Alien Star Could Have Side-swiped Our Solar System Closer Than Voyager

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Our solar system might still bear the scars from an extremely close shave with an alien star. Such an encounter – the closest pass we know of – would have shaken up objects on the outskirts and might even mean there's no Planet Nine after all.

Some of these objects have bizarre behavior that doesn't make sense yet. Some have eccentric , or very elongated, orbits. Others have highly inclined orbits, meaning they swing above and below the orbital disk that most other objects stick to. A few even orbit backwards , traveling against the traffic of almost everything else in the solar system.

To explain these eccentricities, astronomers have proposed the existence of a ninth large planet , with a mass of Mars or bigger, lurking in the dark out there. Its gravitational influence could theoretically cause these signs, but extensive searches have so far turned up no direct trace of such a Planet Nine.

Now, astronomers in the Netherlands have suggested a new explanation – a close encounter with another star billions of years ago. To test the idea, the team conducted more than 3,000 simulations modeling what happens when stars of different masses fly through planetesimal disks of different widths, passing at different distances. And sure enough, they were able to recreate the weird orbits seen in the modern solar system.

"The best match for today's outer solar system that we found with our simulations is a star that was slightly lighter than our Sun – about 0.8 solar masses," said Amith Govind, co-author of the study. "This star flew past our sun at a distance of around 16.5 billion km (10.3 billion miles). That's about 110 times the distance between Earth and the Sun, a little less than four times the distance of the outermost planet Neptune."

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