A popular alien-hunting technique is in doubt

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The third factor is the possibility of an inanimate planet generating the observed signal—an equally serious challenge, researchers now realize, entangled in the problem of unimagined abiotic alternatives.

“That's the possibility that we argue that you can't act responsibly,” Vickers said. “It could be anything from almost zero to 1.”

Consider the case of K2-18 b, a “mini-Neptune” whose size is intermediate between that of Earth and Neptune. In 2023, JWST data revealed a statistically weak signal of dimethyl sulfide (DMS) in its atmosphere. On Earth, DMS is produced by marine organisms. The researchers who tentatively discovered K2-18 b interpreted the other gases discovered in its sky as meaning that the planet is a “water world” with a habitable surface ocean, supporting their theory. The DMS there comes from marine life. But other scientists interpret the same observations as evidence of an inaccessible, gaseous planet with a structure similar to that of Neptune.

Unimagined options have already forced astronomers several times to revise their ideas about what constitutes a good biosignature. When phosphine was discovered on Venus, scientists had no idea how it could be produced on the lifeless rocky world. Since then, they have identified several possible inorganic sources of the gas. One scenario is that volcanoes released chemical compounds called phosphides, which could react with sulfur dioxide in Venus's atmosphere to form phosphines – a plausible explanation given that scientists have found evidence of active volcanism on our twin planet. Similarly, oxygen was thought to be a biosignature gas until the 2010s, when researchers including Victoria Meadows at the NASA Astrobiology Institute's Virtual Planetary Laboratory began finding ways that rocky planets without biospheres could accumulate oxygen. For example, oxygen can be formed from sulfur dioxide, which is abundant on worlds as diverse as Venus and Europa.

Today, astronomers have largely abandoned the idea that a single gas could be a biosignature. Instead, they focus on identifying “clusters” or groups of gases that cannot coexist without life. If anything can be called today's gold-standard biosignature, it is the combination of oxygen and methane. Methane degrades rapidly in oxygen-rich environments. On Earth, both gases co-exist only because the biosphere continually supplies them.

So far, scientists have not managed to provide an abiotic explanation for the oxygen-methane biosignature. But Vickers, Smith and Mathis doubt that this particular pair – or perhaps any mixture of gases – will ever be reliable. “There's no way to be certain that what we're seeing is actually the result of life, as opposed to the result of some 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 astronomer at York University who studies exoplanet atmospheres, is more optimistic. She is actively looking for alternative abiotic explanations for biosignatures such as oxygen and methane. Still, she says, “If we see oxygen, methane, and water, and CO, I will open a bottle of champagne—very expensive champagne.”2On an exoplanet.