Astronomers have believed for decades that the Milky Way is on a collision course with our nearest big neighbor, Andromeda. They seemed all but certain the two galaxies were destined to smash in about 4 to 5 billion years, combining into one colossal galaxy in space.
In that scenario, the merger would trigger a riot of star births and deaths and maybe even thrust the sun into a different orbit. We were told all this was inevitable.
But new data from NASA's Hubble Space Telescope and the Gaia spacecraft, run by the European Space Agency, suggests that the future of Earth's home galaxy is not that cut and dried. The study, which relied on 100,000 computer simulations stretching 10 billion years into the future, appears in the journal Nature Astronomy.
"Based on the best available data, the fate of our Galaxy is still completely open," the scientists wrote.
The Milky Way and Andromeda are part of a cluster of about 100 galaxies, held together by gravity, known as the "local group." For more than a century, scientists have known Andromeda is creeping toward the Milky Way. That led many experts to believe a collision was unavoidable.
Turns out the future is much more murky.
The researchers ran computer simulations with 22 different variables that tested different possible routes for the galaxies, trying to predict where they would end up. Their study found that the two galaxies would remain in the same plane as they circle each other.
"But this doesn't mean they need to crash," said Till Sawala, the lead author, who is based at University of Helsinki in Finland, in a statement. "They could still go past each other."
The researchers found that two other nearby galaxies — the Large Magellanic Cloud and Messier 33 — could have significant influence on whether the Milky Way and Andromeda ultimately collide. Though these galaxies are smaller, they have enough mass to tip the scale.
Here's where things get downright mind-blowing: When Messier 33, sometimes called M33 or the Triangulum galaxy, is included in the simulation, it makes a galactic merger more likely. But the Large Magellanic Cloud, whose orbit intersects those of the Milky Way and Andromeda, makes it less likely.
In short, it's a real "will they, won't they?". With all of the data put together, the team arrived at a near-equal, 50-percent chance of the two galaxies hitting within the next 10 billion years. In about half of the possible outcomes tested, the two main galaxies skirted by each other, with a margin of 500,000 light-years between them or less: That's about five times the width of the Milky Way.
Sawala acknowledges the irony that with more precise Hubble data, scientists are less sure about what will happen. After all, they've only narrowed down the odds of a cosmic cataclysm to a coin toss.
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"That’s because of the more complex analysis and because we consider a more complete system," Sawala said. "But the only way to get to a new prediction about the eventual fate of the Milky Way will be with even better data."
Galaxy mergers happen over an unfathomable timescale for humans to imagine, on the order of several hundred million years. Previous Hubble research has shown that these crashes were more common in the past when the universe was smaller. Regardless, collisions continue to occur because galaxies are thought to be bound by the gravity of so-called dark matter — invisible space material suspected to exist — surrounding them.
There are still a lot of unknown factors. In addition to dark matter, galaxies in the vicinity that have yet to be discovered could also influence whether Andromeda and the Milky Way merge. Future data releases from the Gaia mission, which is mapping the positions and movements of stars, could help improve predictions.
"As it stands," the scientists wrote, "proclamations of the impending demise of our Galaxy seem greatly exaggerated."
