 25 years ago, most people would have told you that there are only 9 planets in the universe. Since then, we've lost one. Sorry Pluto. But we've discovered thousands of others. So what happened? Did astronomers suddenly get a new pair of glasses? And now we're meeting all of our new neighbors? Well, no. It turns out we could have seen these exoplanets all along, but we only recently figured out exactly where and how to look. We call these new planets, exoplanets, or extrasolar planets. These are planets outside of our own solar system. Up until a few decades ago, we only knew about planets in our own solar system, like Venus, Mercury, and Earth. But in the last 20 years, we've discovered over 1,000 confirmed planets outside of our own solar system and over 3,000 candidates that are probably planets. In fact, scientists think that there are quintillions of planets in our universe that we just haven't seen yet. We've known for the last 400 years about our 9, well, 8, planets in our solar system. So how are we now just finding all of these other planets? The simple answer is we've built telescopes that are made to find them, and they use something called the transit method. Here's an easy way to think about it. This light is a star, and I am a transiting exoplanet. When I pass in front of this star, you can't see my features, but you can see my outline, and that's how you know I'm there. And that's exactly how the transiting method works. This is real space telescope data. What scientists do is that they monitor the brightness of a star over time. And what they look for are these tiny dips in the overall light, and that tells you that there may be a planet around that star. The transit method needs planets that orbit their star quickly so that we can get lots of transits and get a good signal. This means that the planets need to be close to their star. Now, in our own solar system, the planets that are closest to the star are the small rocky planets, like Mercury, Venus, and Earth. Because astronomers used to think that every planetary system had to look like our own solar system, they thought that the planets that were close enough to the star to detect with the transit method were too small to see with their old telescopes, so they didn't even bother looking. Had they looked, scientists could have found a Jupiter-sized planet right next to its host star. It turns out, exoplanet systems are nothing like our own solar system. In fact, they're changing the way we see and study planet formation in our universe. Not only have we been able to find large planets next to their stars, but we've built better telescopes that can find these small rocky planets, too. For example, NASA's Kepler telescope has found thousands of exoplanets using only the transit method. And now the next generation is coming. This is Tess, the Transiting Exoplanet Survey Satellite. She's going to be launched in 2017 and is being built right now by Google, MIT, and NASA. Tess's main job is to look for exoplanets around the brightest and closest stars to our own solar system. With any luck, she'll find planets that are close enough that we can follow them up with other space and ground-based telescopes to better understand their atmospheres and climates. It's amazing that such a simple method can help us find so many new planets. Just by looking for dips in brightness, we might be able to find a planet that can host life. All we had to know was exactly how and where to look.