Using Webb, scientists discover carbon dioxide and methane in habitable-zone exoplanet

by Haygen Warren

Using the joint NASA/European Space Agency/Canadian Space Agency James Webb Space Telescope, scientists have discovered the presence of carbon-bearing molecules within the atmosphere of exoplanet K2-18 b. Methane and carbon dioxide are among these carbon-bearing molecules.

The discovery of the molecules is allowing scientists to better understand K2-18 b. The planet has been the subject of several recent studies and is theorized to be a Hycean exoplanet — a planet that could possibly feature a hydrogen-rich atmosphere and a surface covered by water.

What’s more, K2-18 b orbits within 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 at 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 in the constellation Leo, K2-18 b is located approximately 120 light-years away from Earth and is roughly 8.6 times the size of Earth. These types of exoplanets, those that are between the sizes of Earth and Neptune, are not found in our solar system and thus haven’t been studied as extensively as gas-giant planets like Jupiter and Saturn have been.

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 kind of planet does not exist in our solar system, sub-Neptunes are the most common type of planet known so far in the galaxy. We have obtained the most detailed spectrum of a habitable-zone sub-Neptune to date, 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 these 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 in the search for life elsewhere. Traditionally, the search for life on exoplanets has focused primarily on smaller rocky planets, but the larger Hycean worlds are significantly more conducive to atmospheric observations,” said the lead author 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 within the atmosphere of the exoplanet. Interestingly, there’s a shortage of ammonia, which further supports the idea that K2-18 b could feature a water ocean underneath its hydrogen-rich atmosphere.

Furthermore, Webb’s observations hinted at the existence of dimethyl sulfide (DMS). Back on Earth, DMS is a molecule that can only be produced by life. A large majority of all DMS molecules within Earth’s atmosphere are created by phytoplankton within marine environments. However, more observations and data are needed to confirm the existence of DMS within K2-18 b’s atmosphere.

“Upcoming Webb observations should be able to confirm if DMS is indeed present in the atmosphere of K2-18 b at significant levels,” Madhusudhan said.

Image of NIRSpec, one of the instruments that helped collect the K2-18 b spectra data. (Credit: Astrium GmbH)

When looking at the characteristics and features of K2-18 b, all the conditions look right for life to exist on the planet. From being a habitable zone exoplanet to the existence of carbon-bearing molecules to the possibility of a liquid ocean on its surface, K2-18 b is a perfect candidate for a habitable exoplanet. However, there are some characteristics of K2-18 b that could 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 size of Earth. The planet’s large size means that its interior most likely contains a large mantle of highly pressured ice, which is similar to the internal structure of Neptune. Furthermore, K2-18 b’s ocean could 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 is very bright, meaning that some of Webb’s observation data could have been meddled with by the light from K2-18.

To collect the spectrum data, Webb observed K2-18 as the exoplanet crossed in front of the star — a phenomenon known as a transit. When the planet crossed in front of the star, Webb recorded a dip in the star’s brightness and collected data on the planet’s atmosphere as it was illuminated by the star. This data is then sent to the scientists who comb through it and search for signals that represent certain molecules, like carbon dioxide.

This method of using an exoplanet’s transit to collect atmospheric data is employed heavily by scientists researching exoplanets. In fact, many scientists use this method of observing transits to discover exoplanets. K2-18 b was discovered by NASA’s K2 mission in 2015 via a transit.

An example of a light curve generated from an exoplanet transiting its star. (Credit: NASA Ames)

“This result was only possible because of the extended wavelength range and unprecedented sensitivity of Webb, which enabled robust detection of spectral features with just two transits. For comparison, one transit observation with Webb provided comparable precision to eight observations with Hubble conducted over a few years and in a relatively narrow wavelength range,” said Madhusudhan.

Madhusudhan et al. were able to pull the atmospheric data from just two transits of K2-18b in front of its star — once again highlighting the true power and capabilities of Webb in exoplanet science. Webb is set to perform additional observations of K2-18 b in the coming weeks, which will provide the team with even more data to comb through and, hopefully, discover more molecules. Specifically, the team is planning to use 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 way. This means 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 would transform our understanding of our place in the universe. Our findings are a promising step towards a deeper understanding of Hycean worlds in this quest,” said Madhusudhan.

Madhusudhan et al.’s results were recently published in the The Astrophysical Journal Letters journal. 

(Lead image: Artist’s illustration showing K2-18 b and its parent star K2-18. Credit: NASA/ESA/CSA/J. Olmsted (STScI), N. Madhusudhan (Cambridge University))

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