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.

Earth-like Planet Found Orbiting Sun-like Star

This is big news, people!
I'm planning a lengthy post on the subject of exoplanets eventually because it is one of my favorite topics in astronomy. However, with this breaking news just released, I have to fill you in a bit now.
For the last 15 years or so, we've been able to detect planets orbiting stars that are not our Sun. We call them extrasolar planets, or exoplanets for short (meaning when we're too excited to actually say the full name because what we're about to say about them is just too amazing). However, the vast majority that we have found are much larger than Earth; most are in the magnitude of Jupiter, which could fit 1,300 Earths inside it. This is not because most exoplanets are huge; instead, it's because our technology is still limited at this time and we can only detect really massive exoplanets.
That said, technology in this field has dramatically improved in the last decade plus and we are detecting smaller and smaller worlds. In fact, we have detected 336 planets orbiting 285 stars to date. This number will only increase exponentially as we continue to develop more precise and more sensitive equipment.
Today's announcement is really big news. The COROT space telescope has identified a small (less than twice the size of Earth) planet orbiting a sun-like star. We've never seen that before (outside our solar system). Early data suggests that it may well be a rocky planet like Earth, as opposed to a gas giant like most other exoplanets detected to date.
One of the biggest differences between this planet, called COROT-Exo-7b (gotta love those super-creative astronomers!), and our home world is that it is very close to its host star. In fact, its year is about 20 Earth hours. To put that in perspective, Mercury takes about 3 Earth months to orbit the sun. The close proximity to its host star makes this newly discovered world's temperature range from 1,000 to 1,500 degrees C. I'll never complain about South Florida summers again. OK, I might.
This system is 457 lights years from Earth, and COROT-Exo-7b is the first Earth-like exoplanet discovered using the transit method. If that doesn't make sense to you, good! It means you'll just have to check back for more sciency goodness.