Stars that steal give birth to backwards planets

Stealing gas from their siblings could leave stars with a motley crew of planets ? including ones with backwards orbits.

Our solar system is thought to have formed from a collapsing cloud of gas and dust that flattened out as it spun, rather like pizza dough. This explains why the planets all orbit the sun in the same direction as the sun itself spins, and share the same plane.

Exoplanets tell a different story, with some tilted at jaunty angles and others orbiting their stars backwards. Planet-on-planet violence is one explanation, but Ingo Thies of the University of Bonn in Germany and colleagues suggest the culprit is the star itself, before its planets are born.

The team made a computer model of stars forming in a cluster. The stars started out forming proto-planetary discs in the usual way. But if a star veered too close to another clump of matter, like another star's disc or a cloud of gas that hadn't formed a star yet, it sucked huge streams of gas ? up to 30 times the mass of Jupiter ? from its neighbours and into its own nascent disc.

Hot Jupiters

In the model, this stolen material tilted the disc. And when the angle and the mass of material were just right, the final disc ended up spinning in the opposite direction to the star. Any planets that formed in that disc did the same. The work will appear in Monthly Notices of the Royal Astronomical Society.

The influx of gas could also compress the inner part of the disc, making planets form there more quickly. These may be more susceptible to violent crashes, leading to further eccentric orbits.

This in turn could help explain why, unlike our solar system, which keeps the smallest and rockiest planets closest to the sun, many exoplanet systems have bloated gas giants, known as "hot Jupiters", as their innermost planets. When the smaller planets get flung out of the inner part of the disc, an overall conservation of angular momentum means these gas giants could get drawn in closer.

Reference: arxiv.org/abs/1107.2113

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