Solitary Sedna (Image: NASA/JPL-Caltech)

Our sun is a thief. Over four billion years ago, it stole hundreds of frozen mini-planets from a passing star – and the peculiar planetoid Sedna is one of them.

With its extremely elongated orbit taking it 200 times further from the sun than Neptune every 11,400 years, Sedna has been a mystery ever since its discovery in 2003. Its nearest neighbours, the thousand-plus “ice dwarfs” that populate the Kuiper belt beyond Neptune, are believed to be the frozen remnants of our solar system’s formation.

But Sedna, and a dozen other objects with similarly wonky orbits, are harder to explain. A gravitational kick from a planet in our solar system could never have thrown them into such orbits.


One idea was that Sedna could have been jolted out of place by a passing star, but there was little evidence to back it up.

Now Lucie Jílková of Leiden Observatory in the Netherlands and her colleagues claim that the passing star wasn’t a trouble-maker but a victim: Sedna and its siblings were actually stolen from it when it ventured too close to the sun.

Using a low-cost, custom-built supercomputer, the team simulated over 10,000 possible encounters to find out which combination of a star’s mass, fly-by distance and velocity would lead to ice dwarfs being gravitationally captured into Sedna-like orbits.

They conclude that the passing star would have been 80 per cent more massive than the sun, and that it came as close as 34 billion kilometres – 51 times Neptune’s distance. The encounter probably took place when the sun was very young and still a member of a newly born star cluster.

You rob me, I’ll rob you

The passing star would itself have stolen hundreds of ice dwarfs from the sun’s Kuiper belt, and flung hundreds more into interstellar space.

Scott Kenyon of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, who together with Ben Bromley of the University of Utah in Salt Lake City was one of the first to propose the idea, says the simulations are “pretty convincing”.

But Bromley points out that the orbits of Sedna-like objects show a persistent alignment that is hard to explain without the “shepherding” effect of a larger planet lurking in the outer solar system. Such a planet doesn’t seem to sit comfortably within the passing-star scenario.

“It remains to be shown that the massive shepherd – or its building blocks – could have survived the fly-by,” he says. “A big piece of the puzzle is still missing.”

If we could show that Sedna-like objects have a different chemical make-up from the rest of the Kuiper belt, that would be convincing evidence that they were stolen from another star. But as the first spacecraft to visit the Kuiper belt is only just about to reach Pluto after a nine-year journey, it may be a long time before the Sedna mystery is settled.

Journal reference: http://arxiv.org/abs/1506.03105