A听passing star may have buzzed the solar system billions of years ago, sending thousands of small icy worlds beyond Neptune鈥檚 orbit careening into interstellar space and leaving others in wildly tilted paths around the sun.
This scenario, dreamt up by Susanne Pfalzner at the Max Planck Institute for Radio Astronomy in Bonn, Germany and her colleagues, could explain some unusual features of the outer system.
Certain听objects beyond the orbit of Neptune have very inclined and elongated orbits, relative to the rest of the solar system. The most famous of this group is Sedna, a dwarf planet in an 11,400 year orbit. Astronomers have seen about 20 of these objects, but there is no obvious explanation for how they got where they are.
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Some models suggest that the trans-Neptunian objects may have originally formed in the region where Uranus and Neptune are now, around 30 astronomical units from the sun (an AU is the distance between the Earth and the sun). They may then been flung out as Neptune migrated away from the sun, creating an unusual lack of mass in this region, but the Sedna-like objects are too far away for that to work, Pfalzner says.
Star disruption
Pfalzner and her team say they may have formed further out and been disrupted by a passing star. The idea has been proposed before, but dismissed by many astronomers as too unlikely.
To firm it up, the team used recent data about the behaviour of young stars to run thousands of simulations of a stellar flyby. They found the odds of one occurring are about 1 in 4 over a billion-year period, which means such an incident is reasonably likely to have听happened in our solar system鈥檚 past.
They modelled the early solar system as a disc of particles, with the density dropping as one gets further from the sun, much as we see in observations of other stars. They found that a star听of similar mass to the sun passing at a distance of 80-100 AU produces something like our solar system, by disrupting outer worldlets and creating the steep drop-off in the amount of mass past 30 AU 鈥� much like what we see in our own solar system.
The work also adds a twist to the hypothetical Planet Nine, which some astronomers believe exists on the edge of the solar system. First proposed in 2016 to explain the strange orbits of objects like Sedna, Planet Nine is believed to be 10 times the mass of Earth, which is too heavy to have formed so far from the sun.
Pfalzner says her model suggests Planet Nine, if it exists, would be around the same mass as Earth. This would make it massive enough to stabilise Sedna鈥檚 orbit after the dwarf planet was being pulled further out by a passing star.
Michele Bannister of Queens University in Belfast, says the paper is good attempt at explaining the orbits of objects like Sedna, but will need further refinement.
Pfalzner acknowledges her model doesn鈥檛 reproduce every feature of the outer solar system, but says it provides a solid explanation for the orbits of Sedna-like objects that the current models do not.
鈥淵ou could well have a kind of hybrid scenario, where the movement of the planets is responsible for the things we find in the inner solar system, like the low mass of Mars, and听a fly-by [is responsible] for the properties of the outer solar system,鈥� she says.
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