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Is There Life on K2-18b?
LIFE ON A DISTANT PLANET: AS DATA ARE DICED, THE SIGNS GET BLURRED
Context: In April, astronomers announced potential signs of life on a distant exoplanet named K2-18b, located over 120 light years from Earth.
What is K2-18b?
- K2-18b is an exoplanet, orbits the cool red dwarf star K2-18, situated in the Leo constellation.
- Lies within the habitable zone of its star, where liquid water could potentially exist.
- Approximately 2.6 times the size of Earth and 8.6 times its mass.
- Classified as a sub-Neptune—a type of planet not found in our solar system but believed to have hydrogen-rich atmospheres and possibly liquid water oceans.
Why Does K2-18b Matters?
- K2-18b is an exoplanet (a planet that orbits a star outside our solar system) not directly visible to the naked eye or conventional telescopes due to its vast distance.
- Unlike nearby planets like Jupiter, which reflect sunlight, K2-18b requires advanced space telescopes to observe.
The Role of the James Webb Space Telescope
- In 2023, astronomer Nikku Madhusudhan from the University of Cambridge and his team used the James Webb Space Telescope (JWST) to study K2-18b.
- They observed the planet as it transited its host star, using JWST’s near-infrared instruments.
- The starlight passing through the planet’s atmosphere showed signatures of hydrogen, carbon dioxide, and methane.
- Additionally, they found suggestive hints of dimethyl sulfide (DMS) — a molecule on Earth only produced by life, specifically by photosynthetic ocean microbes.
- In 2024, a second round of observations using a mid-infrared instrument on JWST again detected a signature of dimethyl sulfide, seemingly stronger than the first.
Counter-Analyses Question the Finding
- Rafael Luque from the University of Chicago analysed combined data from both the near-infrared and mid-infrared ranges. His team found strong evidence for hydrogen, carbon dioxide, and methane, but no clear evidence of dimethyl sulfide.
- Luis Welbanks, an astronomer at Arizona State University and former student of Madhusudhan, took a different approach:
- His team asked whether the mid-infrared signal had to come from DMS.
- They tested 90 different molecules plausible on a planet like K2-18b.
- Result: The signal could be explained by 59 other molecules.
- The strongest candidate was propyne, a gas commonly used by welders as fuel.
Ongoing Debate and Response
- Welbanks’s team did not claim propyne is present on K2-18b.
- They emphasised that the faint atmospheric data can lead to ambiguous spectral patterns, making it difficult to attribute the signal to a single molecule.
- Conclusion: The available data is too limited to claim any evidence of life.
- In response, Madhusudhan’s team conducted a broader study analysing 650 possible molecules in K2-18b’s atmosphere. Dimethyl sulfide was still among the top-ranked candidates.
