The third factor is the likelihood that a lifeless planet would produce the observed signal—an equally serious challenge, researchers now realize, tied to the problem of unimaginable abiotic alternatives.
“That’s the probability that we’re arguing that you can’t step in responsibly,” Vickers said. “It could be almost anything between zero and one.”
Consider the case of K2-18 b, a “mini-Neptune” intermediate in size between Earth and Neptune. In 2023, JWST data showed a statistically weak sign of dimethyl sulfide (DMS) in its atmosphere. On Earth, DMS is produced by marine organisms. The researchers who tentatively discovered it on K2-18 b interpreted the other gases detected in its sky to mean that the planet was a “water world” with a habitable surface ocean, supporting their theory that the DMS there came from marine life. However, other scientists interpret the same observations as evidence of an inhospitable, gaseous planetary composition more similar to that of Neptune.
Unimaginable alternatives have repeatedly forced astrobiologists to rethink their ideas about what constitutes a good biosignature. When phosphine was discovered on Venus, scientists didn’t know how it could be produced on a lifeless, rocky world. Since then, they have identified several possible abiotic gas sources. One scenario suggests 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. Likewise, oxygen was considered a biosignature gas until the 2010s, when researchers like Victoria Meadows of the NASA Astrobiology Institute’s Virtual Planetary Laboratory began finding ways for rocky planets without a biosphere to accumulate oxygen. For example, oxygen can form from 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 groups of gases that could not coexist without life. If anything can be called the biosignature of today’s gold standard, it is the combination of oxygen and methane. Methane breaks down quickly in oxygen-rich atmospheres. On Earth, the two gases only exist side by side because the biosphere continually replenishes them.
So far, scientists have not been able to find an abiotic explanation for the oxygen-methane biosignatures. But Vickers, Smith and Mathis doubt that this particular pair – or perhaps any mixture of gases – will ever convince. “There is no way to be sure that what we are seeing is actually a consequence of life and not the result of an unknown geochemical process,” Smith said.
“JWST is not a life detector. “It’s a telescope that can tell us what gases are in a planet’s atmosphere,” Mathis said.
Sarah Rugheimer, an astrobiologist at York University who studies the atmospheres of exoplanets, is more optimistic. She is actively searching for alternative abiotic explanations for ensemble biosignatures such as oxygen and methane. Still, she says, “I would open a bottle of champagne – very expensive champagne – if we saw oxygen, methane, water and CO.”2” on an exoplanet.