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Odd balls August 15, 2009

Posted by Jorge Candeias in Extrasolar planets.
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2 comments

Here’s something you don’t see every day: last week brought news of not one, but two extrasolar planets that are far more oddballish than anything we have in our Solar System. It started with the announcement that the newly discovered WASP-17b had two very bizarre characteristics. For starters, a planet with its mass, about half that of Jupiter, should be smaller than Jupiter… but that one is much larger. In fact, at twice Jupiter’s diameter (i. e., about 140 000 km), it is the largest planet known so far.

BANG! Strike one.

But what I find really juicy is another fact: WASP-17, the planet’s star, rotates in one direction, and the planet goes round it in the opposite direction.

WHAM! Whoa!

Why is this vastly interesting? Because planets form out of the same rotating clouds of matter that creates stars, which means that the direction of planetary movement around the star, at the time of its formation, has to be the same as the direction the star itself rotates. That’s what happens in the Solar System: not only all the planets go rond the sun in the same direction the sun itself rotates, but the same is true for the vast majority of planet’s moons and smaller objects… all the way down to comets. The only significant exception is Triton, the largest moon of Neptune, whose retrograde orbital motion (it’s the name this phenomenon has) led scientists to believe that at some point it was captured by Neptune from an orbit around the sun. That is: Triton is thought to have been a dwarf planet at some point. Fun fact: it’s about the same size as Eris.

OK, this may be true for almost all Solar System objects but isn’t for WASP-17b.

What this means is that something major happened to it after it was formed. Astronomers don’t seem to be giving much credit to a capture scenario, probably because the planet is so close to its star: it’s a “hot jupiter”. Instead they are speculating that at some point, probably after it was fully formed, it must have had a close encounter (a very close encounter!) with another giant planet that sent it spinning into the traffic, so to speak. The other, unknown, planet, was either sent into a very highly elliptical orbit, or ejected alltogether from the system. This last sentence is me, speculating, so don’t go thinking it’s the truth, OK?

Fascinating stuff!

And it gets better: the very next day, two teams announced the discovery of another retrograde planet, HAT-P-7b. This one wasn’t a new find; the planet was already known. The novelty here lies in the disalignment between the plane of the planet’s orbit and the rotation of its star. In this case, numbers are somewhat conflicting and unclear, so the only thing that is an absolute certainty is that HAT-P-7b does not orbit along the equator of HAT-P-7, as happens with all the major Solar System planets (i. e., all planets excluding most dwarfs): it may be highly inclined, orbiting along the poles or close to them, or, which seems more likely, it’s another retrograde, orbiting along the equator but in the opposite direction, like WASP-17b.

Double Whoa!

Planets, special, orderly, well-behaved objects? Yeah, right…

Addendum: Take a look at Exoplanetology blog, where the method used to make the discovery is very well explained. The only thing I don’t think is correct is where he talks about a violent collision: no collision with an object moving in the same direction could make a planet move in the opposite direction… but a very strong transfer of orbital momentum could.