It's a pretty solid rule: in most planetary systems, as in our own Solar System, planets orbit their central star in the same direction as that star's rotation. But researchers have recently found a glaring, 100,000 kilometer wide exception.

In nearly every other observed planetary system, the planets spin around the star the same direction as the star itself is spinning. This is because, scientists believe, both the stars themselves and their planets are formed from the same rotating gas clouds, leaving them all spinning in the same direction as that original cloud.

There are exceptions to this general rule, but those are caused by gravitational interference pushing the planets into orbits at very strange angles. A brush with another planet or large gravity source can cause the aligned orbit to push into a strange angle. And the team that discovered this strange new planet, dubbed WASP-17b, wanted to blame this mechanism again.

But gravitational interference might be a bit of a stretch here: the planet's orbit is 150 degrees opposed, or almost directly opposite, the star's rotation. It'd take a pretty significant gravitational shove to get this much of a difference.

WASP-17b is notable for a couple of other reasons, too, all described in a paper submitted to the Astrophysical Journal. WASP-17b is possibly the largest yet discovered exo-planet, at twice the width of Jupiter. It's also pretty light, at only half of Jupiter's mass. That leaves the planet with a consistency similar to polystyrene, a light, puffed up ball of mostly nothing.The research team thinks this might be an effect of its very close (7 million kilometers), very quick (only 3.7 days) orbit yanking materials around inside the body and deforming it into this weird puffy planet.

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The planet is, to say the least, really strange, compared to what scientists have seen before. But such discoveries remind us that we've only seen a very, very small percentage of what exists in the rest of the universe. Maybe what we have deemed to be "normal" isn't normal, but just what we are used to.

Planet found orbiting its star backwards for first time [via NewScientist]
Paper: WASP-17b: an ultra-low density planet in a probable retrograde orbit [arXiv.org]

(Illustration: what such a close-orbiting planet may look like, from ESA/C. Carreau)

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