You’ve been following the exploits of NASA's Kepler spacecraft, you probably already know that the mission finds new planets using what's called the transit method.
In short, Kepler stares at a bunch of stars and records when there's a periodic dip in a star's light caused by an object passing in front. With enough data and some careful followup work, scientists can tell whether the passing object is a planet orbiting the star.
So far, Kepler has confirmed 15 new planets using transits, and an additional 1,200 planetary candidates were recently announced.
And next year, people around the world will be able to watch a transit of an Earth-size planet with their own eyes.
OK, fine, I admit—the planet in question is our own Venus. But that's still pretty cool, because Venus transits are exceedingly rare.
—Image courtesy NASA
Due to its slightly tilted orbit with respect to Earth's, Venus crosses between Earth and the sun on an oddly spaced cycle.
Transits come in pairs spaced eight years apart, but the time between pairs is 122 years, then 105 years. Due to this pattern, only six Venus transits have been seen since the invention of the telescope.
A Venus transit in 2004 was the first of a pair. Before that, the most recent transit had been in 1882—which means no living people on Earth had seen a Venus transit when the 2004 event occurred.
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http://blogs.nationalgeographic.com/blogs/news/breakingorbit/2011/03/watch-planet-transit-2012-venus.htm
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