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This broken space telescope keeps spotting new planets

An artistic rendering of Kepler-452b, one of the planets found using Kepler. (EPA/NASA/JPL-Caltech/T. Pyle)
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NASA's Kepler space telescope just keeps raking in the exoplanets, 1,930 of which have now been confirmed (and 4,696 that haven't). These include an impressive 234 new exoplanet candidates discovered by Kepler in 2014, which were announced at the annual meeting of the American Astronomical Society this week.

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Kepler monitors over 150,000 distant stars, allowing scientists to detect exoplanets by measuring the way they seem to dim from Earth's perspective. We can use those variations in light to infer the existence of planets orbiting the observed stars, and even to measure the size, mass, and atmospheres of the orbiting worlds themselves. In this way, scientists can figure out whether a planet is the right size and mass to be rocky (like Earth) and whether it's the right distance from its host star to hold liquid water.

But the coolest thing about Kepler is that it's broken – and has been since 2013, just four years after its mission began. Kepler used three of its reaction wheels to keep it centered precisely on its target, and one of them failed. Without it, the spacecraft was unstable, and any outside force could knock it totally out of position.

Instead of calling it quits, the mission scientists transitioned into a second wave of observation called K2, which started in 2014. K2 uses the power of the sun to keep Kepler from being unwieldy. The light from the sun acts as a virtual third reaction wheel, physically pushing against the craft's solar panels as the three physical reaction wheels push back. The tension keeps the telescope in place, but it has to be repositioned every 80 days to keep the solar forces hitting it in just the right place.

It's an ingenious fix, to say the least – but one that dramatically changed the way Kepler could operate, the stars it could look at, and how long it could investigate them. So it's incredibly exciting that K2 has been so successful, resulting in over 100 confirmed exoplanets so far and bringing in droves of candidates to investigate.

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