The third factor is the likelihood of a lifeless planet producing the observed signal – an equally serious challenge, researchers now realize, mired in the problem of undiscovered abiotic alternatives.
“That’s the probability that we argue you can’t answer responsibly,” Vickers said. “It can almost range from zero to 1.”
Take the case of K2-18 b, a ‘mini-Neptune’ intermediate in size between Earth and Neptune. In 2023, JWST data revealed a statistically weak sign of dimethyl sulfide (DMS) in the atmosphere. On Earth, DMS is produced by marine organisms. The researchers who tentatively detected it on K2-18 b interpreted the other gases discovered in the air to mean that the planet is a ‘water world’ with a habitable surface ocean, supporting their theory that the DMS there comes from marine life. But other scientists interpret the same observations as evidence of an inhospitable, gaseous planetary composition more similar to that of Neptune.
Unthought of alternatives have forced astrobiologists several times to revise their ideas about what constitutes a good biosignature. When phosphine still existed detected on Venusscientists knew no way it could be produced on a lifeless rocky world. Since then, they have identified several possible solutions abiotic sources of the gas. One scenario is that volcanoes release chemical compounds called phosphides, which could react with sulfur dioxide in Venus’ atmosphere to form phosphine – a plausible explanation given that scientists have found evidence of active volcanism on our twin planet. Similarly, oxygen was considered a biosignature gas until the 2010s, when researchers including Victoria Meadows of the NASA Astrobiology Institute’s Virtual Planetary Laboratory began find ways that rocky planets could do that accumulate oxygen without a biosphere. For example, oxygen can be created of sulfur dioxide, which is abundant on worlds as diverse as Venus and Europa.
Today, astrobiologists have largely abandoned the idea that a single gas could be a biosignature. Instead, they focus on identifying “ensembles,” or sets of gases that could not coexist without life. If anything can be called the current gold standard biosignature, it is the combination of oxygen and methane. Methane breaks down quickly in oxygen-rich atmospheres. On Earth, the two gases only coexist because the biosphere is constantly replenishing them.
So far, scientists have been unable to come up with an abiotic explanation for the biosignatures of oxygen and methane. But Vickers, Smith and Mathis doubt this particular pair – or perhaps a mixture of gases – will ever be convincing. “There’s no way to be sure that what we’re looking at is actually a consequence of life, as opposed to a consequence of some unknown geochemical process,” Smith said.
“JWST is not a life detector. It is a telescope that can tell us which gases are in the atmosphere of a planet,” says Mathis.
Sarah Rugheimer, an astrobiologist at the University of York who studies the atmospheres of exoplanets, is more optimistic. She is actively investigating alternative abiotic explanations for ensemble biosignatures such as oxygen and methane. Yet she says: ‘I would open a bottle of champagne – very expensive champagne – if we saw oxygen, methane and water and CO.2” on an exoplanet.