Looking for aliens in all the right places

Contributed by
Jun 4, 2008
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So we're looking for aliens, right? Listening patiently for signals from aliens takes a long time, and we have to hope they're out there, and broadcasting.

Some folks wonder if maybe we should be trying to talk to them*. We could aim our transmitters (radio telescopes or possibly optical lasers) at some potentially habitable stars, and fire away with our "Here we are!" message.

But the sky is really, really big. There are millions of stars to choose from, even after culling them down to find ones that look better than others. Even looking at stars just like the Sun, for example, means looking at about 200 million stars in the Milky Way. It'd be nice to figure out some way to narrow the playing field.

Well, one problem is that we don't know if aliens are looking our way. It would suck to send a signal, only to have the aliens looking the other way.

The reverse is true as well: even if we only want to listen, the sky is a big place to be looking for a signal. What if the Vulcans are trying to talk to us, but we're looking the other way? We'll never meet Spock!

So maybe we need a way to up the odds, and send the signal to (or listen for ones from) stars where the aliens may already be looking our way. How do we do that?

A slight digression. The Earth orbits the Sun in an flat ellipse. If you draw such an ellipse on a piece of paper and look straight down on it, well, it looks like an ellipse. But from the side (holding the paper edge-on to your eye) it looks like a line, right? If the Sun were in the middle of the piece of paper, then every time the Earth got between you and the Sun you'd see it as a little black disk passing over the Sun's bright disk. This event is called a transit, when a small object passes directly in front of a bigger one.

We know lots of stars have planets, maybe even a large fraction. Odds are good that we're right in the orbital plane of some these planets, so we'd see transits. If we watch lots and lots of stars long enough, then we should see those transits, basically as a dip in the star's sunlight by a fraction of percent. And we do! We know this technique works; many extrasolar planets have been detected this way, and our technology is getting good enough that in a few years we'll be able to detect a planet the size of the Earth if it transits its parent star from our point of view.

So let's change our perspective: imagine you're an alien looking for planets. You do the same thing we are, and search for transiting planets. It's a great way to find Earth-like worlds!

Now let's combine these two concepts: if there are aliens out there, then the ones who are located such that they see the Earth's orbit edge-on are more likely to be looking our way. We should try talking to and listening for them!

From our point of view, the stars that see us as edge-on lie along a circle on the sky (astronomers call this the ecliptic). So if we want a place to aim our transmitters or receivers, looking just along that circle significantly cuts down on the sky's real estate.

This new idea has been put forth by astronomers Richard Henry from Johns Hopkins University, who has a team of folks (including my friend Seth Shostak at SETI) who are keen to try this. It's a long shot, of course, and they seem very aware of that. You have to assume the aliens are out there, and want to find us.

But it makes sense to me. Mind you, there are no guarantees here, of course, and if some alien civilization is located way off the ecliptic they're out of luck. But what Henry and his team are trying to do is make it statistically easier to look for aliens. The odds are already so low that anything we can do to improve our chances is probably a good idea to try, at least at first. And we're still pretty new at this; the game has just started. I think this technique is worth a shot.

*Personally, I think we should keep quiet. But then, I just wrote a chapter in Death from the Skies! about aliens coming to eat us, so I'm biased. But still.