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Giant planet pileups in far-flung star systems: Computer says yes

Boffins crack stellar 'celebrity' migration riddle

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Top brainboxes armed with a British supercomputer say that they've cracked the riddle of just why it is that massive planets - spied across the vasty interstellar gulfs in recent times - tend to prefer certain orbits around their faraway parent stars.

"Our models offer a plausible explanation for the pile-ups of giant planets observed recently detected in exoplanet surveys," says Richard Alexander of Leicester uni.

It seems that known exoplanets, most of which are enormous gas giants on the lines of Jupiter or Saturn here in our solar system, are found mainly at distances around 1 Astronomical Unit (AU) from their own suns. This is the same distance as Earth is from our Sun.

We here on the Reg farflung-planets-glimpsed-across-the-vasty-gulfs-of-interstellar-space desk had sort of formed an unscientific impression that this might be because the techniques used to spot exoplanets struggle to see anything which is not a) vast and b) quite close to its parent star. But according to Alexander and his colleague Ilaria Pascucci of Arizona uni, this isn't true: as star systems form, they naturally tend to accumulate gas-giants at this sort of distance - our system is atypical in having them much further out.

"Our results show that the final distribution of planets does not vary smoothly with distance from the star, but instead has clear ‘deserts' – deficits of planets – and ‘pile-ups' of planets at particular locations," says Pascucci.

The two boffins' theory shows that as a young star system collapses onto its central sun, the interplay between the hot solar wind blasting material outward and gravity sucking it inward changes sharply according to distance bands, which results in a clear band from say 1 to 2 AU out. Gas-giant worlds naturally get moved inwards through this clear band and then achieve orbit once they hit the next band of dust and proto-stuff.

According to a Leicester uni statement announcing the new research:

Giant planets migrate inward before they finally settle on a stable orbit around their star. This happens because as the star draws in material from the protoplanetary disk, the planets are dragged along, like a celebrity caught in a crowd of fans.

However, the researchers discovered that once a giant planet encounters a gap cleared by photo-evaporation, it stays put.

"The planets either stop right before or behind the gap, creating a pile-up," explains Pascucci.

The two boffins confirmed their theory with the aid of some mighty number-crunching from the ALICE High Performance Computing Facility at Leicester uni. The resulting paper is to be published in hefty astro mag Monthly Notices of the Royal Astronomical Society, or you can read it online in advance here. ®

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