The data from JWST and how the models diverge at shorter wavelengths. Image Credit: Credits: NASA, ESA, CSA, Joseph Olmsted (STScI)

The models used by the researchers diverge at shorter wavelengths. This means that it should be possible to confirm the source once it is observed there. The discovery of a water-rich atmosphere on a rocky exoplanet would be fantastic but do not picture planet GJ 486 b as Earth’s twin.


First of all, it’s 30 percent larger than Earth and three times as massive. It also orbits very close to its star, completing a full year in 1.5 Earth days. Red dwarfs are less luminous and cooler than our Sun but at that distance, the planet would still be incredibly hot. On top of that, it is likely to be tidally locked so one side is in constant daylight and the other in perennial night.

The mid-infrared instrument(MIRI) on JWST will soon look at the planet as well. By looking at where the hottest part of the planet is, MIRI could help solve this conundrum. If there is no atmosphere, it will be on its constant day side, but air circulation would move heat creating a shift in this point.

“It’s joining multiple instruments together that will really pin down whether or not this planet has an atmosphere,” explained Stevenson.

The study is accepted for publication in The Astrophysical Journal Letters.


spaceSpace and PhysicsspaceAstronomy
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  • exoplanet,

  • JWST,

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  • super earth,

  • exoplanet atmosphere