In the vast and mysterious universe, black holes are undoubtedly one of the celestial bodies that can most inspire human imagination.
However, discovering new black holes is not an easy task—where to start? Recent research has revealed a new class of black holes, including the closest known black hole to Earth.
As an astronomer, I have been studying black holes for many years. Personal experience tells me that finding new black holes is not as simple as people imagine. The first thing to consider is that black holes are black, and their gravity is so strong that not even light—the fastest substance in the universe—can escape their event horizon (the boundary around a black hole from which nothing can return). This means that astronomers can only detect black holes through their interactions with other celestial bodies around them. For example, we know that there is a supermassive black hole at the center of the Milky Way—Sagittarius A* (abbreviated as Sgr A*)—which has a mass more than four million times that of the Sun. Initially, astronomers observed a star moving rapidly around an invisible object and inferred the existence of a black hole. This astonishing result was enough for the astronomer to win the Nobel Prize in Physics in 2020. Two years later, in May 2022, members of the Event Horizon Telescope announced that they had successfully connected radio telescopes around the world to capture a photo of Sagittarius A*. This photo shows a spectacular ring of hot plasma surrounding the event horizon, formed by the black hole accreting gas from its surroundings.
This dormant black hole is not attracting material from a nearby companion star. It does not have a bright accretion disk, so astronomers have to look for other methods to detect such dark celestial bodies.
Black Holes—The Worst Hosts
While our galaxy has only one supermassive black hole, it is home to many smaller black holes. These black holes are formed from the collapse of massive stars (those more than 18 times the mass of the Sun) during supernova explosions. Thanks to the motion of black hole companions, we know of several black holes that exist in binary systems. The most famous is Cygnus X-1, which, despite being about 7,000 light-years away, is one of the brightest X-ray sources in the sky. Cygnus X-1 was the first source widely accepted as a black hole. In fact, astrophysicists Kip Thorne and Stephen Hawking famously wagered in the 1970s on whether Cygnus X-1 was a black hole. Thorne believed it was, while Hawking did not. By the early 1990s, when the evidence became clear, Hawking was willing to concede that it was indeed a black hole.
Today, astronomers are confident that Cygnus X-1 is a black hole with a mass about 21 times that of the Sun. A blue supergiant star orbits it, with an orbital distance of only 1/5 of an astronomical unit (astronomers refer to the average distance between the Earth and the Sun as an astronomical unit, abbreviated as AU). This blue supergiant has a mass between 20 and 40 times that of the Sun, and it too will eventually become a black hole one day. Its stellar wind continuously blows particles toward Cygnus X-1, a process known as accretion. The bright X-ray radiation we see is caused by this accretion. Researchers have also discovered several other black holes (and even neutron stars) through this method. These compact objects that accrete material from their companion stars form X-ray binaries together with their companions.
After two years of hard work, the Event Horizon Telescope team successfully captured the first image of the black hole at the center of the M87 galaxy in 2019, resulting in an astonishing image. This black hole has a mass of about 6.5 billion times that of the Sun, accreting a rotating disk of luminous material, which is the bright area surrounding the black hole in the image.
X-ray binaries are a special, atypical system. The black hole and its companion star must be very close to detect the material released by the companion star. So what about binary systems where the two stars are farther apart? What about black holes that have no companion star and therefore no material to accrete? Is there a way to find such black holes?
Historically, the answer has been no. I mentioned that isolated black holes are black. But just a few months ago, the European Space Agency's (ESA) Gaia mission and astronomer Kareem El-Badry found a way.
Black holes are one of the most mysterious objects in the universe, but researchers have figured out the main components of accreting black holes. Image credit: European Southern Observatory.
Gaia Satellite: A Source of Precise Data
Kareem is currently an assistant professor of astronomy at Caltech. He began searching for black holes while pursuing his PhD at UC Berkeley. He recalls, "During my PhD, I spent a lot of time looking for non-accreting black holes, but I didn't find any." When I first met Kareem, we were both postdocs in astronomy at Harvard University, and he was already well-known in the field, dubbed the "black hole decoder," although he never intended for such titles. Kareem spent a lot of time verifying the claims of other astronomers who reported discovering black holes. However, when he checked the data in those papers, he found that the authors had not actually discovered any black holes. In other words, Kareem found that these purported black holes could not withstand scrutiny. He stated, "The black holes mentioned in those papers do not exist at all, and the methods for detecting black holes need to be optimized."
However, one potential method for finding black holes caught Kareem's attention. This method involved the Gaia satellite. Launched in 2013, the satellite aims to measure celestial bodies, meaning it collects precise positions of millions of stars. Every few months, the Gaia satellite accurately measures and collects the positions of stars in space. Over time, the positional data for each star becomes increasingly precise. Every few years, newly released Gaia data updates previous star catalogs, causing a sensation.
In June 2022, the latest Gaia satellite data (Gaia DR3) was released, and Kareem was ready: just five minutes after the data was published, he ran a program on this massive new star catalog specifically looking for stars that exhibited "wobbling" in their orbits. This wobbling is precisely caused by the gravitational influence of an unseen black hole's companion star. When a star forms a gravitationally bound system with another body, these two (or even three or more) objects will orbit around a common center called the barycenter. Even if the second object cannot be observed, from the perspective of Earth, the star will move back and forth. Once its orbit is determined, astronomers can calculate the mass of its companion star using Kepler's laws of planetary motion. Astronomers often use this technique to explore exoplanets. However, before the launch of the Gaia satellite, the data was not precise enough to track black holes. Now, the precision of Gaia satellite data is sufficient for astronomers to investigate suspected X-ray binary activity and see if there is any wobbling. Subsequently, astronomers could search for these "wobbling" stars to study whether they might be X-ray binaries.
Now, with the Gaia DR3 data in hand, after his analysis, Kareem found that among the millions of stars in the database, two stars stood out. After analysis, the results showed that these two stars are orbiting the two closest black holes to Earth.
Is this puzzle piece in the right position?
As depicted by artists, isolated black holes without accretion disks are impossible to find throughout the universe because we cannot visually observe isolated black holes.
The first suspected black hole companion star is very similar to the Sun. Its size and mass are comparable to the Sun, located 1,560 light-years away. This star is bright and easily observable with professional telescopes. That is where all the similarities end. Unlike our Sun, this star orbits an invisible massive companion star, completing an orbit every six months. The distance between the two stars is roughly equivalent to the distance from Mars to the Sun. Through follow-up observations, researchers confirmed this orbit and estimated that the mass of this invisible body is ten times that of the Sun, much larger than the visible companion star. Moreover, this body cannot be another star because a star of such a massive size would be brighter and easier to detect than the first star. Furthermore, there are no other known objects that have such a massive size and are dark. By simple elimination, we can conclude that this invisible companion star must be a black hole. Kareem named this black hole Gaia BH1, which is currently known as the closest black hole to Earth.
This discovery made headlines and shocked the astronomical community. Gaia BH1 is not only three times closer to Earth than the previously closest black hole, V616 Monocerotis, but it is also a dormant black hole, meaning it does not accrete material from its companion star to form an accretion disk. Such a celestial body has never been discovered before. "Because the width of the orbit has remained constant, we can study the evolution of the black hole itself," explained Katie Breivik, an astronomer at Carnegie Mellon University who studies star and black hole evolution. Katie stated, "This dormant black hole is a remarkable treasure for studying black hole formation."
One unknown factor is how this binary system originally formed. Breivik stated, "Due to the interactions between the two stars, the orbit will narrow over time. Currently, we believe it is impossible for an isolated black hole to form." In other words, we do not understand how the Gaia BH1 system (which includes the black hole and the Sun-like star) formed. This is because, in previously known binary systems with black holes, they may have shared material with companions during their evolutionary processes when they were still stars. Additionally, there is no evidence that this Sun-like companion star has ever interacted closely with other stars or black holes. "This is a mystery," Breivik said.
Researchers discussed several possibilities: for example, suppose the system was originally a triple star system, where one companion star was ejected or swallowed by the black hole. But confirming this possibility is very difficult. Another possibility is that the Sun-like star and the star that eventually became Gaia BH1 were born in the same crowded star cluster and were pushed into the same orbit. After millions of years, it is now difficult to prove whether this scenario ever occurred.
This discovery also raises other questions: Is Gaia BH1 really a dormant black hole? Is it the only dormant black hole? This is the topic I want to study.
Should We Detect It?
Just as Kareem's scientific reputation was built on discovering black holes, my scientific reputation was earned by detecting the radio emissions from black holes. This radiation is typically emitted when black holes tear apart stars and consume them. At the end of 2022, when the news of Gaia BH1 broke, I was in my office at Harvard University, right next to Kareem, so I quickly ran over to ask if he or his colleagues had considered using the Very Large Array (VLA) in New Mexico to specifically observe this radio emission source. After all, that Sun-like star would emit solar wind particles similar to those from the Sun. At such close distances, just a few hours of VLA time might be enough to detect the radiation when these particles fall into the black hole or to determine that the black hole's accretion rate is very low.
Kareem agreed and arranged for VLA observation time to obtain better results. Unfortunately, no radio waves were detected at the location of Gaia BH1. But it wasn't all bad news. A few days later, the observation results were disappointing. Kareem stated, "In the data returned by the Gaia satellite, I also found another possible black hole, and if it turns out to exist, it will be the second closest black hole to Earth. Moreover, detecting the radio emissions from this black hole might be more appropriate. However, this black hole is located in the Southern Hemisphere. Are you interested in observing it?"
I think no astronomer would refuse such an opportunity. Kareem detailed the situation to me: Gaia BH2 is about 3,800 light-years away, farther than Gaia BH1. Gaia BH2 is separated from its companion star by 5 astronomical units (approximately the distance from the Sun to Jupiter), meaning it takes 3.5 years to complete one orbit. However, fortunately, this pair of stars will come closest to each other in February 2023 (reaching periapsis). More importantly, the star in this binary system is a red giant. The Sun will eventually become a red giant when the hydrogen in its core is nearly exhausted, causing it to expand, cool, and turn red, producing stronger stellar winds. So, although Gaia BH2 is farther from Earth than Gaia BH1, the stronger particle flow it scatters means we are more likely to detect the black hole's accretion radiation. If we still cannot detect any radiation, it would mean we have discovered a new class of black holes, a type that cannot be directly observed with current technology.
If the observation time is long enough, a star forming a binary system with an invisible black hole will exhibit noticeable wobbling in space. The Gaia satellite continuously scans in two directions to detect this wobbling and measures the position of each star with an accuracy of about 20 micro-arcseconds. Image credit: Astronomy: Roen Kelly.
We hurriedly wrote an observation proposal and were granted access to the MeerKAT radio telescope in South Africa. The observation plan was scheduled for the periapsis moment. On a snowy winter weekend, I woke up unprepared to receive an email informing me that the observation was successful. Now, the real fun was just beginning!
There are many reasons why I chose to become an astronomer, but whenever I receive new observational data, only one reason occupies my mind: for that moment, that instant of enduring the anxious wait, finally receiving data transmitted from the other side of the world. In that moment, I become the only person in the world who knows the latest information about the universe. How can I describe this soul-filling feeling? Especially when we explore whether we can detect a new type of black hole, this feeling is particularly intense. I can tell you, this feeling is addictive!
Sometimes, a blank pixel spot appears on my screen, and I am not surprised because detection work is never that smooth. Although detecting radiation is always more exciting than not detecting it, this time, I was surprised that we detected absolutely no radiation. According to our understanding of black holes and stellar environments, we should have been able to detect radio emissions. However, in reality, we did not detect any radio waves at all. This means that the stellar wind particles have not come close enough to the event horizon of Gaia BH2 to produce accretion, and thus cannot generate radio waves. This could mean that some material is preventing the two celestial bodies from getting closer. Perhaps there is a strong wind near the event horizon pushing the stars away? I began to excitedly imagine all the possible reasons why we could not detect radiation.
I thought of a family of black holes that have never been directly detected and only exist in speculation, namely isolated black holes or wandering black holes. It is estimated that there are about 100 million such black holes silently roaming in the Milky Way. Can we detect them through their occasional interactions with stray gas and dust? The detection results of Gaia BH2 show that it is simply not possible. Deep space is not an absolute vacuum, but it is much emptier than the space near Gaia black holes. Since neither Gaia BH1 nor BH2 detected radiation, it can be inferred that with current technology, humans cannot detect the electromagnetic radiation produced by accretion, and thus cannot detect isolated black holes. We still need a considerable amount of time to confirm whether isolated black holes exist. This idea, emerging from science fiction stories, is both fascinating and frightening, and the radio data I collected has turned the idea into reality.
This new family of black holes may be the darkest black holes we can directly detect so far, without observing X-rays or gravitational waves. But excitingly, our research on these black holes is just beginning. The Gaia satellite's mission is far from over. The European Space Agency (ESA) plans to collect more precise data by measuring millions of celestial bodies before 2030. In 2025, when the European Space Agency releases a new round of data, we expect to discover dozens of new members of this black hole family in the star catalog, waiting for us to explore. We will be ready to welcome that day.
Black holes do not suck everything in!
There is a common misconception about black holes: people mistakenly think that black holes are like vacuum cleaners in the universe, sucking in and devouring everything that comes close. However, this is not the case. Although black holes are very small in volume and have immense mass, their gravity corresponds to their mass and is not greater than the gravity of the Sun, Earth, or other massive celestial bodies.
For example, imagine the Sun suddenly collapsing into a black hole. The newly formed "black hole Sun" would have the same mass, but its radius would shrink to only about a mile (approximately 1.6 kilometers). Surprisingly, the Earth and other planets would continue to orbit as before, and everything would remain unchanged. Only when a celestial body gets too close to the black hole will it be torn apart, just as the Sun currently tears apart comets that come too close. But as long as one keeps a safe distance from the black hole, there is no danger of being sucked in.