When a star passes another star, any planets orbiting them can be flung into space, destroying what could once have been hospitable solar systems. And while rare in the outer parts of our galaxy, such events might be common in the inner regions of the Milky Way.
That’s according to researchers who have found that about 80 per cent of stars in our galaxy’s central bulge pass close by another every billion years, getting within 1000 astronomical units – 1 AU is the distance from Earth to the sun. And half of these bulge stars experience more than 35 such encounters in the same time frame.
“I was naively expecting maybe 10 percent of stars to have these types of close encounters,” says Moiya McTier at Columbia University in New York. “I was really shocked when I saw the number.”
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The Milky Way’s bulge is an area in the middle of the galaxy where stars move faster and are more tightly packed than stars in the outer disc, like our sun. The bulge is loosely defined as extending to about 20,000 light years from the galaxy’s centre, with up to 20 per cent of our galaxy’s mass located in it.
To calculate the number of these encounters, McTier and her team simulated the orbits of 100,000 stars in the bulge. They found that 35 percent of these stars are expected to have more than one encounter within 100 AU every billion years, and 0.02 percent within 10 AU – the distance from Saturn to our sun.
These encounter rates dramatically drop as you move out from the galaxy’s centre. “Even before the bulge officially ends, there are virtually no encounters,” says McTier. They are not impossible, however. Our own sun is expected to have a close encounter with a star in 1.5 million years.
For stars that do experience such encounters, the effects on any planets or planetary material around them can be devastating. Previous studies have suggested that as much as 97 per cent of the material orbiting a star can be swept away by an encounter at 100 AU.
The effects of encounters can range from small changes to the orbits of planets to “major disruptions like planets being ejected and becoming free-floating planets,” says Rosalba Perna at Stony Brook University in New York. “This makes the bulge an interesting place where planetary architectures can be changed over their lifetime by encounters.”
These regular encounters may therefore have consequences for the stability and habitability of planetary systems in the centre of our galaxy. “These encounters can be really dangerous for planets around their stars,” says McTier.
For now, much of this work relies on simulations, as the precise orbits of stars in our galaxy are not known. But upcoming data from the European Space Agency’s Gaia space telescope, which is aiming to map a billion stars in our galaxy, could provide valuable new insights.
“Once Gaia releases its full data release, maybe in 2025, we’re going to have a much better insight into where the stars in the bulge are,” says McTier. “So it’s definitely going to help with this type of work.”
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