Warning: Science Content

In the wake of the recent discovery of a relatively Earth-like extrasolar planet, a friend of mine sent me an e-mail asking if I could explain how we know exactly how far away such a planet is. I was delighted to comply, because I'm always happy to be asked questions I actually know the answer to. And because it seems a shame to waste a good lecture, I figured I'd also post a (slightly edited) version here for the edification of the general public. Or at least for the handful of people who read this blog and don't already know this stuff.

So, how do we know how far away this interesting little world is?

The distances to relatively nearby stars (and thus to any planets they might have) is measured using parallax, which -- and you may already know this -- involves looking at an object from two different positions and measuring how it appears to move relative to a more distant background.

Parallax is a familiar phenomenon. If you hold out a finger and close first one eye, then the other eye, you'll see your finger appear to move relative to the wall behind it, because your eyes are a few centimeters apart. Likewise, as you drive down the road in a car, the nearby telephone poles seem to move much more quickly than the distant mountains. The reason why this works is best explained with a diagram, but my scanner seems to be broken, so I can't draw you one, and if I tried to do it in Paint, I think we'd all regret it. You can check out the Wikipedia entry on parallax, though. There's some diagrams there, as well as a much more technical explanation than you probably want. The short answer is that you're seeing the object from a different angle, and the difference is much bigger for things that are close to you than for things that are far away.

Now, because stars, even ones that are close by astronomical standards, are so far away, just moving a few feet, or even a few miles, isn't going to show you much change. What you can do, though, is to use the fact that the Earth moves around the sun to help you. The Earth is 93 million miles from the sun, and it revolves in a nearly circular orbit. This means that in July, Earth is 186 million miles away from where it is in December. (Because it moves in a near-circle with a radius of 93 million miles.) So, if you observe a star, and then observe it again six months later, you can see it move against the background of more distant objects (such as galaxies, which are very, very much farther away). Knowing the orbit of the Earth and observing the amount by which the star seems to shift, you can use some simple trigonometry to calculate the distance to the star.

There. Hopefully that makes sense.