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.
The fringes of the solar system, beyond the orbit of the outermost known planet, Neptune, is a chaotic place. It’s populated by Trans-Neptunian Objects (TNOs) including dwarf planets like Pluto and Sedna, and thousands of smaller rocky and icy bodies like Arrokoth.
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.”
That might sound like a long way out, but on cosmic scales it missed by a whisker – it’s closer than the Voyager probes, which are more than 136 times the Earth-Sun distance (Astronomical Units, or AU). The previous closest known pass was Scholz’s Star, which swept past at more than 50,000 AU. Suddenly a mere 110 AU feels like a graze.
Of course, a star can’t pull a drive-by like that without leaving its mark. Not only could it produce the orbital oddities seen in TNOs today, but the team found that many others would have been ejected from the solar system entirely. Meanwhile, others were flung inwards, where they may still remain to this day.
“Some of these objects could have been captured by the giant planets as moons,” said Simon Portegies Zwart, co-author of the study. “This would explain why the outer planets of our solar system have two different types of moons.”
While it remains unconfirmed whether this scenario actually did take place in our solar system’s deep history, it does have one thing going for it: it’s a neat explanation for several mysteries. A stellar side-swipe could account for the odd orbits of TNOs, the apparent absence of a Planet Nine, and the origin of those gas giant moons.
The answer could come in the next few years, as the Vera C. Rubin Observatory comes online. This telescope is designed to detect transient objects and is expected to find tens of thousands of new, small bodies in the outer solar system. If it finds a lot of them also have eccentric or backwards orbits, it could build more evidence for a star fly-by. Or it could finally find Planet Nine itself.
The research was published in two studies, one in Nature Astronomy and the other in The Astrophysical Journal Letters. Check out the chaotic simulation in the video below.
Source: Forschungszentrum Jülich
