Approximately one in every 50 planets may have been stolen from other stars in their infancy – perhaps even in our own solar system.
We’ve thought for some time that planets in extremely wide orbits around stars may have been born elsewhere, because it is difficult to form planets at such distances from a star. The hypothesised Planet Nine in our solar system, for example, may be a stolen exoplanet snatched from a passing star.
Such events may occur early in the lives of stars when they are born in dense clusters from the same cloud of dust and gas. These clusters can contain thousands of stars, often packed relatively closely together before later spreading out. If any planets form around these young stars, it might be possible for them to jump ship early on as other stars pass nearby.
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Emma Daffern-Powell at the University of Sheffield, UK, and her colleagues worked out how often this might occur. They modelled an example cluster of 1000 stars, each separated by a third of a light year. Half of the stars had a single planet with an orbit at least as far out as that of Neptune – a simplistic model designed to represent how often planet transfer could take place in more complex environments.
The results showed that about 2 per cent of planets were “stolen” in the cluster’s first 10 million years, meaning they were directly transferred between stars before they spread out. A further 2 per cent were “captured”, meaning they became free-floating planets unbound to a star before being grabbed by another star. The rest either survived in orbit around their star or were flung into the galaxy by encounters with other stars.
at the University of Bordeaux, France, says the study provides a novel look at the interactions of planets in young star clusters. “I didn’t think there would be as many chances for stealing planets,” he says.
For a star to grab a planet from another star, it would need to approach at a few hundred astronomical units (AU) – 1 AU is the Earth-sun distance. Captured planets would be in wider orbits than stolen planets because “the encounter is less energetic”, says at the University of Sheffield, a co-author of the study. “If you have a very energetic interaction, that planet has to have a fairly small orbit,” he says.
Both captured and stolen planets would also have less circular and more inclined orbits – ones that are angled to the flat plane of the system.
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By directly imaging star systems, we can look for such planets by mapping their orbits. at Leiden University in the Netherlands says we have already seen evidence for such worlds. “My group has seen three planets where they’re at 100 to 500 AU,” he says. “This paper is saying maybe that’s not their parent star.”
At the moment we have only directly imaged a few dozen planets, but that number should increase as more powerful telescopes like the European switch on in the coming years. It may even be possible to compare the composition of planets with their stars to see if they originated there. “We could see if the chemistry of planets matches the chemistry of the star,” says Kenworthy.
Monthly Notices of the Royal Astronomical Society
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