Personally, I don't think there's intelligent life on other planets. Why should other planets be any different from this one? ~ Bob Monkhouse
When last we met, I was rambling on about how life on other worlds might come in unexpected forms. That got me to thinking about the odds of intelligent life occurring at all and Frank Drake's equation. Think about how we got here.
Planet Earth formed around 4.5 billion years ago. Somewhere along the line, after Earth had oceans and possible life, an object the size of Mars probably came along and smacked into the Earth, destroying any life that was around. On the other hand, the collision (according to the most popular theory these days) gave us our moon, which had a number of salutary effects, the most important of which was to stabilize Earth's wobbling about its axis. This stability lengthens climatic cycles, giving life forms a better chance to get going.
So, on Earth Mark II, life arose again. For an incredibly long time, all the planet had was microbial life and blue-green algae. This stuff may have had to survive a planet-wide ice age. Had any land-based life been around (and there's no evidence there was), it would have been wiped out by such an event, leaving the planet to the microbes again. Eventually, Earth thawed out, and life finally exploded (for reasons that no one has ever been able to explain) in an event called the pre-Cambrian Explosion.
Things were pretty good until the Permian extinction. This event, possibly related to immense volcanic activity, wiped out 95% of the life around at the time. Life is tenacious, though, and gradually made a comeback. Mass extinctions came and went until about 220 million years ago (give or take) when the dinosaurs showed up and had a 165 million year run.
The dinosaurs went extinct thanks most probably to a combination of climatic change, possibly induced by another bout of volcanic activity, and a big honking meteor strike in Mexico about 65 million years ago. Now think about that for a minute. It took around 60 million years for our primate ancestors to show up, but once we did, we made pretty quick strides to getting smart. The dinosaurs were around almost three times as long and evidently never got beyond the clever carnivore stage.
It is, of course, possible that there were actually intelligent dinosaurs that built some sort of civilization. After 65 million years there isn't much chance that you're going to find signs of settlements or pottery fragments. It's hard enough to locate stuff like that from 5000 years ago. Not finding big-brained dinosaurs is no absolute proof either, considering how difficult it is to find ancient hominids. Let's face it: Intelligent beings are crunchy and taste good to carnivores (with or without ketchup).
So, even if you can liberate yourself from being a carbon-based chauvinist and imagine all sorts of exotic life forms, intelligent life seems to be a more difficult thing to come by, even if it breathes methane. So, how does this impact Drakes' Equation?
Based on current information, it appears that planets are a pretty common event. While we haven't found many Earth-like planets, our method of search tends to find the big weird gas giants more easily than it finds good candidates for life. I know I'm saying we should accept the idea of strange life forms, but intelligent life appears to need some sort of planetary stability. Gas giants whirling around their stars close enough to be boiling or in eccentric orbits that send them from freezing voids to blistering passes by their alien suns are not going to be likely habits for technological civilizations.
On the other hand, given the tenacity of living things like microbes and simple creatures, based solely on our sample of one planet (Earth), it's likely that life occurring is quite common. It may not be very common, though, that it ever gets intelligent enough to build radio telescopes.
If you refer back to the article linked above, then, it appears that the factors for the existence of planets and life arising at least once are probably pretty close to one, as shown in the second set of variables. The trouble occurs when you consider the proportion where intelligent life arises and builds a technological civilization. The number in both examples is 0.01, which leads to 1,000,000 (in the second example) as the number of planets in our galaxy with intelligent life.
Frankly, that sounds pretty high.
Given all the ways that life can be extinguished (not counting those that intelligent beings would inflict on themselves), it would seem that the intelligent life factor should be much smaller. How much? I have no idea, but reducing it from 10-2 to 10-6 would not seem to be unreasonable. That would release the number of intelligent civilizations from potentially 1,000,000 to 100 in the entire Milky Way galaxy.
And it's a big galaxy.
Now that doesn't mean that we should quit looking for life, because the odds favor finding some sort of living things out there. The odds might even favor finding creatures that walk around on land surfaces and eat each other, just like they did hear for eons. Just finding that sort of life would be a intellectually stunning event. People would have to come to grips with the fact that the universe is not here just for us.
It also doesn't mean that we should quit looking for intelligent life. Aside from the fact that my numbers could be so much smoke, even if I'm right we could still have someone in our neighborhood. Making contact with some alien civilization would send shockwaves throughout our immensely self-centered societies.
I look at it this way, though. It's difficult to know how many Earth-like planets there are out there, but it's likely that they won't be inhabited by anyone who is going to be put out by a bunch of strangers showing up from another planet. It means that humanity has a chance to survive future disasters like those that have wiped out life before by going to the stars and colonizing other systems. Assuming that we don't obliterate ourselves through our own collective stupidity (no sure thing), our descendants could even survive the death of the our sun by going to another star system.
It's nice to know we have options.