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Some hot Jupiters orbit the wrong way


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All of the major bodies in our solar system orbit in the same direction our sun rotates, and are roughly in the same plane. This is nicely explained by the process that produced them, a combination of rotation and gravity that flattened the dust and gas they originated from into a disk. These models for the formation of planetary systems probably still apply, but some new findings are indicating that what happens afterwards may not be so neat and orderly: nearly a quarter of the hot Jupiters (gas giants orbiting in close proximity to their host stars) found in a recent survey of other stars are orbiting their hosts backwards.

Now, it's worth pointing out that, although the formation of our solar system seems neat and tidy, its evolution probably involved major collisions among most of its rocky protoplanets. And, in fact, the existence of hot Jupiters suggests other systems undergo significant changes during their evolution. There's simply no way to form anything that big close to a star, which indicates that these planets originated towards the outer edges of the systems, and were then pushed inwards by orbital interactions with the dust disk and fellow planets. But the new findings (along with some older ones) suggest that this gravitational pushing and shoving may not be limited to a single orbital plane.

Earlier work had indicated that more than half the hot Jupiters examined in an early survey had orbits that were not in the same plane as the rotation of their host star. The new survey adds a few more, as well as six that were orbiting in the opposite direction. This suggests that the orbital interactions aren't limited to pushing and pulling within the plane—large bodies must get close enough to swing a Jupiter-sized planet out of the plane of orbit, and possibly flip it to the opposite plane. The definition of "close," of course, will vary with the mass of the second body. If that second body is really large—maybe it's an additional star—it can be quite far away, at least relative to the typical distance between planets.

The researchers involved in the announcement helpfully point to an animation, generated by a Harvard researcher, that shows a large collection of odd orbits that can be generated if a stellar companion is orbiting out-of-plane. Any smaller bodies that were in orbit anywhere near this planet would obviously have had an equally chaotic history.

The big message here is that it's tempting to try to slot what we're finding elsewhere in the galaxy into what we know about our own solar system and its history. But it's dangerous to extrapolate too much for a sample size of one, and we need to make sure our observations are able to find things that we might not necessarily expect to see.

Although the paper hasn't been through peer review yet, the processes of detecting extrasolar planets and measuring stellar rotation are pretty well worked out, so the survey results seem reliable.

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it's a parallel universe actually - where everything is in reverse :P

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