What is the likelihood of extraterrestrial life?

Dear Cosmic Enquirer,

Yours is an old, perhaps ancient, and irresistible question. It’s another way of asking: “Are we alone here in the unimaginably vast cosmos?” Unimaginable as it may be, there is a remarkable and reasonable way to take a crack at imagining an answer to this question. In fact, there’s even a well-established scientific discipline to study the origin, evolution, and distribution of life in the universe–it’s called exobiology.

But closer to home, as recently as November 2006, we may have found a very reasonable place to go looking for life on our neighbor world–Mars, right down the cosmic alley, if you will. We’ll get to that a little further down. First …

How to calculate the likelihood of extraterrestrial life
A well-known astronomer named Frank Drake proposed a series of approximations back in 1961 to give us a reasonable estimate of the chances of having an extraterrestrial civilization “out there.” It’s simple. Just use the calculation N = R*fp nefl fi fcL. Easy, right? Here’s what it means:

Called the Drake equation, this calculation starts with total number of stars in a given part of space, say, our galaxy. At any time in the cosmos, some stars are forming. Others are using up their fuel and dying. Still others create more fusion energy than their gravity can contain; they’re exploding. So, astronomers often estimate the total number of stars using a term associated with a rate of star formation. We call it “R*” (R-star). For the Milky Way, it’s around 400 billion; hence the expression, “billions and billions” of stars. (This phrase was used by the extraordinary talk show host Johnny Carson, when he parodied the extraordinary astronomer Carl Sagan.)

Then we can consider the percentage or fraction of stars that would also have planets orbiting them. That’s fp.

Of those fp planets, what portion of them have conditions that could sustain life, akin to the conditions on our own planet? That’s ne, (”n sub-e” for “earthlike”).

Now, the fraction of planets on which self-aware, or “intelligent” life has evolved: fi.

How about the fraction of self-aware-ians (sic) that feel like communicating with other civilizations?: fc.

Then, an important and perhaps sobering one, the lifetime of such a civilization: L

So how many planets could have intelligent life? We write that total number with a capital N.

Again, the calculation N = R*fp nefl fi fcL gives us the likelihood of intelligent life “out there.”

Exobiologists have good estimates for just one of these terms: R* (representing the total number of stars, if you’ve forgotten). Because all the rest of the terms are still pretty unknown, you can go wild with this equation.

So what’s the answer already!?
Let’s say there are and were about 400 billion stars in the Milky Way Galaxy when ours came along. Then, let’s say only a tenth of them have planets (which is a low estimate). Of those, let’s say only a millionth of them are Earth-like. Looking around, life may not be that unlikely, so let’s say of these Earth-like planets, the chances of some kind of life happening are about one in ten. Then only a 10,000th of them have intelligent life. Of those, perhaps a 100th stumble on to radio-wave telescopes. Then let’s say, once a civilization comes into being, it doesn’t blow itself up, catastrophically disrupt its planet’s climate, or lose interest in radio astronomy, so it lasts about 10,000 years. So, let’s see:

N = (400 billion)(1/10)(1/1,000,000)(1/10)(1/10,000)(1/100)(10,000)
= 40 civilizations.

That’s not that many, seeing as how there is so much space in space.

On the other hand, what if these crazy-wild guesses are way off? What if life is very likely? Using clever techniques and telescopes, we have discovered hundreds of planets orbiting other stars, and the search for these planets is in its infancy. What if intelligent civilizations are common? It’s easy to use different guesses for each (fudge) factor, recompute this, and estimate that civilizations number not in the dozens, but in the millions. It’s quite an exercise.

So get this: If there is liquid water on Mars right now, well that might be a good place to go looking for fossilized forms of life–something like Martian bacteria. Oh, and what if those things living underground away from the sterilizing ultraviolet rays of sunlight were still alive?! If we found evidence of life on Mars, it would, dare I say it, change the world!

Meanwhile, the Phoenix is the next spacecraft we’ll be sending to Mars, this time to the north polar region of the planet. It will probably arrive in the late spring of our year 2008. See, the North Pole of Mars is a lot like the North Pole of the Earth. It’s cold. On Earth it’s covered with water–ice, frozen water. Well on Mars, there’s frozen water all right, but it’s below the surface. Phoenix has an arm that can dig down and sniff around. Who knows what it will discover a year from now. One-celled life on Earth is found in a lot weirder places than the Arctic or deep underground. Oooh, it’s exciting!

From time to time, people find meteorites on Earth’s surface that were blasted away from Mars by an impact there about 15 million years ago. The famous meteorite designated ALH84001 landed here about 13,000 years ago, and it’s from this inferred impact on Mars. Well, consider this: What if life actually started as some sort of single-celled organism or collection of amino acids on Mars, and found its way here by an impact not unlike the one that sent us the ALH84001 rock?

It’s not beyond imagining. You might not think it’s likely, but it is absolutely possible. It is, if I may, a cosmic connection. We may all be Martians …

There must be someone else out there
Whatever numbers you put in the Drake equation, and whatever you might wonder about the countless stars visible from Earth, you have to admit that it just doesn’t seem possible that there are no other civilizations out there, not one. It seems like there has to be somebody. I wonder often what they’re like. For our wealthy society with spacefaring robot capability, we just have to see what’s up on Mars.

If we discover life on Mars or anywhere else, everyone on Earth would have to stop and think about life elsewhere, about other forms of life. Then in turn, each of us would have to consider his and her own place here on Earth. Do we matter in the cosmic scheme of things? Are we just insignificant specks orbiting a speck in the middle of specklessness?

I often think about this, and I consider the size of a typical human brain. It’s pretty small. Yet with it, we can imagine and consider all of this. That, my friends, is remarkable. Yours is a wonderful question. It helps us ponder our place near our star, the Sun, and among all the stars of the cosmos.

Perhaps soon, members of the human species, the species that sends its robots to Mars, will change their world.

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