Greetings! I’m Reid Reimers and welcome to SciShow Space. On February 26th 2014, the Kepler science team announced the discovery of 715 new planets outside of our solar system. This almost doubles the number of alien planets confirmed to date. But how did this happen?

Since the Kepler Telescope’s launch in 2009, we’ve been discovering new planets only a handful at a time. So, where did these 715 new planets come from, and are they actually there or are they just 715 pretty good guesses? Well, a lot of this discovery has to do with not just what’s been observed, but also the probability that what we’re seeing actually are planets. Let me step back. The Kepler space telescope has an instrument called a photometer. It’s specifically designed to measure the brightness of light with a lens almost a meter wide, and it’s the largest camera we’ve ever launched into space. And it finds planets by watching for their shadows… kind of.

Planets outside our solar system are too small and too far away to see, but we can see stars. And when a planet crosses between us and a faraway star, it blocks some of the star's light. The Kepler photometer detects that dip in brightness and it records it. But, that dip might also be caused by some kind of solar event or a bit of space junk flying through the shot, like say some asteroid photobombing us like an obnoxious tourist. So, Kepler keeps watching that same star to see if it dims again. If it does days, months, or years later and it dims by the same amount for the same amount of time then we know that this isn’t some weird fluke and instead there’s probably something orbiting it.

Kepler watches more than a hundred thousand stars for these kind of fluctuations. When it finds one, it flags it as a planetary candidate. The job of the Kepler science team is to verify how many of these planetary candidates are actual planets because there is another celestial phenomenon that can cause a star to appear to dim. Imagine that there are two other stars that are orbiting each other and they happen to be directly behind the star that we’re observing. These are called binary stars and they're Kepler’s biggest source of false positives. Think of it this way. When a pair of binary stars lines up with the star that we’re looking at, one of those binary stars is going to eclipse the other as they revolve around each other. When that happens, the whole field including the star in front of it will look less bright. The bummer for Kepler scientists is that binary stars are super common, like so common that if Kepler sees a star with only one planetary candidate around it, there’s only a 25% chance that that’s an actual planet.

So, what if a star has more than one candidate running for office of exoplanet? That’s where probability gets involved. Planets, I mean actual planets, the things that Kepler’s looking for, tend to cluster around the same star. Just look at our solar system. We’ve got eight of the things right here. But whether or not a star is going to have a pair of binary stars behind it is totally random. So those false positives are evenly distributed among all of the planetary candidates that Kepler finds. And that’s why, statistically speaking, observing one planet around a star, an actual we’re-sure-it’s-a-planet planet, makes you thirty times more likely to see another planet. And when you see two planets, you are fifty times more likely to see a third one. So the odds that a single planetary candidate is a real planet are actually pretty low. But the odds that three planetary candidates around a single star are planets are really high. Like we can be 99% sure that those are planets without even taking any other readings.

This kind of logic is how the Kepler science team announced finding 715 new planets at once. It’s called "verification by multiplicity." Using Bayesian probabilities, astronomers can determine that in some cases, it’s so likely for a planetary candidate to be an actual planet that it’s just silly not to call it one. So when the Kepler science team calculates that a given planetary candidate is 100 times more likely to be a planet than something else, at that point they say that planet is officially discovered. So those 715 new planets were flagged as planetary candidates by the Kepler telescope, but they were discovered by the Kepler scientists using math.

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