2023-09-13 10:01:33
This article was originally published in English
Astronomers are looking for signs of life elsewhere in the universe, and the planet K2-18 b is of particular interest. Researchers remain cautious.
NASA says the James Webb Space Telescope may have detected a molecule in the atmosphere of a distant exoplanet that, on Earth, is produced only by life.
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The planet K2-18 b has been the subject of intense study since astronomers announced in 2019 that they had found potential signs of water vapor in its atmosphere.
Orbiting a red dwarf star in the constellation Leo, about 120 light years from our solar system, the planet has a mass equal to 8.6 times that of Earth.
A later study using the same data then suggested that the water vapors could in fact be methane.
Now the even more powerful James Webb Telescope has turned its attention to the planet and provided a wealth of new data.
He discovered the presence of carbonaceous molecules, including methane and carbon dioxide, fueling speculation that K2-18 b may have a hydrogen-rich atmosphere and a surface covered by an ocean of water.
These features could be signs of a planet likely to support life.
Clues of life?
Among the observations made by James Webb was the possible detection of a molecule called dimethyl sulfide (DMS), which, on our planet, is only produced by life.
The majority of DMS in the Earth’s atmosphere comes from phytoplankton, microscopic organisms found in the oceans.
In a statement, NASA said the hypothesis of the presence of DMS in the atmosphere was “less solid” than other results, and that it required further validation.
“The next observations from the Webb probe should confirm whether DMS is indeed present in the atmosphere of K2-18 b at significant levels”explains Nikku Madhusudhan, astronomer at the University of Cambridge and lead author of the article announcing these results.
The space agency, however, clarified that the abundance of methane and carbon dioxide detected in the atmosphere, as well as the shortage of ammonia, support the hypothesis of the existence of an ocean of water under the hydrogen-rich atmosphere of K2-18 b.
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The planet is thought to be an example of a hyceanic planet, a planet larger than Earth but smaller than the gas giants of our solar system, and which is covered by a liquid ocean and a thick nitrogen atmosphere.
There are no planets like K2-18 b in our solar system. They are therefore poorly known, even though scientists believe they are common around red dwarfs.
Some astronomers believe that Hyceaean planets could be promising environments for searching for traces of life.
“Our results highlight the importance of taking into account diverse habitable environments in the search for extra-terrestrial life”underlines Nikku Madhusudhan.
“Traditionally, the search for life on exoplanets has focused primarily on small, rocky planets, but the larger hycean planets are much more amenable to atmospheric observations“.
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Habitable zone exoplanet
Astronomers are particularly interested in studying K2-18 b because it is also in its host star’s habitable zone, meaning it is neither too close nor too far from its sun.
NASA points out, however, that despite the apparent composition of its atmosphere and its proximity to its star, the size of the planet means that its interior likely contains a vast mantle of high-pressure ice, like Neptune, but with an atmosphere finer and rich in hydrogen, and an ocean surface.
The space agency says that while Hyceaean planets are believed to have oceans of water, it is also possible that the ocean is too hot to be habitable or liquid.
“Although this type of planet does not exist in our solar system, sub-Neptunes are the most common type of planet known to date in the galaxy”says Subhajit Sarkar, member of the Cardiff University team.
“We obtained the most detailed spectrum of a sub-Neptune in the habitable zone to date, which allowed us to determine the molecules present in its atmosphere”.
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Measuring the atmosphere
Studying the potential atmospheric composition of an exoplanet is a difficult task, especially since the host star is much brighter than the planet itself.
Astronomers were able to analyze K2-18 b by observing light from its parent star passing through the planet’s atmosphere. As the planet passes in front of the star, telescopes are able to detect the drop in brightness that occurs at that time.
This is a common technique for detecting the presence of a planet around a star, but it also results in the emission of light through the planet’s atmosphere, light which can be captured by telescopes as well. powerful as James Webb.
By studying this light, experts can determine some of the gases that make up the exoplanet’s atmosphere. “This result was only possible thanks to the extended wavelength range and unprecedented sensitivity of the Webb telescope, which enabled robust detection of spectral features with only two transits”assure Nikku Madhusudhan.
“For comparison, one transit observation with the James Webb provided comparable precision to eight Hubble observations conducted over a few years and in a relatively narrow wavelength range.”
The researchers added that this is just the beginning of the James Webb Telescope’s observations, and there will be more “many more to come.”
Their results were published in the Astrophysical Journal Letters.
The team now intends to conduct further research using the telescope’s Mid-Infrared Instrument (MIRI) spectrograph. She hopes that this research will further validate their results and obtain new information on the environmental conditions of K2-18 b.
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