Milky Way may not collide with the Andromeda galaxy in 5 billion years after all: study
But forecasts of the impending "Milkomeda" mega-galaxy may be exaggerated.
According to a new study, published in Nature Astronomy, there's a close to 50 per cent chance they won't collide at all.
Even if they did collide, it would probably take much longer — more like 10 billion years.
This means it will happen long after the death of our Sun, expected in roughly 5 billion years.
It's a finding that surprised the team, said study co-author Ruby Wright, an astrophysicist at the University of Western Australia.
"We expected [the study] to confirm the classic picture that the Milky Way and Andromeda merger is a done deal," Dr Wright said.
The Andromeda and Milky Way galaxies are the two biggest galaxies in our section of the universe, referred to as the Local Group.
Despite being 2.5 million light-years away, Andromeda's huge size, at about 200,000 light-years wide, makes it visible with the naked eye.
From Australia, it can be seen just above the northern horizon in spring and summer.
In 1913, US astronomer Vesto Slipher discovered Andromeda was getting closer to the Milky Way — even before we knew it was a separate galaxy, or how far away it was.
We now know Andromeda is closing the distance to the Milky Way at about 100 kilometres per second.
Astronomers have generally agreed that the two spiral galaxies would crash into each other within 5 billion years, leaving behind an elliptical pile of stars.
To predict the fate of the galaxies, the researchers used datasets from the Gaia and Hubble space telescopes to model how they would move over the next 10 billion years.
While it's fairly straightforward to learn whether something is getting closer or further in space, it's harder to tell if it's moving in any other directions.
"At the moment, the galaxies are being drawn together by the influence of their own gravity, but obviously these galaxies don't exist in isolation," Dr Wright said.
While Andromeda and the Milky Way are the biggest players, there are about 100 other smaller galaxies in the Local Group.
The largest of these are the Triangulum Galaxy and the Large Magellanic Cloud (LMC), a major satellite galaxy of the Milky Way in the southern sky that is visible to the naked eye.
The team ran 100,000 different simulations to test how the movements of the different galaxies affected each other.
Although it is only about 15 per cent of the mass of the Milky Way, the study found the LMC could potentially tug our home galaxy out of Andromeda's path.
In just under half of the simulations, the galaxies passed each other uneventfully: Andromeda got closer and then further away from the Milky Way, like watching a plane fly across the sky.
In only 2 per cent of the simulations, the galaxies got close enough that they'd be pulled together within 5 billion years.
"But if they miss by a certain amount, they end up in this almost galactic dance," Dr Wright said.
"The gravity between them is still quite strong, so they'll pass by each other, reach another point where they are very far apart, and then they'll start coming back together again."
These scenarios, which were roughly half of those modelled, saw the galaxies colliding within 10 billion years.
Most simulations, though, predicted the Milky Way and the smaller LMC would merge within 2 billion years.
But Dr Wright said there were huge uncertainties in each simulation.
Lisa Harvey-Smith, an astrophysicist at the University of New South Wales who wasn't involved with the study, said predicting the future of Local Group movements was a very difficult thing to do.
"It's almost like you're playing snooker, you've got 100 balls on the table, and you smash them and see where they're going to go in 10 billion years," she said.
"The problem is the snooker balls are then soft and squishy, and they change shape, and there's all these invisible balls on the table as well — which is dark matter."
Professor Harvey-Smith, who wrote a book centred on the collision of the Andromeda galaxy with our own, said the new study was "a really good paper to get people thinking".
"You constantly have to update science knowledge," she said.
"It's great to see a fresh perspective on things that sometimes become embedded."
But she suspected that, because of the huge uncertainties involved, future research might come to different conclusions about the Local Group's movements again.
"We in astronomy are always grasping at data beyond our reach," Professor Harvey-Smith said.
Dr Wright believed data from newer telescopes would "absolutely" challenge the team's findings.
"We're definitely not claiming to have predicted the future with certainty, but rather we're showing it's more uncertain than previously appreciated."
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