Using spectral data from Webb’s Near-Infrared Spectrograph (NIRSpec), astronomers determined that the warm super-Earth Gliese 486b either has a water-rich atmosphere or the exoplanet’s spectrum is contaminated by water present in cool starspots of the parent star Gliese 486.
This graphic shows the transmission spectrum obtained by Webb of the warm super-Earth exoplanet Gliese 486b. Image credit: NASA / ESA / CSA / J. Olmsted, STScI / S. Moran, University of Arizona / K. Stevenson, JHUAPL / R. MacDonald, University of Michigan / J. Lustig-Yaeger, JHUAPL.
Gliese 486 is a red dwarf star located around 26.3 light-years away in the constellation of Virgo.
Also known as GJ 486, Wolf 437, LHS 341, and HIC 62452, the star is much fainter and cooler than the Sun.
Gliese 486 hosts at least one exoplanet, Gliese 486b, which belongs to a class of planets called super-Earths.
The planet has a radius of 1.31 Earth radii, a mass 2.8 times that of our home planet, but has a similar density.
Gliese 486b orbits the hosy star once every 1.5 days at a distance of 2.5 million km.
With an equilibrium surface temperature of 700 K (427 degrees Celsius, 801 degrees Fahrenheit), the planet is too hot to support life as we know it.
The new observations of the Gliese 486 system show hints of water vapor.
If the water vapor is associated with the planet, that would indicate that it has an atmosphere despite its scorching temperature and close proximity to its star.
However, astronomers cautions that the water vapor could be on the star itself — specifically, in cool starspots — and not from the planet at all.
“We see a signal, and it’s almost certainly due to water,” said Dr. Sarah Moran, an astronomer at the University of Arizona.
“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.”
“Water vapor in an atmosphere on a hot rocky planet would represent a major breakthrough for exoplanet science,” added Dr. Kevin Stevenson, an astronomer at the Johns Hopkins University Applied Physics Laboratory.
“But we must be careful and make sure that the star is not the culprit.”
The study authors observed two transits of Gliese 486b, each lasting about an hour.
They then used three different methods to analyze the resulting data.
Their results from all three are consistent in that they show a mostly flat spectrum with an intriguing rise at the shortest infrared wavelengths.
The astronomers ran computer models considering a number of different molecules, and concluded that the most likely source of the signal was water vapor.
While the water vapor could potentially indicate the presence of an atmosphere on Gliese 486b, an equally plausible explanation is water vapor from the star.
Surprisingly, even in our own Sun, water vapor can sometimes exist in sunspots because these spots are very cool compared to the surrounding surface of the star.
The host star is much cooler than the Sun, so even more water vapor would concentrate within its starspots. As a result, it could create a signal that mimics a planetary atmosphere.
“We didn’t observe evidence of the planet crossing any starspots during the transits,” said Dr. Ryan MacDonald, an astronomer at the University of Michigan.
“But that doesn’t mean that there aren’t spots elsewhere on the star.”
“And that’s exactly the physical scenario that would imprint this water signal into the data and could wind up looking like a planetary atmosphere.”
The findings were published in the Astrophysical Journal Letters.
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Sarah E. Moran et al. 2023. High Tide or Rip-Tide on the Cosmic Shoreline? A Water-Rich Atmosphere or Stellar Contamination for the Warm Super-Earth GJ 486b from JWST Observations. ApJL, in press; arXiv:2305.00868
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