Scientists detect water vapor on rocky exoplanet GJ 486 b: Is it from the planet or the star?

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Monday, May 15, 2023 07:00 PM

It is known that GJ 486 b is about 30% larger than Earth and has a mass of about 2.8 times the mass of our planet, which means that it is a rocky world with gravity stronger than Earth. It orbits a red dwarf star in less than 1.5 Earth days. The planet is expected to be facing its host star by orbital locking or tidal restriction, with a permanent day side and a permanent night side.

It is too close to its star to be within the habitable zone, with a surface temperature of about 430 degrees Celsius (800 degrees Fahrenheit).

However, James Webb’s observations using the Near Infrared Spectroradiometer (NIRSpec) that the rocky exoplanet about 26 light-years from Earth appear to contain water vapor.

It could be water vapor from the planet’s atmosphere, in which case it would have to be constantly replenished due to losses from stellar radiation. But a similar possibility says that the water vapor is in fact from the planet’s cool host star’s outer layer.

James Webb’s additional observations will help answer the question: Can a rocky planet maintain or recreate an atmosphere in an extreme environment near a red dwarf star?

The most common stars in the universe are red dwarfs, which means that rocky exoplanets orbiting such a star are more likely to be found.

And red dwarf stars are cold, so a planet must hug them into a tight orbit to stay warm enough to host liquid water (meaning it falls into the habitable zone).

These stars are also very active, especially when they are young, and they emit ultraviolet and X-rays that can damage planetary atmospheres. As a result, an important open question in astronomy is whether a rocky planet can maintain or recreate an atmosphere in such an extreme environment.

To help answer this question, astronomers used NASA’s James Webb Space Telescope to study a rocky exoplanet known as GJ 486 b.

And if the water vapor is associated with the planet, this indicates that it has an atmosphere despite its scorching temperature and its proximity to its star.

Water vapor has been seen on gaseous exoplanets before, but until now no definitive atmosphere has been detected around a rocky exoplanet. However, the team cautions that the water vapor could be on the star itself, more precisely, in the cooler stellar quarters, and not from the planet at all.

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

“Water vapor in the atmosphere on a hot, rocky planet would represent a major breakthrough for exoplanet science. But we have to be careful and make sure the star is not the culprit,” added Kevin Stephenson of the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, a co-author of the study.

The team observed two crossings, each lasting about an hour. They then used three different methods to analyze the resulting data. All three results were consistent in that they show a mostly flat spectrum with an interesting spike in the shorter infrared wavelengths.

The team ran computer models studying a number of different molecules, and concluded that the most likely source of the signal was water vapor.

While water vapor could indicate the presence of the atmosphere on GJ 486 b, water vapor from the star is also a plausible explanation. Surprisingly, even in our Sun, water vapor can sometimes be present in sunspots because these spots are very cold compared to the surface around the star. GJ 486 b’s host star is much cooler than the Sun, so more water vapor will be concentrated within its starpoints. As a result, it can create a signal that simulates the planet’s atmosphere.