JWST Detects Atmosphere on Super-Earth
The search for habitable conditions outside our solar system has taken a significant leap forward. NASA’s James Webb Space Telescope (JWST) has analyzed the atmosphere of K2-18 b, an exoplanet 8.6 times as massive as Earth. The telescope detected clear signatures of carbon-bearing molecules, specifically methane and carbon dioxide. These findings support the hypothesis that K2-18 b could be a “Hycean” world—a planet with a hydrogen-rich atmosphere covering a global water ocean.
The Discovery of Methane and Carbon Dioxide
In observations reported by NASA in September 2023, astronomers used JWST to peer through the atmosphere of K2-18 b. The data returned distinct chemical fingerprints for methane and carbon dioxide.
The presence of these carbon-based molecules is exciting, but what makes this discovery truly compelling is what the telescope did not find. The spectral data showed a shortage of ammonia. According to atmospheric models, the combination of high methane, high carbon dioxide, and low ammonia is consistent with a planet that possesses a surface ocean under a thick hydrogen atmosphere.
This specific chemical cocktail helps astronomers classify K2-18 b. It is not quite a rocky planet like Earth, nor is it a gas giant like Neptune. It sits in a category known as “sub-Neptunes,” which are the most common type of planet known in the galaxy, even though our solar system lacks one.
Understanding K2-18 b and the "Hycean" Theory
K2-18 b orbits the cool dwarf star K2-18 in the constellation Leo, roughly 120 light-years from Earth. It resides within the “habitable zone” (or Goldilocks zone) of its star, meaning it is located at a distance where liquid water could theoretically exist on the surface without freezing or boiling away.
While the planet was first discovered by the Kepler Space Telescope, and later observed by Hubble in 2019 to contain water vapor, those earlier instruments lacked the sensitivity to determine the atmosphere’s full composition.
The new data from Webb supports the “Hycean” planet theory. The term Hycean is a combination of “hydrogen” and “ocean.” These worlds are massive, hot, and covered in water. Because they are generally larger than Earth and have extensive atmospheres, they are easier for telescopes like JWST to study. The discovery proves that researchers do not need to limit their search for life solely to rocky, Earth-sized planets.
The Intriguing Hint of Dimethyl Sulfide (DMS)
The most provocative aspect of the JWST data is a weaker, yet potential, detection of a molecule called dimethyl sulfide (DMS).
On Earth, DMS is produced almost exclusively by life. The primary source of DMS in Earth’s atmosphere is phytoplankton in marine environments. The presence of this molecule on K2-18 b could theoretically suggest biological activity.
However, the team led by Nikku Madhusudhan, an astronomer at the University of Cambridge, urges caution. The signal for DMS is not as strong as the signals for methane and carbon dioxide. It requires further validation to be confirmed. Madhusudhan noted that while the inference of DMS is fascinating, more observations with Webb’s Mid-Infrared Instrument (MIRI) will be necessary to definitively confirm or rule out its presence.
How JWST Analyzed the Planet
Astronomers analyzed K2-18 b using a method called transmission spectroscopy. The planet does not emit its own light; instead, astronomers wait for the planet to pass in front of its host star.
As the starlight passes through the planet’s atmosphere, different gas molecules absorb specific wavelengths of light. By analyzing the light that makes it through to the telescope, scientists can read the “barcodes” of the chemicals present in the atmosphere.
For this study, the team utilized two specific instruments on the Webb telescope:
- NIRISS: Near-Infrared Imager and Slitless Spectrograph
- NIRSpec: Near-Infrared Spectrograph
These instruments provided a spectrum resolution far clearer than what was possible with the Hubble Space Telescope. While Hubble detected the presence of water vapor, it was the precision of JWST that allowed scientists to quantify the abundance of methane and carbon dioxide.
Challenges for Habitability
While the term “habitable zone” sounds promising, K2-18 b is not necessarily a pleasant place to visit. The planet is 2.6 times the radius of Earth and has a powerful gravity.
Furthermore, a “Hycean” world creates a greenhouse effect. The thick hydrogen atmosphere likely traps immense amounts of heat. Even if a water ocean exists, the temperatures could be high enough to make the ocean boil or reach a supercritical state where the distinction between liquid and gas blurs.
Researchers must also consider the host star. K2-18 is a red dwarf. These stars are known for being active and emitting high-energy flares that can strip atmospheres away or sterilize planetary surfaces. However, the detection of a stable atmosphere suggests K2-18 b has survived its star’s activity so far.
Why This Changes Exoplanet Research
This finding marks a shift in strategy for astronomers. Previously, the search for biosignatures focused heavily on finding “Earth 2.0”—rocky planets with thin, nitrogen-rich atmospheres.
The K2-18 b findings suggest that life could potentially survive in conditions vastly different from Earth. By targeting Hycean worlds, which are larger and have “puffier” atmospheres, scientists can gather data much faster than they can with smaller, rocky worlds.
Upcoming observations with the MIRI instrument will focus on the mid-infrared range. This will help clarify the DMS detection and provide more data on the planet’s thermal structure. If the DMS detection is confirmed, K2-18 b will become the primary target for exoplanet biology research for decades to come.
Frequently Asked Questions
Did JWST find life on K2-18 b? No. The telescope found methane and carbon dioxide, which suggests an ocean might exist. It also found a possible hint of dimethyl sulfide (DMS), a molecule created by life on Earth, but this specific detection is weak and not yet confirmed.
How far away is K2-18 b? The planet is approximately 120 light-years from Earth located in the constellation Leo.
What is a Hycean planet? A Hycean planet is a theoretical class of exoplanet that is hot, up to 2.6 times larger than Earth, and possesses a hydrogen-rich atmosphere capable of supporting a global water ocean.
Why are methane and carbon dioxide important? Finding these two molecules without ammonia suggests a chemical equilibrium consistent with liquid water oceans. Without an ocean, scientists would expect to see different ratios of these gases.
Can humans live on K2-18 b? Likely not. The gravity is significantly stronger than on Earth, the atmospheric pressure is likely crushing, and the temperatures may be boiling. The search for habitability refers to potential microbial life, not necessarily human colonization.