Using the joint NASA/European Space Agency/Canadian Space Agency James Webb Space Telescope, scientists have discovered the presence of carbon-bearing molecules inside the atmosphere of exoplanet K2-18 b. Methane and carbon dioxide are amongst these carbon-bearing molecules.
The invention of the molecules is allowing scientists to higher understand K2-18 b. The planet has been the topic of several recent studies and is theorized to be a Hycean exoplanet — a planet that might possibly feature a hydrogen-rich atmosphere and a surface covered by water.
What’s more, K2-18 b orbits inside the habitable zone of its parent star, which is the region of space surrounding a star where conditions are suitable for the formation and existence of liquid water. Data from NASA’s Hubble Space Telescope first hinted on the unique properties of the exoplanet’s atmosphere, which then led to more teams performing research on the exoplanet.
Orbiting the cool dwarf star K2-18 within the constellation Leo, K2-18 b is positioned roughly 120 light-years away from Earth and is roughly 8.6 times the dimensions of Earth. Some of these exoplanets, those which might be between the sizes of Earth and Neptune, are usually not present in our solar system and thus haven’t been studied as extensively as gas-giant planets like Jupiter and Saturn have been.
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Graphic showing the spectra of K2-18 b. The spectra data was collected using Webb’s Near-Infrared Imager and Slitless Spectrograph and Near-Infrared Spectrograph instruments. (Credit: NASA/ESA/CSA/J. Olmsted (STScI), N. Madhusudhan (Cambridge University))
“Although this type of planet doesn’t exist in our solar system, sub-Neptunes are essentially the most common variety of planet known up to now within the galaxy. We have now obtained essentially the most detailed spectrum of a habitable-zone sub-Neptune to this point, and this allowed us to work out the molecules that exist in its atmosphere,” said team member Subhajit Sarkar of Cardiff University.
Many astronomers debate and research the characteristics of the atmospheres of those sub-Neptune exoplanets, with some even believing that, if the conditions are right, these planets and their atmospheres could possibly sustain life.
“Our findings underscore the importance of considering diverse habitable environments within the search for all times elsewhere. Traditionally, the search for all times on exoplanets has focused totally on smaller rocky planets, however the larger Hycean worlds are significantly more conducive to atmospheric observations,” said the lead writer Nikku Madhusudhan of the University of Cambridge.
So, what exactly makes K2-18 b so special?
Webb’s data on K2-18 b shows an abundance of methane and carbon dioxide inside the atmosphere of the exoplanet. Interestingly, there’s a shortage of ammonia, which further supports the concept K2-18 b could feature a water ocean underneath its hydrogen-rich atmosphere.
Moreover, Webb’s observations hinted on the existence of dimethyl sulfide (DMS). Back on Earth, DMS is a molecule that may only be produced by life. A big majority of all DMS molecules inside Earth’s atmosphere are created by phytoplankton inside marine environments. Nevertheless, more observations and data are needed to verify the existence of DMS inside K2-18 b’s atmosphere.
“Upcoming Webb observations should find a way to verify if DMS is indeed present within the atmosphere of K2-18 b at significant levels,” Madhusudhan said.
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Image of NIRSpec, considered one of the instruments that helped collect the K2-18 b spectra data. (Credit: Astrium GmbH)
When the characteristics and features of K2-18 b, all of the conditions look right for all times to exist on the planet. From being a habitable zone exoplanet to the existence of carbon-bearing molecules to the opportunity of a liquid ocean on its surface, K2-18 b is an ideal candidate for a habitable exoplanet. Nevertheless, there are some characteristics of K2-18 b that might prevent it from sustaining life.
One such characteristic is the planet’s immense size. As mentioned, K2-18 b is roughly 8.6 times the dimensions of Earth. The planet’s large size signifies that its interior most definitely incorporates a big mantle of highly pressured ice, which has similarities to the interior structure of Neptune. Moreover, K2-18 b’s ocean could possibly be too warm to be a liquid or too hot to sustain life.
Obtaining the spectrum data on K2-18 b’s atmosphere didn’t come without its challenges, though. The exoplanet’s parent star may be very brilliant, meaning that a few of Webb’s statement data might have been meddled with by the sunshine from K2-18.
To gather the spectrum data, Webb observed K2-18 because the exoplanet crossed in front of the star — a phenomenon often called a transit. When the planet crossed in front of the star, Webb recorded a dip within the star’s brightness and picked up data on the planet’s atmosphere because it was illuminated by the star. This data is then sent to the scientists who comb through it and seek for signals that represent certain molecules, like carbon dioxide.
This approach to using an exoplanet’s transit to gather atmospheric data is employed heavily by scientists researching exoplanets. Actually, many scientists use this approach to observing transits to find exoplanets. K2-18 b was discovered by NASA’s K2 mission in 2015 via a transit.
![](https://www.nasaspaceflight.com/wp-content/uploads/2022/01/280_656348main_ToV_transit_diag.jpeg)
An example of a light-weight curve generated from an exoplanet transiting its star. (Credit: NASA Ames)
“This result was only possible due to the prolonged wavelength range and unprecedented sensitivity of Webb, which enabled robust detection of spectral features with just two transits. For comparison, one transit statement with Webb provided comparable precision to eight observations with Hubble conducted over a number of years and in a comparatively narrow wavelength range,” said Madhusudhan.
Madhusudhan et al. were capable of pull the atmospheric data from just two transits of K2-18b in front of its star — once more highlighting the true power and capabilities of Webb in exoplanet science. Webb is ready to perform additional observations of K2-18 b in the approaching weeks, which can provide the team with much more data to comb through and, hopefully, discover more molecules. Specifically, the team is planning to make use of Webb’s Mid-Infrared Instrument spectrograph for the observations.
“These results are the product of just two observations of K2-18 b, with many more on the best way. This implies our work here is but an early demonstration of what Webb can observe in habitable-zone exoplanets,” said team member Savvas Constantinou of the University of Cambridge.
“Our ultimate goal is the identification of life on a habitable exoplanet, which might transform our understanding of our place within the universe. Our findings are a promising step towards a deeper understanding of Hycean worlds on this quest,” said Madhusudhan.
Madhusudhan et al.’s results were recently published within the journal.