Exoplanet or its star? Webb telescope detects water vapor, perplexing astronomers about source

The mystery remains

GJ 486 b orbits its red dwarf star in just 1.5 Earth days, indicating that it is in the habitable zone. Furthermore, this rocky world is roughly 30 per bigger than Earth and has stronger gravity than our planet.

The presence of an extremely hot surface temperature of 800 degrees Fahrenheit (430 degrees Celsius) makes it a bit perplexing to understand how it could support any atmosphere.

That’s why the team suspects water vapor may exist on its host star, particularly in cool starspots. In fact, our Sun sometimes contains water vapor in cooler sunspots than the hotter surface. And, GJ 486 b's star is noted to be much cooler than the Sun, implying the possibility of more water vapor within its starspots. 

“We didn't observe evidence of the planet crossing any starspots during the transits. 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,” said Ryan MacDonald of the University of Michigan in Ann Arbor, one of the study’s co-authors.

Using Webb's Near-Infrared Spectrograph (NIRSpec), the team observed GJ 486 b transiting its star. Two transits lasting about an hour were observed for this study. Following that, the data was examined using various methods. According to the statement, "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 team also used computer models to determine the signal, concluding that water vapor was the most likely source. The computer models, on the other hand, suggested that the signal could be coming from the planetary atmosphere or starspots. 

According to the team, additional research is needed to determine the exact source of the water signal.

“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,” concluded Kevin Stevenson of the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, principal investigator on the program.

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

Study Abstract:

Planets orbiting M-dwarf stars are prime targets in the search for rocky exoplanet atmospheres. The small size of M dwarfs renders their planets exceptional targets for transmission spectroscopy, facilitating atmospheric characterization. However, it remains unknown whether their host stars’ highly variable extreme-UV radiation environments allow atmospheres to persist. With JWST, we have begun to determine whether or not the most favorable rocky worlds orbiting M dwarfs have detectable atmospheres. Here, we present a 2.8—5.2μm JWST NIRSpec/G395H transmission spectrum of the warm (700 K, 40.3× Earth’s insolation) super-Earth GJ 486b (1.3 R⊕ and 3.0 M⊕). The measured spectrum from our two transits of GJ 486b deviates from a flat line at 2.2 − 3.3σ, based on three independent reductions. Through a combination of forward and retrieval models, we determine that GJ 486b either has a water-rich atmosphere (with the most stringent constraint on the retrieved water abundance of H2O > 10% to 2σ) or the transmission spectrum is contaminated by water present in cool unocculted starspots. We also find that the measured stellar spectrum is best fit by a stellar model with cool starspots and hot faculae. While both retrieval scenarios provide equal quality fits (χ2ν = 1.0) to our NIRSpec/G395H observations, shorter wavelength observations can break this degeneracy and reveal if GJ 486b sustains a water-rich atmosphere.