Last year Stephen Hawking raised some eyebrows when he said that humans should avoid drawing attention to ourselves, because interaction with aliens would turn out poorly for us. Others, like skeptic Michael Shermer, pooh-poohed the idea that aliens are dangerous. Shermer says evil aliens are a "myth" and we have nothing to worry about.
So, how likely is it that there are planets like Earth, and that there are intelligent aliens living there, that those aliens can travel between stars, and that aliens will come here?
These days astronomers are finding Earth-sized planets at an amazing clip. Since its launch in 2009 the Kepler telescope has identified almost a hundred earth-sized planets, hundreds of super Earths (rocky planets bigger than earth), and many hundreds of gas giants like Neptune and Jupiter.
Kepler orbits the sun in the same orbit as Earth, trailing millions of miles behind us. The telescope is pointed at a small patch of sky in the area of the constellations Cygnus, Lyra and Draco. Kepler detects planets when they "transit" their stars. That is, when the planet comes between its parent star and Kepler. A planet transiting its star reduces the star's observed brightness ever so slightly. The amount of light blocked roughly indicates the size of the planet. Results are often verified by examining the tiny wobble that the planets cause in the star's position, to give an estimate of the planet's mass. (Stars actually orbit their planets too, but the amount of motion is relatively small.)
For Kepler to detect the planet, it has to be in a very particular orbit around its star: it must be orbiting its star in the same plane in the direction of us and Kepler. The odds of that happening are, uh, astronomically small. Which means that if we're detecting hundreds of planets by their transits around the hundred thousand or so stars we're observing, there are probably many, many thousands more that we can't see because their orbits are in a different plane.
So now we know there are almost certainly billions of planets out there, and that planets similar in size to Earth are probably very common. What we don't know is how many of them have atmospheres like ours, and how many have life, and how many have intelligent life. Astronomers like Frank Drake have tried to calculate this, with his famous Drake Equation. The honest truth, though, is that we have no way of knowing what values to assign to the terms of that equation.
So, is Hawking justified in fearing that aliens would be dangerous? His basic thesis:
“If aliens ever visit us, I think the outcome would be much as when Christopher Columbus first landed in America, which didn’t turn out very well for the Native Americans.”
Shermer represents the other view:
I am skeptical. Although we can only represent the subject of an N of 1 trial, and our species does have an unenviable track record of first contact between civilizations, the data trends for the past half millennium are encouraging: colonialism is dead, slavery is dying, the percentage of populations that perish in wars has decreased, crime and violence are down, civil liberties are up, and, as we are witnessing in Egypt and other Arab countries, the desire for representative democracies is spreading, along with education, science and technology. These trends have made our civilization more inclusive and less exploitative. If we extrapolate that 500-year trend out for 5,000 or 500,000 years, we get a sense of what an ETI might be like.
Shermer is being naive. Even if we posit that galactic civilizations advance socially and morally, not every group or individual will be so enlightened. We have plenty of examples today of dictators who murder their own people, oil companies that turn countries like Nigeria into a swamp of toxic oil residues, and criminals who traffic in sex slaves -- even in our own country.
Any galactic civilization capable of interstellar travel would have the technological wherewithal to give small gangs of thugs or even one individual the capacity to drive our entire civilization into the stone age. Given a base on the moon, we could do this ourselves. With lunar mass drivers (electromagnetic catapults) we could bombard terrestrial cities with rocks that would strike with the force of an atomic bomb (as described in The Moon is a Harsh Mistress).
And even if an alien civilization is truly benign, the discovery of its existence could have a very destructive effect on the fragile psyche of humanity. If we detected an alien spacecraft heading our direction, reactionary elements on Earth could very well bring about Armageddon before the aliens even got here.
So Hawking is right that we shouldn't be intentionally sending radio signals into the void. It's very unlikely that anyone will pick them up, but we certainly shouldn't be trying to draw any attention to ourselves. At least, as long as we have no means to protect ourselves from a space-faring civilization. Why take an unnecessary risk?
But it seems doubtful that any alien civilization would bother coming to Earth. If they have the capacity to travel between stars, there's nothing here that they couldn't get in their own solar system, or a closer one, with less effort. It would be far easier for them to terraform other planets and moons in their own solar system, or build their own habitats in space from raw materials in their asteroid belts or the moons of gas giants. No matter how you slice it, they'll never be able to send enough colonists to other solar systems to relieve their own population pressures; in the end they'll have to learn how to live within the means of their own solar system or perish. The only ones that would come here would be small groups of explorers, exploiters, or people with an ax to grind.
But still and all, Earth is the only planet we've got. We should be very careful with it.