Astronomy
Detecting Biosignatures in Rocky Exoplanet Atmospheres with JWST
Quick fact
In 2023, JWST tentatively detected carbon dioxide in the atmosphere of a rocky exoplanet (TRAPPIST-1 c), a milestone that brings us closer to finding gases that could signal life.
Why this is interesting
What if we could smell the air of a planet hundreds of light-years away? Scientists are doing exactly that with the James Webb Space Telescope.
Read the full explanation
Understanding Detecting Biosignatures in Rocky Exoplanet Atmospheres with JWST
Imagine a planet orbiting a small, cool star. When the planet passes in front of its star (a transit), some starlight filters through the planet's atmosphere. This light carries fingerprints of the gases it encountered. By splitting that light into a spectrum, astronomers can see which wavelengths are missing, revealing which molecules are present. For rocky planets, the atmosphere is thin, so the signal is faint. JWST, with its large mirror and infrared sensitivity, can capture this faint signal. The goal is to find combinations of gases that are hard to produce without life, like oxygen and methane together, which are a strong biosignature.
A deeper explanation
The mechanism is transmission spectroscopy. As starlight passes through the planet's atmosphere, each gas absorbs specific wavelengths, creating dark lines in the spectrum. The depth and pattern of these lines tell us the atmosphere's composition and density. For rocky planets, the atmosphere is small, so the signal is tiny—something like a 1% dip in the planet's transit depth. JWST's precision is needed to detect this. Moreover, not every gas is a good biosignature. Oxygen alone can be produced without life, but oxygen plus methane together are unstable in the atmosphere unless something continually produces them, which often points to biology. The challenge is that M-dwarf stars, which host many of these rocky planets, are active and can mimic biosignatures with false positives. The concept matters because it's our first practical way to search for life beyond our solar system.