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Researchers published a paper last month, exploring the possibility that our sun might have once had a stellar twin! Could our solar system have once been a binary, or even a multi-star system?

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It’s been all over the internet lately: Astronomers discover that the Sun has an evil twin!

And that’s what killed the dinosaurs! Which would be an incredible discovery... if it were true.

These articles are based on a paper that was published last month in the Monthly Notices of the Royal Astronomical Society, where researchers predicted that every star could’ve started with a partner, like in a binary star system, and that most pairs just split up as time goes on. Which means that our very own Sun might have a long-lost sibling out there somewhere in the Milky Way. But we haven't actually found that sibling.

And even if this other star is out there, it isn’t what killed the dinosaurs. This is just one of those cases where people have gotten a little carried away with their headlines. So, let’s start with the facts.

A lot of stars are singletons like our Sun: They’re on their own, with no nearby stellar neighbors to keep them company. But astronomical surveys have found that tons of stars are in multi-star systems, which happens when two or more stars form from the same cloud of gas and dust and keep orbiting together.

Until now, astronomers figured that most loner stars were always only children, while stars in multi-star systems had always had siblings. The paper’s authors wanted to learn more about how multi-star systems evolve over time, so they observed 24 young, multi-star systems in the Perseus molecular cloud, a stellar nursery a few hundred light-years away that has lots of new stars.

Most of the systems were binaries, with two stars. Astronomers have found that binary stars tend to be pretty far apart when they’re young, and get closer together as they age. And that’s exactly what the team saw in the Perseus star systems. But they also found something interesting about the way that the binaries orbited each other at different points in their lives. Young binaries that were just a few hundred thousand years old, which practically makes them newborns to astronomers, tended to orbit in the same direction their parent clouds were spinning. That’s not too surprising: If they just formed, they should be moving the same way as whatever they formed from. But the fewer older binaries orbited in ways that were more random, and didn’t really have anything to do with the way their clouds spun. So something had to have changed their orbits as they aged.

To figure out what might cause these trends, the team did what astronomers often do best: They turned to computer simulations. They simulated stars forming under lots of different conditions, but only two of the models they tried actually fit the data. In both models, all stars had to form with a distant sibling, instead of just some of them, and the orbits would naturally shrink as the stars aged. In the first model, about 60% of the time, something like another star would get too close, and its gravity would fling the siblings apart. In the second model, the gas clouds around them would sometimes split into pieces and drag the stars apart. Either way, these binaries would become single stars like our Sun. So if either model is right, that would mean that every star started with a sibling, and that loners like our Sun lost their neighbors as they grew up. Still, the paper’s based on pretty limited data, so we don’t know for sure if every star actually does start with a sibling. We’ll have to wait and see if future observations match up.

But in the meantime, all of this has dredged up an old hypothesis about a brown dwarf star named Nemesis. Some scientists over the last 50 years have claimed that astronomical events like the asteroid that killed the dinosaurs happen too regularly to be by chance, about once every 27 million years. In the 1980s, that led astronomers to calculate that something like a brown dwarf, a small, failed star that couldn’t start a fusion reaction, could cause that pattern if it orbited the Sun from about one and a half light-years away. Its gravity could affect the orbits of things like asteroids and fling them toward Earth. The researchers named this star Nemesis. Because every good star needs an archenemy. So some people have been saying that this new paper proves that Nemesis has been out there, causing chaos in the inner solar system since it formed four and a half billion years ago.

But that’s not what it says. Like, at all. The models predict binaries either break up really early on or that their orbits shrink as they get older. So, yeah: the Sun might have once had a partner. But if it did, they would’ve split billions of years ago, and they wouldn’t be anywhere near each other now. The confusion probably comes from people calling the long-lost partner from this paper Nemesis. But again: this star doesn’t fit the theory that a star might have killed the dinosaurs. Even if the Sun’s sibling was still around, it should be hundreds of times closer than the 1.5 light-years astronomers predicted back in the 80s. Which means we almost definitely would’ve seen it by now. But astronomers have searched incredibly carefully for Nemesis, and they’ve come up with a whole lot of nothing. We just haven’t found anything that big anywhere near us. So it’s no surprise that this new paper never mentions Nemesis. Because even though it makes for a great headline, it’s just not relevant.

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