If you look at all the colors of the light given off by a star in fine detail, you'll see lines that are characteristic of atoms and molecules that make up the star. These lines can be compared with similar lines when the same atoms or molecules are heated in the lab back here on Earth. So, you can tell what objects are made of anywhere in the Universe. If an object is moving toward you, these lines move to higher frequencies - towards the blue end of the spectrum (at least in visible light). If the object moves away, these lines move to lower frequencies - towards the red end of the spectrum. It's similar to the way a fire truck's siren is higher pitched when it comes towards you, but falls to a lower pitch when it has passed by and moves away from you. Looking at the detail of the colors of light is called spectroscopy.
The first planet discovered around a star other than the Sun was announced in 1995. That's about 16 years ago. Since then, 528 such planets have been discovered (a number that changes almost daily). The method used at the time was the "wobble" method. Most of these discoveries were made using this technique. It's based on spectroscopy. The idea is that as a planet orbits it's star, it tugs on it's star with gravity. So the star wobbles. With spectroscopy, the movement of the star towards or away from us can be detected. It's not just if it's moving away or towards us, but how fast. So, if the planet is closer to us, it tugs the star towards us. If it's on the far side, it tugs it away. The planet has to go around it's star at least once, but more is better.
Now, it's easier to detect movements of a big star if the planet is big. And, it's easier to detect movements of a planet if the planet is closer to it's star. That's because planets closer to the star pull on the star with more force if it's closer. And, shorter, quicker orbits mean that you can get one or more full orbits quicker. So, most of the planets detected this way are large - as big as Jupiter, and close in to their parent star - sometimes closer than Mercury is to the Sun. And, it's easier if the star is smaller than the Sun. A planet can move a small star easier than a big one.
What we'd like find is Earth sized planets in orbit around stars like the Sun. That's because what we'd really like to know is if there are planets like ours. At the moment, the only place we know of for sure with life on it is the Earth. We'd like to know if there are other places with life. Do aliens exist? (If they're on their own worlds, they aren't aliens - they're natives).
There are other ways to discover planets around other stars. One might expect that a picture could be taken of a star at high resolution, and all the planets would show up. Unfortunately, stars shine through their own light, and planets shine mostly through reflected light. So, stars are something like a billion times brighter than planets. It's like looking for a firefly next to a search light, only harder. But it has been done. At least twice. This technique favors planets that are far from their host star, big, and it helps if they're very young, so they can shine in infrared light by virtue of being hot. You have to take at least two images to show that the planet moves with the star. Three images gives you more confidence, and can show the planet arc around in it's orbit.
Another way detect a planet around another star is to watch a star often, and look for a small drop in light as the planet comes in front of the star. You have to look very often to catch it in the act. You have to have pretty good sensitivity, like a part per 50,000. Both of these issues suggest that you need a telescope in space. In space, you can look at the same spot on the sky 24x7. You don't have to worry about poor weather. And, you don't have a boiling Earth atmosphere making changes to your star's brightness every few seconds. But there's another issue. Most planets won't happen to pass in front of their stars from our point of view. It's a geometry thing. If we're looking down on the pole of the star, then we'll never see any planets come in front. And the farther the planet is from it's star, the fewer stars will be aligned close enough to get one cross in front. So, if you want to detect planets this way, you have to look at lots of stars. This method, called the 'transit method', is used by the Kepler space craft. It's looking at the same patch of sky with a keen interest in about 150,000 stars.
And, the Kepler mission has 15 confirmed planet discoveries. In February of 2011, the team announced 1,235 planet candidates. Estimates are than perhaps 80% of these candidates will be confirmed as real planets. That suggests that perhaps 976 new planets will have been found. If confirmed, it more than doubles the current number of known planets. And, this preliminary data is from when the Kepler mission has more or less just gotten started, nothing like sixteen years. Since Kepler hasn't been looking very long, the data favors planets that orbit close to their stars. Bigger planets are easier to spot. But planets as small as the Earth and smaller are among the candidates. And yet, fifty four of the candidates orbit their star at a distance where liquid water might exist on the surface. These kinds of orbits are smaller for smaller stars. Five of these palnets are near the size of the Earth.
The Kepler mission is currently funded for an initial mission of 3 1/2 years. That's because the goal is to find Earth-like planets in orbit around Sun-like stars. We can tell if a star is Sun-like through spectroscopy. We can tell if a planet is Earth-sized by the amount of dimming. The spacecraft needs to detect three transit events to give us confidence that it's really a planet. Three orbits of a planet in an Earth-like orbit around a Sun-like star will take three years. You'll need a little extra to make sure you get three. The mission could easily be extended longer. The spacecraft doesn't run out of fuel at the 3 1/2 year mark. It's is hoped that the data from Kepler will lead to solid statistics on how common habitable planets are in the Universe. Or, at least, in our part of the galaxy.
What can we do with such statistics? We'll, in 1961, Frank Drake proposed a simple formula for estimating how many civilizations there might be in the galaxy. At the time, we didn't know much about what numbers to plug into the formula. The formula has terms like "the fraction of stars with planets". This is a number where Kepler data can help. Better numbers help give us a better estimate.
We also might be able to discover not only if there might be water, but if there actually is water on these planets. The Spitzer infrared space telescope was used to detect a variety of compounds in the atmospheres of a couple extrasolar planets. And while it is no longer capable of this feat, it demonstrates that it can be done. Transit data tells you when to look to pull it off.
The Kepler discovered planets will miss something we would really like. And that is that none of them will be very near to the Earth. If a planet discovered by Kepler has intelligent life on it, communication (by radio) would still take thousands of years, each way. It's tough to hold up much of a conversation with that sort of delay. We'd really like to find habitable planets that happen to be closer to us. But to do that, you have to look in almost every direction at once. And, we'll likely have to use direct imaging. That's going to require very large telescopes in space. In principal, it can be done. In practice, it will be expensive. But the results will definitely be exciting.