Image: NASA
If aliens aren't going to visit us in a flying saucer, we've got to find a way to determine whether they're out there, minding their own business on some faraway exoplanet. Unfortunately, one of the best proposed methods of doing it is likely to result in a whole bunch of false positives, according to a researcher at MIT.There's a lot of ways that astronomers and SETI researchers are looking for inhabited planets—we're looking into intercepting radio transmissions, sending some out ourselves, looking for biological components on Mars, and even looking for alien pollution—but analyzing exoplanets' atmospheres for "biosphere gases" (the ones given off by life) is one of the more promising and realistic techniques.At least, that's what we thought. But a new paper published inProceedings of the National Academies of Sciences by Sara Seager, a planetary researcher at MIT, suggests that maybe atmosphere analysis isn't all it's cracked up to be.That's because exoplanets are incredibly diverse, and the gases we're likely to run into (and that could be suspected to be coming from living things) could have perfectly normal geological origins."False positives will, in many cases, be a problem, and in the end, we will have to develop a framework for assigning a probability to a given planet to have signs of life," she wrote.In other words, it's entirely possible that we find a planet in which the atmosphere is made up of oxygen or methane or some other gas we're looking for, and it turns out to have no life whatsoever. And, when we're talking aliens, false positives are a pretty huge deal, as is any sort of uncertainty.Rather than making contact with aliens or having some sort of definitive knowledge that they exist, instead we'll probably enter a time period where humans are pretty sure there's life out there somewhere, and where we're pretty sure where it might be living, but one in which we don't really know for sure. Which, while exciting, is kind of a bummer.But let's back up for a quick second. Part of the problem with measuring atmosphere composition is the fact that we're way over here, on Earth, and exoplanets are way out there, light years away.Despite that fundamental problem, astronomers have managed to do rudimentary atmospheric analysis in much the same way that we discover planets in the first place—by tracking their transit across its star. Basically, we can measure the wavelengths of light and fumes that are coming from a star before and after a planet crosses in front of it (relative to a telescope), and can do some fancy computations to figure out what changed. So far, we've done it for a couple dozen planets, a development that as Seager writes, has "raised more questions than answers."That's because we've learned that many of the gases that are produced by life also have geological or other bases: "Many different gases are produced by life, but the anticipated diversity of exoplanet atmosphere composition and host star environments may yield different detectable biosignature gases than the terrestrial examples," she wrote.In other words, we know what gases can be attributed to life here on Earth, but we have a less clear idea of what gases could be attributed to life elsewhere.In any case, atmospheric analysis isn't going anywhere, and NASA is doubling down on it. The planned James Webb Space Telescope will help us more accurately detect atmospheric makeup, and a mission planed for 2017, the Transiting Exoplanet Survey Satellite, has the same general idea. The thinking is, if we can get a catalogue of atmospheric makeups that's large enough, we can figure out which ones are anomalous more easily.To be "realistic about assessing the probability for life under the multitude of false positives, we need larger numbers of Earth-like planet candidates and not just one or two crudely measured planet atmospheres to argue over," she wrote. In turn, we might be able to argue all about where life might be—but we probably won't know for sure. And that's kind of a bummer.
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