Water in the gas shell of a nearby planet

GJ 9872d

This is what the exoplanet GJ 9827d could look like, the smallest exoplanet to date where water vapor has been detected in the atmosphere. © NASA/ESA, Leah Hustak (STScI), Ralf Crawford (STScI)

Exoplanets with water resources are considered the best chance for extraterrestrial life. But detecting water on distant worlds is not easy. Now, for the first time, astronomers have succeeded in detecting the spectral signature of water vapor in the gas envelope of a nearby exoplanet. The planet GJ 9872d is only around 97 light-years away and is around twice the size of Earth. It is considered a possible water world because of its low density. The current data is also the first spectral detection of water on such a small exoplanet, the team reports. However, this water world is not life-friendly - the planet is about as hot as Venus.

The search for life in space is closely linked to the search for water on exoplanets. Because this molecule is considered an important prerequisite for biological life as we know it. Cells need water as a solvent and as a helper for biochemical reactions. However, although H2O is one of the most common molecules in the cosmos, it is difficult to clearly detect on exoplanets using current astronomical tools. Most previous discoveries of potential water worlds have been based only on data on the density of these planets, but this may also be reduced for other reasons. It is much clearer when astronomers can detect the presence of water vapor in the atmosphere of an exoplanet - especially if it also has a density that suggests extensive oceans.

Clear spectral signature of water in the gas envelope

This is exactly what a team led by Pierre-Alexis Roy from the University of Montreal may have achieved. For their study, the astronomers used the Hubble Space Telescope to examine the light spectrum of the exoplanet GJ 9872d, 97 light-years away. This planet is around twice as big as Earth and weighs around 3.4 Earth masses. This means it could be a gas-rich super-Earth or a small sub-Neptune. Because GJ 9872d passes in front of its sun-like star as we see it, the spectral signatures of its gas envelope can be detected in starlight. The starlight shines through its gas shell and the atoms and molecules present there leave absorption lines in the light spectrum. “Our observation program was developed with the aim of not only detecting the molecules in the planet's atmosphere, but also specifically searching for water vapor,” explains Roy.

The spectral analyzes showed a noticeable absorption line at 1.4 micrometers - this is considered an indicator of the presence of water. However, this signature alone does not clearly reveal how much water vapor GJ 9872d has in its gas shell. Theoretically, it could be a small amount of water vapor in an inflated hydrogen-helium atmosphere, but it could also be a shell consisting largely of water vapor. However, given the exoplanet's relatively old age of around six billion years and its close proximity to its parent star, astronomers consider the latter to be more likely. Because of the intense radiation from the star, GJ 9872d must have long since lost most of its original hydrogen. This probably left behind an atmosphere characterized by water vapor, as Roy and his team explain.

Hot counterpart to the icy moons?

In any case, the spectral detection of water in GJ 9872d represents an important advance: “This would be the first time that we can directly show by studying atmospheres that these water-rich planets actually exist in the vicinity of other stars,” says Roys Colleague Björn Benneke. “Until now, we have not been able to directly detect the atmosphere of such a small planet.” GJ 9872d is the smallest exoplanet to date where the spectral signature of water in the gas envelope has been detected. “Water on such a small planet is a groundbreaking discovery,” adds co-author Laura Kreidberg from the Max Planck Institute for Astronomy in Heidelberg. “It brings us closer than ever before to characterizing truly Earth-like worlds.”

The astronomers suspect that GJ 9872d originally formed further away from its star - beyond the snow line at which water- and ice-rich planets can form. “The planet GJ 9827d could be half water, half rock,” says Benneke. This would make GJ 9872d similar, for example, to Jupiter's moons Europa and Ganymede in our solar system. However, after its formation, the exoplanet migrated inward to its current position near the star. This caused it to heat up to temperatures of around 400 degrees and a large part of its ice and water evaporated. “GJ 9872d could be a larger, hotter and closer to the star version of the icy moons in our solar system,” explain the astronomers. Analyzes conducted recently by astronomers using the James Webb Space Telescope could show how much water there is on this exoplanet. Using its high-resolution infrared spectrometers, they searched in more detail for traces of water and other molecules. “We can’t wait to see what this data reveals,” says Kreidberg. “Hopefully we can now settle the question of water worlds once and for all.”

Source: Pierre-Alexis Roy (Université de Montreal) et al., The Astrophysical Journal Letters, doi: 10.3847/2041-8213/acebf0

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