Super-Earths and sexy aliens

"Exoplanets" -- planets outside our own solar system -- are now routinely being discovered, and as our instruments grow more sensitive, smaller and smaller planets are being detected. Indeed, some of the most recently-found exoplanets are almost as small as Earth, a few even being described as "Earth sized". Unfortunately MSM reports of these discoveries suffer from an odd and naïve misplacement of emphasis. A recent example:

Astronomers using a telescope in Chile have discovered 50 previously unknown exoplanets. The bumper haul of new worlds includes 16 "super-Earths" - planets with a greater mass than our own, but below those of gas giants such as Jupiter. One of these super-Earths orbits inside the habitable zone - the region around a star where conditions could be hospitable to life.

The news piece is illustrated with an artist's conception of a very Earthlike-looking world, and speculation about life occupies a good chunk of its length.

In these media reports, the prospect of life is always at the fore- front. It's understandable, in a way; we have more than enough lifeless rocks, big and small, in our own solar system, and these stories have to be made appealing to a public raised on a diet of science fiction rich with alien beings (usually scary or sexy or both) often by authors with little knowledge of either astronomy or evolution. One could easily conclude that the discovery of an abundance of verdant Earthlike worlds, complete with analogs of the Ewoks or the Na'vi or at least the Klingons, is just around the corner.

Er, not really.

First off, the term "super-Earth" just means a planet with a certain mass range, larger than Earth but smaller than the gas giants of our solar system. It says nothing about other characteristics we associate with Earth. Most of these planets are several times as massive as Earth, implying much higher gravity, a very different atmosphere, and other drastic differences.

The "habitability zone" (H-zone) means the range of distance from the local sun within which temperatures would allow liquid water to exist on a planet. Being in the H-zone doesn't mean a planet actually has liquid water, nor that conditions are otherwise at all suitable for life.

Nevertheless, it's clear that planets are a common phenomenon, and given the number of stars in our galaxy, there must be billions of planets in the H-zones of their local suns -- even billions of such planets which are similar in mass to Earth -- whether we happen to have spotted them yet or not. (There are, of course, also billions of other galaxies.) That doesn't mean that planets with complex life are likely to be common.

To start with, most stars are smaller and dimmer than our own Sun, meaning that the H-zone is closer in. A planet within the zone would likely be "tidally locked", with one side permanently facing the local sun and the other permanently facing away, as our own Moon is "tidally locked" to Earth. This would produce permanent extremes of heat and cold respectively on the two hemispheres, and the atmosphere would all freeze out on the cold side. Not very Earthlike.

Also, evolution takes time. Earth is 4.6 billion years old and life has existed for at least 3.6 billion years, yet organisms big enough to see have existed for probably less than 1 billion years. How common is it for a planet to stay within the H-zone for billions of years? Most stars exist in parts of the galaxy where stars are much closer together than in our own area; close encounters between stars, which would disrupt planetary orbits, must be frequent (by stellar-lifetime standards) events. Gas giants bigger or closer to the local sun than Jupiter (which seem to be common) would be even more disruptive to the orbits of smaller planets.

Then there are factors such as a planet's chemical composition, axial tilt, the make-up of its atmosphere (if any), the likelihood of a magnetic field strong enough to screen out cosmic rays, frequency of meteorite impacts releasing enough heat to sterilize the entire surface, etc., and, again, the probability of all these factors staying within life-friendly limits for billions of years. It's very likely that Earth is actually a freak case. Finally, there's the problem of the low probability of an initial appearance of life, even if conditions are perfect.

Remember, no matter how ordinary Earth seems to us, it is not necessarily typical. No matter how freakishly rare planets with complex life are, we must by definition be living on one of them.

However, we won't need to rely on such assessments for much longer. Soon, we'll be able to actually detect the presence of complex life -- if it exists at all -- by studying the composition of exoplanets' atmospheres. The clue will be free oxygen. Oxygen is very chemically reactive and would not remain in a planet's atmosphere for a long period of time unless some process is constantly replenishing it -- and the only plausible such process is photosynthesis. That is, the presence of a significant amount of free oxygen in an exoplanet's atmosphere would be pretty solid evidence of an ecosystem with something like Earthly plant life, which would mean other complex life could well be present too. I'm betting that we won't find any such cases -- but in a few years we'll actually know.

If we did find one or more such cases, it would actually be bad news. We would still be stuck with the Fermi paradox -- the total absence of any evidence for technological civilizations elsewhere in the universe. Remember, the universe is billions of years old. If technological civilizations are even somewhat abundant, some of them should be millions, or even hundreds of millions, of years older than we are. Consider how far human technology has advanced in the mere 400 years since real systematic science began, and how the pace of progress has steadily accelerated. A civilization a million years older than ours, anywhere in the galaxy, ought to be producing effects we could easily detect and identify. Since we don't see any such effects, such civilizations probably aren't out there.

If complex life is fantastically rare, Fermi's paradox would be instantly explained. But if it's common, we have a mystery. It might be that, for some reason, complex life almost never evolves high intelligence, but that doesn't seem very likely. Intelligence is too useful. We'd pretty much have to conclude that there's some unknown factor that always destroys technological civilizations before they get much more advanced than we are now -- and that we ourselves are therefore doomed. There is already something of a cottage industry of speculation along these lines, most of it scientifically-worthless venting of whatever form of cynicism about humanity happens to obsess a given writer.

But I don't think that's the case. Everything points to complex life, or even life in general, being freakishly rare.

One last point -- about those sexy aliens. Here's a photo of a female chimpanzee in estrus:


Recall that all life on Earth -- humans, apes, spiders, trees, mold, bacteria, everything -- descends from one common ancestor, whereas life on another planet would have a different origin and would have followed different evolutionary pathways. To a male chimpanzee, the image above would be the epitome of sexy, but frankly it doesn't do a thing for me, and if you're a typical human male, you feel the same. Now, not only is this a fellow Earthly animal, but of all the millions of animal species on Earth, it's the one most closely related to ourselves. Yet the standards of sexual attractiveness are obviously quite different. So, no, an organism that had evolved independently on a different planet would not look like Deanna Troi, or even like anything that the notoriously omnivorous Captain Kirk would give a second glance.

(Chimpanzee image found via PZ Myers, who makes a similar point.)