The third component is the probability that a lifeless planet will produce the observed signal. Researchers have recognized that an equally serious challenge is involved in the issue of unconsidered abiotic alternatives.
“That's the probability that we say you can't fill it out responsibly,” Vickers said. “It ranges from about zero to one.”
Consider the case of K2-18 b, a “mini-Neptune” that is halfway in size between Earth and Neptune. In 2023, JWST data revealed a statistically weak signature of dimethyl sulfide (DMS) in the atmosphere. On Earth, DMS is produced by marine organisms. Researchers who tentatively detected it on K2-18 b interpreted other gases found in the sky to mean the planet is a “water world” with a habitable surface ocean. , corroborating their theory that DMS there originates from marine life. However, other scientists have interpreted the same observations as evidence for an inhospitable gaseous planetary composition more similar to Neptune.
Unthinkable alternatives have already forced astrobiologists many times to revise their ideas about what makes a good biosignature. When phosphine was detected on Venus, scientists didn't know how to produce it in a lifeless, rocky world. Since then, they have identified several viable abiotic sources of the gas. One scenario is that volcanoes release compounds called phosphides, which can react with sulfur dioxide in Venus' atmosphere to form phosphine. That makes sense, considering scientists have found evidence of active volcanism on the twin planets. Similarly, oxygen was considered a biosignature gas until the 2010s, when researchers including Victoria Meadows of the NASA Astrobiology Institute's Virtual Planets Laboratory began figuring out how rocky planets could accumulate oxygen without a biosphere. It was being done. For example, oxygen is produced from sulfur dioxide, which is abundant on worlds as diverse as Venus and Europe.
Today, astrobiologists have largely abandoned the idea that a single gas can be a biosignature. Instead, they focus on identifying “ensembles,” or collections of gases that cannot coexist without life. If there is one thing that can be considered the gold standard for biosignatures today, it is the combination of oxygen and methane. Methane decomposes rapidly in oxygen-rich atmospheres. On Earth, these two gases only coexist because the biosphere continually replenishes them.
So far, scientists have been unable to find an abiotic explanation for the oxygen and methane biosignatures. But Vickers, Smith and Mathis wonder whether this particular combination, or perhaps any mixture of gases, is convincing. “There's no way to be sure that what we're seeing is actually the result of life, rather than the result of some unknown geochemical process,” Smith says.
“JWST is not a life detector. It's a telescope that can tell us what kind of gases are in a planet's atmosphere,” Mattis said.
Sarah Lugheimer, an astrobiologist at the University of York who studies the atmospheres of exoplanets, is more optimistic. She is actively researching alternative abiotic explanations for her ensemble biosignatures, such as oxygen and methane. Still, she says: “If I saw oxygen, methane, water and carbon dioxide, I would have opened a bottle of champagne – very expensive champagne.”2” on an exoplanet.