K2-18b

K2-18b was first detected because it passes in front of its host star, causing tiny, regular dips in brightness. When the planet transits, some starlight filters through its atmosphere, imprinting spectral fingerprints that telescopes can measure. Interpreting those spectra requires careful modelling, and small differences in assumptions can change whether a signal looks like water vapour, methane, or something else. The observations show how we can learn about distant worlds without ever visiting them. By analyzing the light that passes through an exoplanet's atmosphere, we can infer its composition and potential for life.

If life exists on K2-18b, it grew under a red sun, in oceans and atmospheres unlike our own. Stromatolite builders on Earth turned light and water into stone records. On a distant world, some other chemistry might leave different traces for future observers. Thinking about those possibilities sharpens our appreciation of the single biosphere we can study up close. K2-18b represents the possibility that life might exist elsewhere in the universe. It challenges us to think about what life might look like in environments very different from Earth, and how we might recognize it if we find it.