The James Webb Space Telescope (JWST) viewed GJ 486 b, a rocky exoplanet. Water vapor suggests this exoplanet has an atmosphere. If true, this might revolutionize planetary science.

Now the twist. The exoplanet’s star or its water vapor signal is unknown.

“We see a signal, and it’s probably water. In an official statement, lead author Sarah Moran of the University of Arizona in Tucson said, “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.”

Mystery persists.

Habitable GJ 486 b orbits its red dwarf star in 1.5 Earth days. This rocky planet is 30 percent larger than Earth and has higher gravity.

How it supports an atmosphere is puzzling given its 800 degrees Fahrenheit (430 degrees Celsius) surface temperature.

The team thinks its host star may have water vapor, especially in cool starspots. Indeed, cooler sunspots contain water vapor. GJ 486 b’s star is colder than the Sun, suggesting more water vapor in its starspots.

The planet didn’t intersect any starspots during transits. However, the star may have other spots. “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,” said co-author Ryan MacDonald of the University of Michigan in Ann Arbor.

Webb’s NIRSpec observed GJ 486 b transiting its star. This investigation saw two hour-long transits. The data was then analyzed. The statement says “The results from all three are consistent in that they show a mostly flat spectrum with an intriguing rise at the shortest infrared wavelengths”.

The scientists employed computer models to identify water vapor as the signal’s source. However, computer models suggested the signal came from starspots or the planetary atmosphere.

The team needs more investigation to identify the water signal source.

“Water vapor in an atmosphere on a hot rocky planet would revolutionize exoplanet science. But we must be careful and make sure the star is not the culprit,” concluded Kevin Stevenson of the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, the program’s principal investigator.

The Astrophysical Journal Letters accepts this novel work.

Study Abstract

Rockier exoplanet atmospheres are sought around M-dwarf stars. M dwarf planets are ideal transmission spectroscopy targets for atmosphere assessment. It is unclear if their host stars’ very fluctuating extreme-UV radiation conditions support atmospheres.

JWST is determining if the most suitable rocky worlds orbiting M dwarfs have atmospheres. We provide a 2.8—5.2μm JWST NIRSpec/G395H transmission spectra of the heated (700 K, 40.3× Earth’s insolation) super-Earth GJ 486b (1.3 R and 3.0 M). Based on three independent reductions, our two GJ 486b transit spectrum deviates from a flat line at 2.2 − 3.3.

We establish that GJ 486b either has a water-rich atmosphere (with the most rigorous constraint on the recovered water abundance of H2O > 10% to 2) or the transmission spectrum is polluted by water in cool unocculted starspots using forward and retrieval models. Cool starspots and hot faculae best fit the measured stellar spectrum. Shorter wavelength measurements can break this degeneracy and indicate if GJ 486b has a water-rich atmosphere.

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