Will a “wandering black hole” hit the Earth?

A "wandering" supermassive black hole was recently captured by the Hubble Space Telescope. After analyzing a variety of possible scenarios, researchers believe that the most appropriate explanation should be a large black hole rapidly moving away from its galaxy.

"Unlike previous discoveries of stellar-mass 'wandering black holes', this should be the first observational evidence of a supermassive 'wandering black hole' being ejected from the center of a galaxy." Gou Lijun, a researcher at the National Astronomical Observatory, told Science Times.

There is a supermassive black hole at the center of almost every galaxy. As a celestial body in Einstein's theory of relativity, black holes are notoriously elusive. And "wandering black holes" are even more solitary. Scientists have always believed that there are many black holes "wandering" in interstellar space, but have never found definite evidence.

Why do black holes wander?

There are black holes with stellar masses and supermassive black holes in the universe. The former are generally 3 to 100 times the mass of the sun, while the latter are generally more than 1 million times the mass of the sun and exist in the center of galaxies.

"Stellar-mass black holes are mainly formed when massive stars collapse at the end of their nuclear burning because there is no force to resist gravity. The formation mechanism of supermassive black holes is more complicated and is generally believed to be formed by the gradual growth of other medium-mass black holes through mergers, accretion and other processes." Xie Fuguo, a researcher at the Shanghai Astronomical Observatory, told reporters.

As for why black holes "wander", Xie Fuguo explained that the gravitational wave radiation brought by the merger of two black holes may make the merged black hole gain a higher speed, or when three black holes interact, one of the black holes will also gain a higher speed. At least when the speed reaches several hundred kilometers per second, the black hole can truly break free from the gravitational bondage of the galaxy and start the journey of a "wandering black hole". "The location of the 'wandering' depends on how fast it runs and how long it has been running." Xie Fuguo further explained.

This coincides with Gou Lijun's view. "Supermassive black holes have several ways to escape from the center of a galaxy, but the first step is always a galaxy merger, which leads to the formation of a binary system in the center of the merger remnant." Gou Lijun said that the special thing about the supermassive black hole discovered this time is that it may have originally had a black hole "companion" with a mass comparable to its own, belonging to a binary black hole system. The third black hole arrived at the center of the galaxy before the binary black hole merged. Under the action of the three bodies, one of the black holes with the smallest mass was ejected at a very fast speed and began its "wandering" life.

How to determine the identity of a "wandering black hole"

In the vast universe, how can we discover a completely black object?

"For supermassive black holes, researchers can infer the mass of the central black hole through the luminous gas surrounding it." Gou Lijun said that the reason for speculating that there is a black hole at the center of the gas is because it is dense enough.

Research has found that the "tail" of this supermassive black hole is a band of light formed by stars, which is an arrangement of strings of stars. Because black holes have extremely strong gravitational force, they can make nearby matter gather towards them, and the gravitational concentration of supermassive black holes can make some nebulae condense quickly and accelerate the formation of stars. At the same time, it is estimated that the movement speed of this black hole may be around 1,600 kilometers per second.

Therefore, researchers believe that this should be the ejection caused by the merger of black holes. If this view is verified, it will be the first time that humans have clearly confirmed that supermassive black holes can escape from the center of a galaxy.

This is different from the way the "wandering black holes" of stars in the Milky Way were discovered. Gou Lijun introduced that although we have never seen a black hole, we can observe its gravitational attraction on distant celestial bodies, which is called gravitational lensing.

Einstein's general theory of relativity assumes that massive objects will cause bending in space-time, bending the light near them, a process known as gravitational lensing. British astronomer Jonathan Eddington proved the existence of this phenomenon in an experiment observing a total solar eclipse.

"The gravitational lens effect is a method to find isolated black holes." Xie Fuguo said that black holes will change the space-time around them, causing the bending and amplification of light propagation. If a distant celestial body is moving and there is a black hole between it and the earth in the line of sight, a gravitational lens effect will occur.

There is no supermassive "wandering black hole" in the solar system

"We have discovered that the black hole closest to the Earth is about 3,000 light-years away, so the impact of the black hole on the Earth is almost non-existent." Gou Lijun said that the supermassive black hole discovered this time occurs in a dwarf galaxy RCP28 about 7.5 billion light-years away from us.

In Xie Fuguo's opinion, there are definitely no "wandering" supermassive black holes like the one discovered this time in the Milky Way (at least around the solar system). There are a large number of stellar-mass black holes in the Milky Way, and although the probability is very low, these stellar-mass black holes may indeed visit the solar system.

"Humanity certainly does not welcome any 'wandering black hole' to visit the solar system, because it will seriously affect the gravitational field structure of the solar system, causing the motion orbits of all celestial bodies in the solar system to become chaotic." Xie Fuguo speculated that it may also make the Earth's orbit become elliptical, causing a year to far exceed 365 days, and at the same time the positions of the equator and the North and South Poles will undergo earth-shaking changes.