JWST Exoplanet Atmosphere: Carbon Molecules Found on K2-18 b

The search for life beyond our solar system recently took a significant leap forward thanks to the James Webb Space Telescope (JWST). Astronomers using the observatory have detected definitive evidence of carbon-bearing molecules—specifically methane and carbon dioxide—in the atmosphere of K2-18 b. This exoplanet is located 120 light-years away in the constellation Leo and sits squarely within the habitable zone of its host star.

The Discovery of Methane and Carbon Dioxide

The data returned from Webb was analyzed by a team at the University of Cambridge and provides a fascinating chemical profile of K2-18 b. The telescope’s Near-Infrared Imager and Slitless Spectrograph (NIRISS) and Near-Infrared Spectrograph (NIRSpec) captured the chemical fingerprint of the planet as it passed in front of its star.

The analysis revealed two massive findings:

• Abundant Methane: The presence of methane is often a byproduct of geological or biological processes. On gas giants like Jupiter, it is abiotic. However, on a smaller, cooler world, it raises questions about internal chemistry.
• Carbon Dioxide: This is a key indicator of the atmosphere’s composition and evolution (metallicity).

Crucially, the telescope did not detect significant amounts of ammonia. This specific chemical ratio—high methane and carbon dioxide combined with a shortage of ammonia—supports the hypothesis that K2-18 b may possess a water ocean underneath a hydrogen-rich atmosphere.

Understanding K2-18 b and "Hycean" Worlds

K2-18 b is not a second Earth. It is classified as a “sub-Neptune,” with a mass approximately 8.6 times that of our planet. Sub-Neptunes are the most common type of planet found in the galaxy so far, yet our own solar system does not have one. This makes K2-18 b a mystery that astronomers are eager to solve.

The chemical findings support the theory that K2-18 b is a “Hycean” exoplanet. The term Hycean is a combination of “hydrogen” and “ocean.” These hypothetical worlds are hot, covered in water, and possess thick hydrogen atmospheres.

Why Hycean Worlds Matter

Traditionally, the search for life has focused on rocky planets similar to Earth or Mars. However, Hycean worlds broaden the target list significantly.

• Larger Habitable Zones: Hydrogen atmospheres trap heat effectively. This means Hycean planets can be much farther from their stars than Earth is from the Sun and still maintain liquid water.
• Easier to Study: Because these planets have thick, puffy atmospheres and are larger than Earth-sized rocks, they are much easier for telescopes like Webb to analyze.

K2-18 b orbits a cool dwarf star K2-18 in the habitable zone (the “Goldilocks” zone), where temperatures allow for the existence of liquid water.

The Potential Hint of Life: Dimethyl Sulfide (DMS)

While the confirmation of carbon dioxide and methane is scientifically solid, the data provided a tantalizing hint of something else: Dimethyl Sulfide (DMS).

On Earth, DMS is produced almost exclusively by life. The primary producers are phytoplankton in marine environments. The presence of DMS in the atmosphere of K2-18 b would be a massive indicator of biological activity.

Important Caveat: The detection of DMS on K2-18 b is not yet confirmed. The signal in the data is weak and overlaps with other chemical signals. NASA and the research team, led by Nikku Madhusudhan, have been extremely cautious. They state that while the data suggests the presence of DMS, much more observation is required to validate it. It is entirely possible that future readings will rule this molecule out.

How Webb Analyzes Atmospheres

Understanding how scientists know what is in the air of a planet 120 light-years away requires looking at a technique called transmission spectroscopy.

1. The Transit: As K2-18 b orbits, it passes between its star and the James Webb Space Telescope.
2. Filtering Light: A tiny fraction of the starlight passes through the planet’s atmosphere on its way to the telescope.
3. Absorption: Different gas molecules absorb specific colors (wavelengths) of light. Methane absorbs one specific wavelength, while carbon dioxide absorbs another.
4. The Spectrum: Webb splits the light into a spectrum (a rainbow of data). By looking at which colors are missing or dimmer, scientists can determine exactly which gases are present and in what quantities.

Webb’s sensitivity is roughly 100 times greater than previous telescopes like Hubble, allowing it to detect these distinct carbon signatures that were previously invisible.

Implications for Future Research

The findings on K2-18 b demonstrate that the James Webb Space Telescope is fully capable of characterizing the atmospheres of habitable-zone exoplanets. This was a primary goal of the mission, and K2-18 b serves as a successful proof of concept.

The team intends to conduct follow-up research using the telescope’s MIRI (Mid-Infrared Instrument) spectrograph. These future observations aim to validate the DMS finding and provide a clearer picture of the planet’s environmental conditions.

Even if K2-18 b turns out to be lifeless, the confirmation of carbon molecules on a habitable-zone sub-Neptune fundamentally changes our understanding of planetary formation. It proves that complex carbon chemistry exists on worlds that look nothing like our own.

Frequently Asked Questions

Is K2-18 b habitable? K2-18 b is located in the habitable zone, meaning liquid water could exist on its surface. However, the planet likely has a massive hydrogen envelope and a high-pressure ocean. While the conditions might support microbial life (specifically extremophiles), the pressure and temperature could be too intense for life as we know it on the surface.

Did NASA find life on K2-18 b? No. NASA found methane and carbon dioxide, which are carbon-bearing molecules. They also found a potential signal for Dimethyl Sulfide (DMS), a molecule associated with life on Earth. However, the DMS detection is weak and requires further verification before any conclusions can be drawn.

How far away is K2-18 b? The planet is approximately 120 light-years from Earth in the constellation Leo.

What is a Hycean planet? A Hycean planet is a theoretical class of exoplanet that is hot, covered in a global ocean, and possesses a thick hydrogen-rich atmosphere. K2-18 b is the leading candidate for being the first confirmed Hycean world.

Why is the lack of ammonia important? The absence of ammonia is significant because, in a hydrogen-rich atmosphere, ammonia should naturally be abundant. If it is missing, it implies that liquid water is dissolving the ammonia. This supports the theory that K2-18 b has a surface ocean.