One of the strongest signs of life outside Earth was announced this week, but some astronomers cautioned that it is extremely difficult to verify. That raises the question: will there ever come a point where we have definitive evidence of extraterrestrial life, and when might that be?
The supposed signs of life were picked up by the James Webb Space Telescope (JWST) from the exoplanet K2-18b, 124 light years away. at the University of Cambridge and his colleagues of dimethyl sulphide (DMS), a molecule that exists in Earth鈥檚 atmosphere only as the result of phytoplankton in the ocean, as well as the related biomolecule dimethyl disulphide. They say the signal has a confidence level of three sigma, equivalent to a 3-in-1000 chance that it turns out to be a fluke.
To prove that a signal is really a biosignature, we would need to answer three questions, says at the Massachusetts Institute of Technology. The first two are 鈥淚s the signal real?鈥� and 鈥淚s the molecule causing it DMS?鈥� Confirming these points would need independent verification from other research groups, using the same dataset that Madhusudhan and his team used, and also further, high-quality data from fresh JWST observations.
Advertisement
Producing a clear signal and identifying the molecules that may be producing it isn鈥檛 a straightforward task. When an exoplanet passes in front of its host star, JWST can measure the light that travels by the planet and use that to infer information about molecules in its atmosphere, but this comes in the form of noisy and unfiltered data. Astronomers then need to 鈥渞educe鈥� the data, which involves removing background noise, and plot it on a graph, which they can then try to fit to simulated or laboratory data to confirm what molecules may have been producing it.
鈥淚f a handful of other groups find that three-sigma detection and they think it鈥檚 resilient to all the different assumptions that are reasonable during the data reduction process, that would be even more convincing,鈥� says at the NASA Ames Research Center in California.
The third, more difficult, question is whether DMS can be produced by non-biological sources or whether it really is coming from life, says Seager. 鈥淥nly when the first two questions are confidently answered does a planet become a viable biosignature gas candidate, and it may remain in that category for decades since the third question may never be fully resolved with the limited data exoplanets offer.鈥�
While the independent analysis and confirmation could feasibly happen in the coming weeks and months, and future JWST observations are likely to occur over the coming years, the debate over what is making DMS 鈥� if it is verified 鈥� could drag on for far longer.
In 2020, at Cardiff University, UK, and her colleagues announced they had spotted phosphine, another molecule associated with life on Earth, in the clouds of Venus. But five years later, astronomers are still tussling over exactly how much phosphine might exist in Venus鈥檚 atmosphere, as well as whether there are more prosaic explanations, such as volcanic eruptions.
Back in 2004, astronomers became excited at potential signs of methane on Mars, when three independent teams found signs of the gas in the planet鈥檚 atmosphere, a potential biosignature. But later instruments, like the Curiosity rover, have failed to make significant detections.
鈥淚t鈥檚 valuable to look at the other debates over molecules associated with life in the solar system, such as methane on Mars or phosphine on Venus,鈥� says . 鈥淭hese debates have not been nicely solved in five years 鈥� they are continuing.鈥�
K2-18b is tens of millions times further away from Earth than Venus or Mars, and establishing proof of any sort of biosignature is likely to be just as tricky as the thorny arguments over phosphine or methane. If the time does come when we can say we have discovered indisputable proof of active alien life, it is likely to be a slow, inching process of building consensus between scientists, rather than the result of a single, headline-grabbing finding.
