Mystery Planet Packs Enormous Surprise

The Kepler spacecraft scans the stars, looking for signs of habitable, Earthlike planets. A recently discovered Jupiter-size planet, however, turned out to be something far more surprising.

Kepler rides far above us, looking for other worlds like our own. They are rare – of the 2,740 planets discovered so far, only 122 might, repeat might, be similar to Earth. Most of the planets are closer to Jupiter in size, and, like Jupiter, most are gas giants.

At the beginning of April, 2013, however, a run-of-the-mile Jupiter-sized gas giant orbiting a red dwarf star turned out to be anything but run-of-the-mill. Scientists were astounding at the strange dance between this presumed planet and its host star.

Kepler examines stars for two phenomena – occlusion and wobble. When the light from a star dims regularly, it stands to reason that a body must be orbiting the star. The amount of starlight reduced tells us the size, while the frequency of the dimming tells us the diameter of the body’s orbit. Nearly every planet tugs on its host star, causing it to wobble in its passage through the galaxy. The degree to which the star wobbles in its orbit tells us the mass of the planet tugging on it.

Imagine Kepler scientists surprise, then, when they found that the red dwarf star discovered in April was actually orbiting the planet! The planet, in fact, turned out not to be a planet at all, but a white dwarf star – a star similar to our sun that had burned out and shrunk to the size of Jupiter. For all that tiny size, however, it is still just as massive as the sun, and has the same gravitational power.

The red dwarf star has been captured by the gravitational field of the white dwarf, even though the white dwarf is unbelievably tiny, as stars go. The two stars form a pair of binary stars.

There is one more twist to the story: When the tiny white dwarf star passes on the far away side of the red dwarf, the red dwarf’s light gets dimmer. Repeat: dimmer. When the white dwarf passes between the ref dwarf and us, its light is concentrated in an Einsteinian gravitation lens and shines at us far more brightly.

Not all the planets Kepler finds are Earthlike. In fact, after this most recent discovery, it’s safe to say that not all the planets Kepler finds are even planets!

You can learn more about the Kepler project at Nasa.gov/Kepler.

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