Springboarding off yesterday's post, which suggested that -- from a biochemical standpoint, at least -- extraterrestrial life might be way more common than we'd thought, today we look at how we might find out where it lives.
This is a thornier problem than it might seem at first. Despite hopeful movies like Contact, picking up an alien radio signal makes looking for a needle in a haystack seem like child's play. Consider the difficulties; you have to have your radio telescope pointed at exactly the right place in the sky, at exactly the right time, and tuned to exactly the right frequency, to pick it up as it sweeps by the Earth at the speed of light. Even if you posit an extremely simple message, which repeats indefinitely (like Ellie Arroway's string of prime number blips), there's the problem that any kind of electromagnetic signaling follows the inverse-square law, meaning if you double the distance between the sender and the receiver, the intensity of the received signal goes down by a factor of four. Triple it, and it goes down by a factor of nine, and so forth.
And the fact is, the distances we're talking about here are...
... astronomical. (*rimshot*)
So the possibility of detecting some sort of radio signal (whether or not deliberately sent to attract our attention) is not zero, but pretty damn small. And the other downside is that if that's all we're looking for, we're going to miss a huge slice of the living creatures that could be out there -- we'd only see the ones that have a technological civilization that uses radio waves to communicate. From that approach, Earth itself would have appeared to be barren and lifeless until the use of radio became widespread, back in the 1930s.
Is there another way?
An alternate approach -- one that avoids at least some of these pitfalls -- is to look for biosignatures, chemical traces that might indicate the presence of life on a planet even if it hasn't reached the point of being technological. The studies done on Mars that attempted to find Martian microbes took this approach; take a sample of soil, add some likely nutrients, and look for a sign of metabolism. But this, too, has its inherent difficulties. How do you tell the difference between Martian microbes chowing down on the food you gave them, and some exotic but abiotic chemical reaction?
A team of astronomers and biologists from the University of Birmingham and MIT have come up with a possible answer. According to a paper in Nature Astronomy last week, there is a pair of dead giveaways; an atmosphere depleted in carbon dioxide but enriched in ozone.
Carbon dioxide is a highly stable compound, and on lifeless, dry planets like Venus and Mars, it makes up a significant percentage of the atmosphere. (96.5% on Venus, 95.3% on Mars.) The fact that despite the amount of carbon on the Earth, the quantity in the air is only 0.04%, is due mostly to the fact that the water in the oceans acts as a huge carbon sink, first dissolving the carbon dioxide, then reacting it with dissolved metal ions like calcium and magnesium to form minerals like the calcite and magnesite in limestone. Without the oceans, all of that carbon would stay in the atmosphere -- and we'd be a lot more like the inferno that is Venus than the temperate world where we reside.
As far as ozone, the real tipoff for the presence of life would be gaseous oxygen, which is a highly reactive substance that, in the absence of something producing it pretty much continuously, would all be bound up chemically. Ozone -- a chemical relative of oxygen, O3 instead of O2 -- is expected to be present in small amounts in any atmosphere with free oxygen, but is the astronomers' choice because its spectral signature is much easier to detect than oxygen's.
Likewise, carbon dioxide's spectral fingerprint is obvious because of its strong absorption in the infrared (a property that is directly related to the greenhouse effect and carbon dioxide's warming effect on atmospheres).
So it should be possible to analyze the light reflected from the surface of exoplanets that seem to be in the right temperature range, and look for two things -- low carbon dioxide (indicating liquid water on the surface) and high ozone (indicating something, possibly life, keeping molecular oxygen in the atmosphere). See both of those things, the team said, and you're very likely looking at a planet that is inhabited.
Like I said yesterday, of course, "inhabited" doesn't mean "inhabited by bipedal humanoids with spaceships and laser guns." But even so, the technique is intriguing in its simplicity. The team suggested starting with relatively nearby planetary systems like TRAPPIST-1, which has seven known exoplanets and is only a little over forty light years away from Earth.
So this is all tremendously exciting -- that astronomers are now taking the possibility of extraterrestrial life seriously enough to start proposing methods for searching for it other than just scanning the skies and hoping for the best. After all, to go back to the movie Contact -- "if we're all alone in the universe, it seems like an awful waste of space."