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MIT researchers publish a legend for alien hunting

If alien life is nearby and intelligent, finding it likely won’t be a problem for long. Intelligent life is assumed to always work its way toward some form of radiation-producing technology that we will someday see and recognize. But what if the only life within a reasonable distance is non-intelligent, or early in its cultural development? Finding the Romulan Empire would certainly be cool, but for scientists the prospect of a world covered in alien algae could be just as exciting. So, if we’re looking for alien algae worlds, or primitive forest moons, or even major societies intentionally hiding away, we can’t wait for life to call out to us. We’re going to need to figure out what life looks like all on its own.

That’s what astrobiologists have been trying to do for decades — figure out the signatures of life they ought to be looking for on alien worlds. Traditionally, this has meant looking for certain telltale components in the planet’s atmosphere, compounds that are assumed to be impossible, or at least improbably, in a non-living environment. Finding a planet with an atmosphere rich in oxygen gas, for instance, certainly won’t prove the planet hosts life, but it does provide a nice flag for worlds worthy of further investigation. This search for life has often concerned itself with such human-style products, things like O2 and methane gas. But now astrobiologists from MIT have a new approach to solving this old problem: Stop biasing the search for life based on what life looks like here on Earth, and look for literally every biosignature molecule that’s chemically possible.

Actually, the researchers do put some limits on their enormous database of hypothetical molecules. The core assumption is that any volatile, stable compound can be a potential signature of life — that is, any chemical that will both enter the atmosphere as a gas (volatility, or the tendency to vaporize) and stay there long enough to build up to detectable levels (stability, or the tendency to not spontaneously break apart). But that’s a really unmanageable number of molecules — remember we only collected the first direct image of an extra-solar planet in 2004, so we can’t make just any request of our still bleeding-edge planet detectors.

Exoplanet Kepler-452bSo, the researchers start by compiling a list of all these compounds, algorithmically combining atoms in every possible combination until it came up with all molecules with between 2 and 6 non-hydrogen atoms — a cutoff chosen purely to avoid the next exponential step up the complexity scale to 7 non-hydrogen atoms. Their algorithm combined the elements most crucial for life — Carbon, Nitrogen, Oxygen, Phosphorus, Sulfur, and Hydrogen, or CNOPSH — and then it recombined them in different physical arrangements, or isomers. The resulting lest of tens of thousands of molecules long, then rated along a volatility scale to weed out the bad candidates.

The enormous breadth of targets creates the opposite problem we had before. We’re unlikely to miss any potential signifiers for life, but it could also bog down the search for life by introducing literally thousands of wild geese to chase around the galaxy. In the end, the very fair assumption is that it’s better to err on the side of searching too many planets than too few, to be too open minded rather than too closed.

We can also easily imagine a biome that defies the expectations of this model to slip by even its wide net. Perhaps one species eats and sequesters the gaseous products of another and silencing the effect on the atmosphere. Or maybe it outputs only boring non-life-signifying gasses, or thickens the upper atmosphere so much that we can’t see the life underneath. We will almost certainly go into the hunt for life with narrower expectations than we ought to. But if that’s unavoidable, then this sort of wide searching is a good start.

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