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Methane, carbon dioxide discovered in exoplanet atmosphere

An illustration of what exoplanet K2-18 b could look like based on science data of its atmosphere. Credit: NASA/ESA/CSA/Joseph Olmsted (STScI)

An illustration of what exoplanet K2-18 b could look like based on science data of its atmosphere. Credit: NASA/ESA/CSA/Joseph Olmsted (STScI)

NASA says data from the James Webb Space Telescope shows evidence of “carbon-bearing” molecules in the atmosphere of K2-18 b, an exoplanet located some 120 light years away around its host star’s habitable zone.

During two recent observations, Webb revealed the presence of methane and carbon dioxide in K2-18 b’s atmosphere, adding to previous Hubble Space Telescope observations that showed evidence the planet possesses a hydrogen-rich atmosphere and a water ocean-covered surface, according to NASA.

K2-18 b was discovered in 2015 by the Kepler Space Telescope. It orbits its host red dwarf star every 33 days at a distance of about 13 million miles, roughly one-third the distance between Mercury and the Sun in our solar system.

However, because red dwarf stars are smaller and cooler compared to the Sun, this exoplanet is in the star’s habitable zone, an area where water could be liquid on a planet’s surface under the right circumstances.

K2-18 b is about 8.6 times as massive as Earth and classified as a “mini-Neptune,” meaning it has a mass greater than Earth, but less than Neptune. This type of planet does not exist in our solar system and, according to NASA, this category of exoplanets is poorly understood and the nature of their atmospheres “is a matter of active debate among astronomers.”

“Our findings underscore the importance of considering diverse habitable environments in the search for life elsewhere,” Nikku Madhusudhan, an astronomer at the University of Cambridge and lead author of the paper announcing these results, said in a Sept. 11 NASA news release. “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.”

K2-18 b is thought to potentially be a “hycean” world, which is a planet that is hot and water-covered with a hydrogen atmosphere. NASA said this type of exoplanet is believed by astronomers as having promising environments for life.

NASA said the abundance of methane and carbon dioxide with a shortage of ammonia supports the hypothesis that there might be water oceans underneath K2-18 b’s hydrogen rich atmosphere.

Another molecule potentially detected was dimethyl sulfide, also called DMS. On Earth, this molecule is only produced by life, typically from phytoplankton in marine environments, according to NASA.

The spectra of the atmosphere of K2-18 b obtained by the Webb telescope. Credit: NASA

The spectra of the atmosphere of K2-18 b obtained by the Webb telescope. Credit: NASA

But scientists say there is not enough data to confirm the presence of DMS and additional observations of the planet will be required to understand if it is actually present and at significant levels.

NASA cautioned, however, that even if all of these molecules are confirmed, it doesn’t mean the planet can support life.

The agency said the planet has a radius 2.6 times that of Earth and may have a mantle of high-pressure ice, like Neptune, but a thinner hydrogen rich atmosphere and an ocean surface. Still, NASA said its possible a K2-18 b ocean might be too hot to be habitable, or even liquid.

K2-18 b is outshined by its host star and cannot be directly observed by Webb. As such, astronomers have to use the “transit method” to study the exoplanet.

When a planet passes in front of its parent star, the star’s brightness dims ever so slightly. Detectors on Webb, or any other telescope observing such a transit, can measure the difference in brightness, as well as the composition of light that passes through the planet’s atmosphere as the photons make their way to our solar system.

NASA said an analysis of that light can be done to piece together the gasses of the exoplanet’s atmosphere.

“These results [from K2-18 b] are the product of just two observations of K2-18 b, with many more on the way,” team member Savvas Constantinou of the University of Cambridge said in NASA’s news release. “This means our work here is but an early demonstration of what Webb can observe in habitable-zone exoplanets.”

Video courtesy of Cambridge University


Derek Richardson has a degree in mass media, with an emphasis in contemporary journalism, from Washburn University in Topeka, Kansas. While at Washburn, he was the managing editor of the student run newspaper, the Washburn Review. He also has a website about human spaceflight called Orbital Velocity. You can find him on twitter @TheSpaceWriter.

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