It may seem strange at first, but if you ask professional astronomers if there’s alien life, you’ll find that most of us do think it exists, but not the kind that’s supposed to have already visited us.
Perhaps the biggest evidence against the X-files kinds of aliens that swoop down and either abduct people or just carve enormous circles in cornfields, are the worldwide army of amateur astronomers, often more familiar with the night sky than professional astronomers, who basically never report any odd lights in the sky even though they spend more time looking at the sky than anyone. Barring bizarre alien psychology, too, it doesn’t make much sense to travel trillions of miles in an (invisible) spaceship, hover over San Diego for a few hours, and leave no other sign.
On the other hand, when you consider that our galaxy is enormous and contains perhaps a trillion stars, and accept our current assumption that the Earth and our solar system are nothing particularly special (basically, a complete reverse of Aristotle’s ideas), there should be countless thousands of places in the universe with life. To paraphrase Carl Sagan, if there’s nothing else out there, it’s an enormous waste of space.
The SETI Institute, whose name stands for “Search for Extra-Terrestrial Intelligence”, is a privately funded research and outreach organization that has spent the last 50 years trying to detect radio transmissions from extraterrestrial sources. The project is, depending on who you talk to, either a stupid sci-fi idea or the most important thing we could possibly be doing with radio telescopes. One of its astronomers, Dr. Seth Shostak, is an amazing public speaker, and I highly recommend watching his TED talk.
(If you have never heard of the TED series, it’s got all kinds of people talking about cool projects)
In 1961, the astronomer Frank Drake (a founder of SETI, along with Carl Sagan) put together an equation (the Drake Equation) to compute the number of alien civilizations we could potentially contact right now.
N = R* • fp • ne • fl • fi • fc • L
If you’d like to play along, N is the number of intelligent alien civilizations in the galaxy.
R* is the rate at which the galaxy forms stars (in stars/year). We actually know this one; the galaxy is forming roughly 20 stars per year.
fp is the fraction of stars which have planets. Recent results suggest this is about 1. Not every star has planets, but there are a large number of star systems with multiple planets that balance it out.
ne is the fraction of planets which are habitable. Recent results (http://phl.upr.edu/projects/habitable-exoplanets-catalog) suggest 10 potentially “habitable” planets, out of ~900 confirmed, and that’s probably a bit low because habitable planets have long orbital periods and require a lot of patience to find.
Of course, over the last 50 years, biologists have discovered organisms (deep sea vent creatures, highly resistant creatures like the Water Bear, and so on) that thrive in such extreme conditions that a planet with life might not look terribly friendly or habitable to us. Astronomers now tentatively consider SIX worlds in our own Solar System as potentially habitable: Earth (of course), Mars, Jupiter’s icy moons Europa and Ganymede, Saturn’s icy moon Enceladus (each with more liquid water under their surfaces than all the Earth’s oceans put together), and Saturn’s moon Titan, which has a thick atmosphere and oceans of liquid methane on the surface… and those last four would not traditionally be considered habitable.
fl is the fraction of habitable planets that develop life. We have no idea. In our Solar System, it appears to be 1/6.
fi is the fraction of life-bearing planets that develop intelligent life. Here, again, we don’t really know the number. We don’t even know what constitutes intelligence, or a consciousness. It’s becoming clear that our one life-bearing planet, Earth, has developed MULTIPLE intelligent species – we should probably be counting chimpanzees, dolphins, elephants, crows, octopi, and probably cats and dogs (and many more).
fc is the fraction of intelligent species that develop a communicative (via radio) civilization. We don’t know.
L is how long the average communicative civilization lasts. We’ve only been transmitting powerful broadcasts for the last 80, although that’s dropped dramatically now that TV is mostly digital. The Roman Empire lasted from 27 BCE to 476 CE (a healthy 503 years); various Chinese dynasties have lasted up to 700 years; the Catholic Church is over a thousand years old; but who really knows when it comes to aliens?
For a bonus calculation, if we assume that the galaxy is a uniform cylindrical disk (it’s not; it’s got a central bar, arms, and most stars cluster near the middle, but just go with it) with a radius of 50,000 light years and a height of 2,000 light years, it has a volume of 15.7 trillion cubic light years.
D = (15,700,000,000,000/N)^(1/3) (that’s a cube root) should be the average distance between intelligent civilizations.
I got 9,200 light years when I tried it, but I might be a bit pessimistic on the chances of life. That might explain why SETI has had so little success thus far…