What will happen to Earth if a rogue star comes in close proximity?

Stars are kind of stuck to their galaxies because of gravity and go along with the flow of their surroundings. But every now and then, something messes up this connection. Like, if a star gets too cozy with a supermassive black hole, the black hole might kick it out into space, turning it into a lone wanderer.

What if one of these random stars barged in too close to Earth? It’s not super likely, but it’s not impossible either. Over a gazillion years, our Solar System has settled into a pretty chill routine. The planets do their thing, and the Sun just chills in the middle of it all.

But if some other star decides to crash the party and comes too close, the invisible gravitational ties that keep everything running smoothly would be seriously messed up. Earth’s just a puny rock, packing only about a tiny fraction of the Sun’s hefty mass. Our whole existence relies on the Sun and its gravity game, so if another star muscles in on our cozy setup, Earth would be totally at the mercy of this new gravitational rulebook.

Solar System won’t always be calm and steady

There’s this new paper that dives into what could go down if a renegade star gets within 100 astronomical units (AU) of our Sun. The paper’s called “Future Trajectories of the Solar System: Dynamical Simulations of Stellar Encounters Within 100 au.” It’s set to be published in the Monthly Notices of the Royal Astronomical Society. Sean Raymond, an astronomer from the Laboratoire d’Astrophysique de Bordeaux, CNRS (National Center for Scientific Research), and the Université de Bordeaux, is the main brain behind it.

So, here’s the deal: the calm and steady routine in our Solar System won’t stick around forever. The Sun is on the move, and, in the next billion years or so, it’s going to amp up its brightness. Earth is hanging out pretty close to the inner edge of the habitable zone. Just a smidge closer to the Sun, and the delicate equilibrium that keeps liquid water chillin’ on the surface will be thrown out of whack.

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What happens when Earth encounters a rogue star?

So, in that same billion-year ballpark, there’s like a 1% shot of running into a rebel star. If that goes down, what’s the scoop on Earth? Is it gonna get pushed out of the comfy habitable zone?

“Earth has about a billion years of habitable surface conditions remaining,” the authors write. That’s in a closed system, which, for the most part, is our solar system. “While the orbital evolution of the planets is largely determined by secular and resonant perturbations,” the authors explain, “passing stars can have a consequential influence on the planets’ orbits.”

Alright, so these wandering stars, they go by names like intergalactic stars or hypervelocity stars, mainly because their paths shoot them out of the Milky Way. But don’t sweat it; most of these cosmic drifters, like Kappa Cassiopeiae, are a whopping 4,000 light-years away and won’t be swinging by anytime soon. Some rebels, like the 675 discovered by the astronomers at Vanderbilt University in 2012, got the boot after messing with the Milky Way’s supermassive black hole, but they’re on trajectories that won’t be giving Earth a close encounter.

These rebel stars, sometimes called intergalactic or hypervelocity stars because they’re on paths that boot them out of the Milky Way, aren’t getting anywhere close to Earth. Take Kappa Cassiopeiae, for instance – it’s a whopping 4,000 light-years away and won’t be making a pit stop here. Then you’ve got the troublemakers found by the Vanderbilt University astronomers in 2012, those 675 rogue stars? They got the boot for tangling with the Milky Way’s supermassive black hole, but their routes aren’t bringing them anywhere near Earth.

What will happen to Earth in such a scenario?

Okay, so it’s not super likely, but it’s on the table. Considering the entire galaxy, chances are pretty high that at some point, somewhere, there’ll be a stellar drive-by, getting within 100 astronomical units (AU) of another star. Now, if that star happens to be our Sun, what’s the deal for Earth?

The crew ran some serious N-body simulations to figure out what could go down for Earth. They kicked things off with our Solar System’s eight planets and threw in one rogue star for good measure. They made sure these rogue stars matched up with the masses and speeds of the stars nearby in our hood. Then, they played around with various speeds and paths for the star to see the whole spectrum of possibilities for Earth. All in all, these researchers cranked out a whopping 12,000 simulations.

“If a star passes within 100 au of the Sun, there is still a very high chance that all 8 Solar System planets will survive,” the authors write.

So, what goes down next pretty much depends on this thing called angular momentum deficit (AMD). It’s like a gauge for how excited a planetary system’s orbits are and whether it can keep its act together in the long run.

So, what happens when we lose one of our Solar System’s planets? Well, the simulation gave us a wild mix. Mercury’s kinda on the edge, sometimes getting smacked into the Sun. Then there’s Earth, possibly crashing into Venus, or the ice giants Uranus and Neptune getting kicked out. There are even scenarios where only Earth and Jupiter make it, or just Jupiter on its own. And, brace yourself: in one seriously apocalyptic scenario, all eight planets get booted out.

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The chances of such events are slim

Now, the other outcomes aren’t as flashy. In some, all eight planets just chill with no major issues, or they get a little nudge but nothing too crazy. But here’s the twist—even if all eight planets technically survive in most scenarios, survival isn’t a walk in the park. Their orbits might go haywire, with some getting flung way out into the Oort Cloud and things getting seriously chaotic.

“We determined the most common pathways through which planets may be lost, keeping in mind that there is a greater than or equal to 95% chance that no planet will be lost if a star passes within 100 au,” they wrote.

In the grand scheme of things, the chances of a stellar flyby within 100 AU are crazy tiny. And guess what the simulations say? If, by some cosmic coincidence, it does go down, the overwhelmingly likely scenario is that all eight planets make it through, just cruising in orbits that are a tad different from what they’re used to.