One In a Million

Well, really it's more like one in a hundred billion. We're talking about our Earth, and estimates of how many Earth-like planets there could be in our galaxy.
If you're as old as I am (or older, or a bit younger), you'll recall being taught in school that there were only 9 planets in the universe. Not our solar system, not our galaxy, only 9 in the entire universe. That's a total of nine worlds that we knew of, in all of space, everywhere we knew of. That was a pretty lonely prospect. And if you're like me, it made you think that solar systems like ours must be extremely (almost impossibly) rare.
That all changed when I was in high school. In 1995, the first year I worked at the Santa Barbara Museum of Natural History, I came into the Space Lab for my work shift and saw a newspaper clipping on our news board. The article itself didn't seem anymore spectacular than the others we had up at the time, but when I read it, my heart skipped a beat: scientists had found a planet orbiting a star called 51 Pegasi. The planet was a gas giant about half the mass of Jupiter that orbited at an incredible 4.23 days! To put that in perspective, Mercury takes 88 days to orbit the sun.
This was the beginning of a revolution (no pun intended) in planetary science. First of all, we knew there were other solar systems out there. Secondly, this first one was dramatically different from our own. Finally, and perhaps most incredibly, no one could see this world - its existence was extrapolated using mathematics.
51 Pegasus b (as this planet has been dubbed - "51 Peg a" would be the host star) was found using a method called spectroscopy. If you're familiar with this technique, skip to the next paragraph. If not, it's not too complicated; read on. Anything with mass has gravity, and the more mass you have, the more gravity you have. That is to say, the sun exerts its gravity on us, but we also exert a lesser gravitational force on it. In fact, although we say we orbit the sun, both the sun and Earth orbit a central point that is within the physical space of the sun, but not at the sun's center. If you watched to sun from another point in space, you would see the sun orbit around this central point; it wouldn't make a huge arcing circle, it would just seem to wobble in place. Spectroscopy measures the wobble of stars by looking for very regular (but extremely slight) changes in its distance to Earth. This method is sometimes called Doppler spectroscopy because it relies on measuring the Doppler effect of these stars, light red-shifting or blue-shifting as it comes to Earth. Here's a diagram from Wikipedia:

Now astronomers use a variety of methods for detecting planets that orbit other stars. We'll discuss later in the week, as we get closer to the launch of the Kepler mission. You'll be hearing about it in the news, and I have no less enthusiasm for the topic than the rest of the world. So stay tuned!
In the meantime, console yourself with this thought - Earth may be one in a hundred billion, but at least it's not alone.