Rogue Black Hole Wandering the Milky Way Could Kick Planets Into Deep Space

Astronomers estimate that there are around 100 million black holes roaming around our Milky Way galaxy, and in the very unlikely scenario that one were to pass through our solar system the Earth could be flung out into deep space.

It's almost certainly not going to happen—not in any time scale that matters to us anyway—although scientists have recently estimated that the closest isolated stellar-mass black hole may be as close as 80 light-years away.

Black hole
A stock image shows an artist's depiction of a black hole in space. Scientists can detect black holes by the warping effect they have on the light from other objects. Cappan/Getty

The new approximation follows the discovery of what is thought to be direct evidence of a lone black hole wandering through space.

Astronomers think that many such objects exist, but they're hard to detect. Usually, the existence of black holes can be inferred by the effect they have on their companion stars.

Sometimes though, a black hole can end up single. This might happen during the black hole's birth, when a huge star collapses in on itself. When this happens, the star's core is crushed by gravity to form a black hole, but if the accompanying explosion is not symmetrical then the black hole can be flung out into the galaxy "like a blasted cannonball," according to NASA.

Using Hubble telescope data, two teams of astronomers think they have detected one such lone black hole located around 5,000 light-years away in the Carina-Sagittarius spiral arm of our galaxy, the spacy agency said last week. The potential black hole is known as MOA-11-191/OGLE-11-462 and has been studied before.

The astronomers think the black hole is there because of the detection of warped starlight, known as a macrolensing event, which suggests a black hole had passed between a star and Earth. By measuring this light deflection very precisely, the teams were able to estimate that the black hole weighs between 1.6 and 7 times as much as the sun and is moving at around 100,000 miles per hour.

The object is not certainly a stellar-mass black hole—macrolensing events can be caused by other roaming objects like stars. Still, it's an interesting possibility that could allow for more accurate estimations of how many roaming black holes there are.

"Our discovery of a black hole is consistent with the theoretical calculations which suggest that there should be about 100 million black holes in our Galaxy," said Kailash Sahu, an astronomer at the Space Telescope Science Institute in Maryland who led one of the studies investigating the potential black hole. He told Newsweek: "Then, assuming black holes follow similar distribution as stars, one expects, statistically, that the nearest black hole may be about 80 light years away."

Do these invisible cosmic wanderers pose any sort of threat to Earth? In theory they could, but the chances are quite literally astronomically small according to Jessica Lu, associate professor of astronomy at the University of California, Berkeley, who co-authored one of studies investigating the Hubble data.

"Small black holes, the ones born from dying massive stars, are very compact," she told Newsweek. "Their event horizons are only 40 km [25 miles] across—city-sized. Even though there are a lot of black holes in the galaxy, our chances of running into one directly are astronomically small."

But let's say our solar system was dealt a very unlucky hand. In the case of a rogue black hole zooming through our region of space, it could be enough to seriously disturb the orbits of our planets.

"If a wandering black hole or neutron star passed anywhere close to our solar system, we would certainly see its effects on orbits of the outer planets and comets in the Oort cloud," said Lu. "We might expect to see some comets kicked out of our system and some kicked into the inner solar system. Perhaps we would see an increase in the number of comets.

"If the black hole passed through the middle of our solar system, it could be more disruptive, especially to the outer solar system. Some minor planets and asteroids would be kicked around and maybe even kicked out of our solar system."

Of course, this is nothing to lose sleep over, and scientists are still working to understand more about these free-floating black holes that, while thought to be abundant, remain elusive.

Lu's study and Sahu's study were both published as pre-prints in May and have been accepted into The Astrophysical Journal and The Astrophysical Journal Letters.