In Life through Time and Space, Wallace Arthur brings together the latest discoveries in biology and astronomy to examine our deepest questions about where we came from, where we are going, and whether we are alone in the cosmos. The answer to that last question is almost certainly negative, but, below, Arthur addresses a slightly different puzzle: life is probably out there—but can we see it?
For sure it’s there. The odds of it not being there are negligible. By “there” I mean somewhere in our home galaxy, the Milky Way. But can we see it? Strange question perhaps, since the obvious answer would appear to be “no.” But it’s always worth poking under the skin of seemingly obvious answers. And sometimes the first step in this poking process is refining the question.
One version of our question here is “do any of the stars we can see with the unaided eye have inhabited planets?” The likely answer to this version is “yes.” This positive answer is yielded by doing a few exceedingly simple sums. To a rough approximation, we can see about 10,000 stars from a dark sky location. Well, that’s for folk with really good eyesight. But even for others, like me, the answer is still in the thousands. The proportion of those stars that have planets orbiting them is probably well over 90%, based on exoplanet searches to date. And most of those stars will have multiple planets, not just one. Last month, the first 8-planet system other than our own was discovered, when an extra planet was found orbiting the star Kepler-90.
If the average number of planets per star is 5 (though it’s probably higher) then we can see the host stars of about 50,000 planets. That’s great, but now we need to guestimate what fraction of these have life. If we use our own solar system as a yardstick, the answer is 1⁄8th, which works out at 6250. But that’s probably too high. To err on the cautious side let’s assume that our initial answer was out by an order of magnitude in the wrong direction. In this case the right answer is 625. And if we want to be really cautious we could go with two orders of magnitude, in which case the right answer is still over 60. Could it in fact be zero? Well, yes, but that’s really not likely. So in one sense we can see the home of alien life, but only in the sense of its host star rather than its host planet.
Another version of the question with which we started is: can we see life from other planetary systems because it has come to see us? The probable answer to this question—at least in my view—is “no.” However, our recent visit from the elongated interstellar object called 'Oumuamua might make us think again. I wouldn’t urge belief in the accuracy of all those YouTube depictions that see it as an alien spacecraft disguised as a long thin rock. But then again, no one has come up with a convincing explanation of how a natural object in space comes to be approximately cylindrical. In our own system there are more than a million asteroids. Not one of them, as far as we know, is of this shape.
A third version of the question emerged from discussion at the end of a talk I gave at a famous second-hand bookshop—Barter Books—in the ancient English town of Alnwick, a talk that was tied in with the launch of Life through Time and Space. A member of the audience took issue with my carbon chauvinism, but not in the usual way, by proposing silicon-based life. Rather, he asked if it might be possible for life to be made of dark matter. What a fascinating question!
My gut feeling is that such a form of life isn’t possible. But just supposing that it is—this would mean that we couldn’t see it even if it landed here on Earth. Of course, we can’t go too far with this line of thought because we don’t yet know what dark matter is made of; we just know that MACHOs are less likely than WIMPs. Nevertheless, the vast majority of matter in our galaxy is dark, not light, so we shouldn’t altogether ignore it.
So, to summarize: We can see the home stars of as-yet undiscovered life. And we can see a very odd-shaped interstellar visitor, but it’s probably uninhabited. We can’t see any DML (to coin a new acronym) if it exists, even if it’s currently sitting just across the table. What will change in the next decade or so? One distinct possibility is that we will see the signatures of life in exoplanet atmospheres. Here’s a specific hypothesis: we will first see such signatures in the year 2030. This, like all good hypotheses, is ultimately testable—but it won’t be so until December 31st 2030, or, more realistically to allow for data analysis, early in the year 2031.