“It is dangerous to assume life is common across the Universe.” These were the words of Charles Cockell at a Royal Society event on March 11 this year. While many people have freely debated the existence of extraterrestrial life, Cockell’s words carry a bit more weight than most. He happens to be the director of the U.K. Center for Astrobiology, based at the University of Edinburgh.

Bringing to mind the argument made by Fermi’s paradox — if the universe is teeming with life, where exactly is everyone? — this may seem at first to be a slightly pessimistic outlook. Evidently, however, the intention is not so much to pour cold water on the astrobiology research community, but to call into question our assumptions in the search for life elsewhere.

As Cockell went on to explain, ”People are encouraged to think that not finding signs of life is a ‘failure’ when in fact it would tell us a lot about the origins of life.”

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In fact, this one single statement sums up what is potentially the biggest problem in astrobiology. We, quite simply, have no idea how life started. We don’t know where, how, when, or why molecules managed to replicate themselves into ever-more-complex forms and, eventually, living organisms.

Earth is the only example of a living planet that we have in the entire universe, and we don’t even understand it. Even now, biologists can still argue over what really constitutes a living thing, highlighted a couple of years ago by the discovery of giant viruses with genomes larger than some bacteria. And even if we did fully understand life here, there’s nothing to say it would be the same elsewhere.

Purely because we know life can exist on a planet like ours, with liquid water and at an appropriate distance from our star, we’re forced to search for life in similar situations. Habitable “goldilocks” zones are the target of planet-hunting missions like Kepler. The hope is that if we can find another Earth, then we can find another place that might have life.

But what if life doesn’t want to play ball? In his statement, Cockell warned that simply finding such a planet may not be enough: “On our planet, carbon leaches into most habitat space and provides energy for microorganisms to live. There are only a few vacant habitats that may persist for any length of time on Earth, but we cannot assume that this is the case on other planets.”

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Is our planet a verdant oasis in an arid desert of a galaxy? Or, like in a desert, is there life we can’t readily see? In questioning how effective our current approach may be, Cockell highlights that there are currently three criteria that astrobiologists are really paying attention to.

First are biomarkers, signature gasses that we might recognize as having been created by living things. For example, on Earth, the oxygen, which makes up 21 percent of our atmosphere, is all created by living things. If there was no photosynthesis, Earth’s atmosphere would rapidly become devoid of oxygen.

Then there’s the criterion that alien life will widely colonize its home world. Again, on Earth, living things exist everywhere. “There are only a few vacant habitats that may persist for any length of time on Earth, but we cannot assume that this is the case on other planets,” Cockell said, pointing out that the pervasiveness of life on Earth leads us to make assumptions about life elsewhere.

Third, and most importantly: there will be enough alien life present to emit enough of those signature gasses that we would be able to detect them from Earth with our telescopes. Otherwise, we wouldn’t see them even if we were looking right at them.

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And this is, perhaps, the point of Charles Cockell’s statement. This approach makes our attempts to find alien life a little blinkered, limiting us to only a very specific definition of alien life. Anywhere those three conditions are not met, we would not find anything regardless of whether or not it was there.

Cockell isn’t saying that life is necessarily rare in the galaxy. We may even have already looked at planets with life. Perhaps we simply didn’t look closely enough.

Image: NASA’s 2012 “Black Marble” showing the obvious signs of human civilization on Earth. Credit: NASA Earth Observatory/NOAA NGDC