The scoop: Believing that intelligent extraterrestrial life -- better known as alien life -- exists is one thing.
Believing that they have visited Earth in our short time on the planet is another. Astronomer, author and blogger Phil Plait explains.
When I give public talks, I can almost guarantee that during the Q&A I'll get asked: Do I believe in aliens and UFOs?
My answer usually gets a laugh: "Yes, and no."
As far as aliens go, I suspect pretty strongly that there's life in space. We know of over 300 planets orbiting other stars, and we've only just started looking. In our Milky Way Galaxy alone there are probably literally billions of planets. Life on Earth got started pretty rapidly, relatively speaking, after the crust cooled and liquid water formed, so we know it's not tough for life to get its start... and it's entirely possible there is microbial life inside icy moons orbiting Jupiter and Saturn.
So thinking aliens exist has a pretty decent scientific basis. But them coming here is an entirely different beast.
There are tens of thousands of UFOs reported every year. That's one of the reasons a lot of people think aliens are visiting us: there's no way that there could be that many reports if some of them weren't real!
But that's bad reasoning. In fact, the vast majority of reported UFOs are mundane things in the sky. The planet Venus is incredibly bright; most people don't believe me when I point it out to them. They think it's a nearby airplane, or some other bright earthbound object.
Not only that, but if you're driving, it appears to follow you through the trees because it's so far away. If it's low to the horizon, turbulent air makes it flicker and change color. Does this sound familiar? How many UFO reports have you heard that say a huge object (people often mistake brightness for size) was following someone in their car, and it was rapidly changing color?
Manmade satellites pass overhead several times an hour, and some brighten tremendously as a solar panel or mirrored surface catches the sun. Meteors blaze across the sky, ice crystals refract sunlight and moonlight, atmospheric effects make a distant object appear distorted and weirdly shaped. All of these have been mistaken for alien spacecraft.
So I know that most people misinterpret what they see. But there's something else too. If alien spaceships are really out there abducting us and playing chicken with our airplanes, then you'd expect that people who spend more time looking at the sky would see more of them. And who spends lots of time looking up?
Amateur astronomers. They are dedicated observers, out every night peering at the sky. If The Truth Is Out There, then amateur astronomers would be reporting far and away the vast majority of UFOs.
But they don't. Why not? Because they understand the sky! They know when a twinkling light is Venus, or a satellite, or a military flare, or a hot air balloon, and so they don't report it.
That, to me, is the killer argument that aliens aren't visiting us. If they were, the amateur astronomers would spot them.
Of course, you might say "But just because they don't see UFOs doesn't mean they aren't real. It just takes one to prove aliens are coming here!" That might be correct, but remember, we started off thinking they're coming here because so many UFOs are reported! Once you realize that the overwhelming majority of UFO cases are just everyday things, then that "it just takes one" argument gets a whole lot weaker.
But I'll surprise you, though: I agree. It really only does just take one. But that one better have good proof! Something better than a single eyewitness, a badly sketched object, a fuzzy photograph, or out-of-focus video (heck, with digital effects the way they are today, you can't even trust video that's crystal clear). It needs a sample of non-terrestrial metal. An actual alien. Some incontrovertible evidence that is impossible to deny.
But we never get that. Why not? I think it's because we're not being visited. When Klaatu comes and lands on the White House lawn, I'll be willing to change my mind. But until then, well, keep watching the skies. Learn what's up there, and what isn't. You might someday spot the genuine article.
But even if you don't, you get to discover what's really up there... and there's treasure aplenty in the sky to be had, even by us folks stuck here on planet Earth.
Phil Plait is an astronomer, lecturer and author who worked on the Hubble Space Telescope for 10 years. He is the creator of the Bad Astronomy blog and president of the James Randi Educational Foundation.