James Webb Space Telescope observes hints of water vapor on rocky exoplanet

GJ 486 b is a rocky world that orbits a red dwarf star in just under 1.5 Earth days. Despite being too close to its star to be within the habitable zone, this planet is intriguing scientists due to its size, mass, and hints of water vapor detected through observations from the Webb telescope.

This raises the question: Can a rocky planet survive in the harsh environment near a red dwarf star and maintain, or reestablish, an atmosphere? This is a pressing issue as red dwarf stars are the most common stars in the universe, and planets orbiting them are more likely to be rocky.

Red dwarf stars are cooler than other types of stars, which means planets have to be situated in a tight orbit to remain warm enough for potential liquid water. However, these stars are also more active, especially when they are young, emitting ultraviolet and X-ray radiation that could destroy planetary atmospheres.

Further observations from Webb will help determine if the water vapor detected around GJ 486 b is due to an enveloping atmosphere or from the outer layer of the planet’s host star. This finding will help us better understand the survival and evolution of rocky planets orbiting red dwarf stars.

Astronomers have utilized NASA’s James Webb Space Telescope to investigate a rocky exoplanet, named GJ 486 b, to understand if rocky planets can maintain or reestablish an atmosphere in a hostile environment near a red dwarf star. Although the planet is too close to its star to be habitable, with a surface temperature of around 800 degrees Fahrenheit (430 degrees Celsius), the observations using Webb’s Near-Infrared Spectrograph (NIRSpec) suggest the existence of water vapor.

The presence of water vapor in the atmosphere of a hot rocky planet would represent a significant discovery for exoplanet science. However, the team is careful in their interpretation, as the water vapor could be on the star and not the planet. It is essential to determine whether the water vapor is associated with the planet’s atmosphere or just from the star’s cool starspots.

GJ 486 b is approximately three times more massive and 30% larger than the Earth, making it a rocky planet with stronger gravity than Earth. It has a tight orbit around a red dwarf star, completing one revolution in just under 1.5 Earth days. It is likely tidally locked, meaning one side always faces the star, while the other side remains in perpetual darkness.

These findings will enhance our understanding of rocky planets orbiting red dwarf stars and their potential for habitability.

This graphic shows the transmission spectrum obtained by Webb 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 background illustration of a planet is an artist concept. Webb has not taken an image of the planet.. Credit: ILLUSTRATION: NASA, ESA, CSA, Joseph Olmsted (STScI). SCIENCE: Sarah E. Moran (University of Arizona), Kevin B. Stevenson (APL), Ryan MacDonald (University of Michigan), Jacob A. Lustig-Yaeger (APL)

GJ 486 b crosses in front of its host star, which allows astronomers to study its composition through transmission spectroscopy. The team conducted two observations of the exoplanet transiting its star, and the resulting data suggest the presence of water vapor. However, the team is cautious and acknowledges that the water vapor could also come from the star’s cool spots rather than the planet’s atmosphere.

The team used three different methods to analyze the data, all of which indicate a mostly flat spectrum with a significant rise at the shortest infrared wavelengths, which is indicative of water vapor. The team ran computer models to consider different molecules, and water vapor was deemed the most likely candidate.

The possibility of the water vapor originating from the star is a plausible explanation. Sunspots in our own Sun can harbor water vapor because they are much cooler than the surrounding surface. GJ 486 b’s host star is even cooler than the Sun, which could concentrate even more water vapor within its spots. The absence of evidence of the planet crossing any starspots during the observations does not rule out this scenario. The water signal in the data could be due to the physical conditions of the star rather than the planet’s atmosphere.

If GJ 486 b does have a water vapor atmosphere, it would likely be in a constant state of erosion due to the star’s heat and radiation. To maintain such an atmosphere, the planet would need a mechanism for continuously replenishing it, such as volcanic activity that ejects steam from its interior. More observations are needed to determine the exact amount of water present in the atmosphere.

In the future, the James Webb Space Telescope will observe GJ 486 b’s day side using the Mid-Infrared Instrument (MIRI). If the planet lacks an atmosphere, or has only a thin one, then the hottest part of the day side should be directly under the star. If it’s shifted, however, that could indicate an atmosphere that circulates heat.

To distinguish between the planetary atmosphere and starspot scenarios, the Near-Infrared Imager and Slitless Spectrograph (NIRISS), another instrument on the James Webb Space Telescope, will be needed to observe at shorter infrared wavelengths.

“It’s through the combined use of multiple instruments that we can truly determine whether or not this planet has an atmosphere,” said Stevenson.

The Astrophysical Journal Letters has accepted the study for publication.

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