In June, astronomers reported a disappointing discovery: The James Webb Space Telescope failed to find a thick atmosphere around the rocky planet TRAPPIST-1 C, an exoplanet in one of the most tantalizing planetary systems in the search for alien life.

The finding follows similar news regarding neighboring planet TRAPPIST-1 B, another planet in the TRAPPIST-1 system. Its dim, red star hosts seven rocky worlds, a few of which are in the habitable zone—at a distance from their star at which liquid water could exist on their surfaces and otherworldly life might thrive.

What it would take to detect that life, if it exists, isn’t a new question. But thanks to the JWST, it’s finally becoming a practical one. In the next few years, the telescope could glimpse the atmospheres of several promising planets orbiting distant stars. Hidden away in the chemistry of those atmospheres may be the first hints of life beyond our solar system. This presents a sticky problem: What qualifies as a true chemical signature of life?

“You’re trying to take very little information about a planet and make a conclusion that is potentially quite profound—changing our view of the whole universe,” says planetary scientist Joshua Krissansen-Totton of the University of Washington.

To detect such a biosignature, scientists must find clever ways to work with the limited information they can glean by observing exoplanets.

Chemicals in context

Even the most powerful telescopes, including the JWST, almost never “see” exoplanets—by and large, astronomers know these distant worlds only by the flickering of their stars.

Instead of viewing planets directly, astronomers train their telescopes on stars and wait for planets to “transit,” or pass between, their sun and the telescope. As a planet transits, a bit of starlight filters through its atmosphere and dims the star at certain wavelengths, depending on the chemicals in the atmosphere. The resulting dips and peaks in the star’s brightness are like a chemical barcode for the transiting planet.

Perhaps the most intuitive way to look for a biosignature in that barcode is to scour it for a gas that was clearly produced by life. For a time scientists thought that oxygen, which is abundant on Earth because of photosynthesis, served as a stand-alone biosignature. But oxygen can arise from other processes: Sunlight could break apart water in the planet’s atmosphere, for example.

And that problem isn’t unique to oxygen—most of the gases that living things produce can also arise without life. So instead of treating single gases as biosignatures in their own right, scientists today tend to consider them in context.

Methane, for instance, can be produced both with and without life. It wouldn’t be a convincing biosignature on its own. But finding methane and oxygen together “would be hugely exciting,” says planetary scientist Robin Wordsworth of Harvard University; it’s very difficult to produce that combination without life. Likewise, work by Krissansen-Totton and colleagues recently showed that finding methane along with the right amounts of other gases, such as carbon dioxide, would be hard to explain without life.

Watching how an exoplanet atmosphere changes over time might also provide valuable context that could strengthen otherwise weak biosignatures. Seasonal variations in the concentration of ozone, for example, could be a fingerprint of life, scientists reported in 2018.

Surprises, not assumptions

Of course, “if you’re looking for individual gases like oxygen or methane, then built into that are assumptions about what type of life is elsewhere,” says Krissansen-Totton. So some scientists are developing agnostic biosignatures that don’t assume alien biochemistry will be anything like Earth’s biochemistry.

One possible agnostic biosignature is an exoplanet atmosphere’s degree of chemical “surprisingness”—what scientists call chemical disequilibrium.

An atmosphere close to equilibrium would be chemically uninteresting, a bit like a closed flask of gas in a laboratory. Of course, no planet is as boring as a lab flask. Chemical reactions in a planet’s atmosphere can be powered by their stars, and geological processes like volcanic activity can increase disequilibrium, and thus increase the chemical surprisingness of the atmosphere.