About 120 light-years away from Earth in the constellation Leo, an exoplanet 8.6 times as massive as Earth revolves around a cool dwarf star K2-18 in the habitable zone of the Universe. Also known as K2-18 b, this planet has a size between Earth and Neptune and is unlike anything in our Solar System. This lack of an equivalent planet in our celestial neighborhood makes it poorly understood, and the nature of its atmosphere is still debated among astronomers.
Discovery of Carbon-Bearing Molecules
A new investigation with NASA's James Webb Space Telescope (JWST) into K2-18 b has unveiled the presence of carbon-bearing molecules like methane and carbon dioxide in its atmosphere. This discovery adds to previous studies that suggest that K2-18 b could be a Hycean exoplanet, a hypothetical planet with a hydrogen-rich atmosphere and a surface covered with water ocean.
Observations with NASA's Hubble Space Telescope provided the first insight into the atmospheric composition of this habitable exoplanet. The data provided prompted further studies that have changed our understanding of our universe.
Since K2-18 b is abundant in methane and carbon dioxide and has a shortage of ammonia, scientists assume there could be water oceans under its hydrogen-rich atmosphere. However, the suggestion that K2-18 b could be a Hycean exoplanet is intriguing, as some astronomers believe that such planets are promising worlds to look for evidence of life.
The initial observation by JWST provides possible detection of dimethyl sulfide (DMS) molecules. This compound can only be produced here on Earth by living organisms such as phytoplanktons in marine environments.
DMS has less robust inference that requires further validation, so the upcoming JWST observations should be able to verify if DMS is present in K2-18 b's atmosphere at significant levels.
Astronomers identify their physical conditions and the types of gases present in characterizing the atmosphere of similar exoplanets. Since they are outshone by the glare of their large parent star, exploring the atmosphere of exoplanets has become particularly challenging.
In the case of K2-18 b's atmosphere, researchers addressed this challenge by studying the light from its parent star as it passes through its atmosphere. Since K2-18 b is a transiting exoplanet, scientists can detect a drop in brightness as it travels across the face of its host star. This was the approach used when this exoplanet was first discovered in 2015. During transits, a tiny amount of starlight passes through the exoplanet's atmosphere before reaching telescopes such as JWST. This passage leaves traces that can be collected to identify the gases in the exoplanet's atmosphere.
Can K2-18 b Support Life?
Although K2-18 b lies in the habitable zone and is known to contain carbon-bearing molecules, this does not necessarily mean that this planet can support life. It has a large size with a radius that is 2.6 times the radius of the Earth, which means that its interior likely contains a thick mantle of high-pressure ice. Like Neptune, it may include a thinner hydrogen-rich atmosphere and an ocean surface. Although Hycean planets are assumed to have a water ocean, it is also possible that the ocean of K2-18 b is too hot to be habitable or to even exist as a liquid.
According to astronomer Nikku Madhusudhan from the University of Cambridge, it is important to consider diverse habitable environments when searching for life elsewhere. Traditionally, the search for extraterrestrial life has primarily focused on smaller rocky planets, but the larger Hycean planets are more conducive to atmospheric analysis.
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