Every time a planet passes in front of its star, a tiny fraction of starlight filters through the planet’s atmosphere. Molecules imprint that light with spectral lines—chemical barcodes that telescopes can read from light‑years away. With these clues, astronomers are building weather reports for alien worlds.
Single molecules rarely prove biology. Methane can arise from life but also from volcanoes and hydrothermal chemistry. The power comes from context: mixtures, abundances, and the planet’s environment. A combination such as methane with carbon dioxide and very little carbon monoxide could be hard to sustain without continuous biological replenishment.
Clouds complicate matters by muting spectral features, and stellar activity can masquerade as atmospheric signals. Careful modeling, repeated observations, and comparisons across wavelengths are essential to avoid false positives.
Even a strong biosignature would be an inference, not a photograph of microbes. That’s why researchers emphasize cautious language and independent checks. Over the next decade, coordinated campaigns with space and ground telescopes will expand the sample from hot giants to temperate Earth‑sized planets around small stars.
If we do see a persuasive chemical imbalance on a rocky world, it will not answer every question. But it will tell us that life’s chemistry is not unique to Earth—and that alone would be a profound shift in our place in the cosmos.