You're standing outside, and it's a sunny day. You bask in the sunlight, enjoying its warmth, when suddenly it dims a bit. That catches your attention, so you look up, and see a cloud scudding across the Sun. It's not a thick cloud, but enough to notice.
Congratulations! You understand one of the basic principles of looking for extrasolar planets.
Astronomers at the Harvard/Smithsonian Center for Astrophysics (CfA) have announced that they have discovered a planet orbiting a star 120 light years away, using roughly the same technique. Called "transit searching", they use small telescopes to look at lots and lots of stars all at the same time. They look for tiny drops in the amount of starlight. This can be caused by a myriad of events: the star might be a variable, or the detector in the camera might be faulty, or an asteroid might have passed between us and the star, or, yes, a cloud may have crossed the 'scope's field-of-view.
But sometimes, a planet orbiting that star has its orbit aligned so that we see it edge-on, and that planet crosses the face of the star. When that happens, the star light dips a bit.
In cases like that, the star is flagged for more observations. If the culprit is a planet, and it's close to the star so that its orbital period is short, it'll quickly pass in front again on the next orbit, and the dimming repeats. And again, and again.
That's what happened in the case of one star in the constellation of Cygnus. The CfA telescopes bagged a planet dimming the star's light. We know what kind of star it is, so we know how bright it is, and how big it is. By measuring the amount the light drops, we can say how big the planet is (a bigger planet makes the light drop more than a smaller one).
Better yet, by carefully measuring the spectrum of the star over time, we can figure out how hard the planet pulls on the star; that depends on its gravity which depends on its mass.
So the planet, called HAT-P-11b, has a size about 4.7 times the Earth's, and has about 25 times our mass. To compare, Neptune is 3.8 times the size of Earth, and 17 times the mass, so this new planet is like a beefier Neptune. Doing a little math, we can even determine that this new planet is slightly less dense than Neptune. So "beefier" is the wrong word. Puffier, maybe.
So we know its size, mass, and density... not bad for a planet we can't even see directly! And knowing its orbital distance from the star (measured from the orbital period) we can guess at its temperature, too. Incredible. All that, from a 0.4% dip in the light from a star (well, that plus some pretty sharp spectral measurements).
What's perhaps most amazing about this are the telescopes involved: six of them, each with a mirror only 11 cm (4 inches) in diameter! You don't need huge telescopes to look for transits. Small ones with electronic detectors (similar to what's in off-the-shelf cameras) can pretty easily look at bright stars and see a brightness drop of 1% or less. In many ways they're better than big 'scopes: they see bigger chunks of the sky, their detectors are cheaper, they're cheaper, it's easy to get time on them -- heck, you can build 'em yourself -- and therefore they can stare for long periods of time at a single star or collection of stars, increasing your chance of seeing something interesting.
This is a fantastic time to be an astronomer. Cutting-edge stuff is now cheap enough that it's not hard to set up. Looking for transiting planets would have been incredibly hard before digital detectors, and now you can literally do it from your back yard. And who knows, the next super Neptune discovered might even be the one you find.