Is a black hole stuck inside the sun? No, but here's why scientists are asking

Just before the end of 2023, astrophysicist Matt Caplan announced the release of a new paper that boasts a rather outlandish title: "Is there a black hole in the center of the Sun?" I asked for the short answer, and, as you may expect, he said "probably not." But that's far from the end of the story. 

Though the new study, currently available to view on arxiv, is yet to be peer-reviewed, it accompanies a paper published in The Astrophysical Journal that tip-toes around the same strange question as a means to a fascinating end. 

The point of all this is to provide a hook that reels you into an epic theory concerning the elusive nature of dark matter, arguably the biggest and most confounding space mystery of our time. This is a substance that couples with its equally bewildering counterpart, dark energy, to account for over 95% of the universe. Yet, the two remain hidden to human eyes. Over the years, scientists have come up with quite a few possible explanations for the dark universe, ranging from strange new particles all the way to the suggestion that our math is just wrong. Nothing has panned out.

To that end, Caplan is part of a crew that posits the dark matter portion of the dark universe could very well be made up of not particles like we imagine, but instead a huge number of atom-size black holes produced during the dawn of the universe, each of which is about as massive as a typical asteroid in our own solar system. "I think all dark matter candidates are just a little bit wild," Caplan, who is an assistant professor of physics at Illinois State University, told Space.com. "Some guesses are better than others, and primordial black holes are taken seriously. I'll go so far as to say I think they're popular."

But to turn the hypothesis into fact, he says, scientists have to actually find one of these miniscule ancient voids — which brings us to this new black-hole-sun conversation. Potentially, Caplan and his co-authors say in their papers, some of those ultrasmall black holes might've gotten caught up in dust clouds in the midst of forming stars. Potentially, they might've ended up literally lodged in those eventual sparkling oceans of plasma. Potentially, they might still be there. 

So, no, there is probably not a black hole in the center of our star — but there might be other stars gallivanting through space with black holes indeed wedged within their hearts. 

Related: We still don't know what dark matter is, but here's what it's not

The phone call that started it all

During the COVID-19 pandemic, astronomy understandably experienced a slew of hurdles that led to many delays, but also to much innovation. 

"I became friends with Earl Bellinger, who is an asteroseismologist," Caplan said. "We were just having a very ordinary research call, talking about stars as one does. And he just mentioned this sort of off-the-cuff, throwaway remark. He was like, 'I always thought it'd be fun to put a black hole in the center of these simulations, just to see what happens.'"

And when Bellinger ended his statement with "But there's no reason to do that," Caplan recalls correcting him with "No, actually, this is a testable dark matter candidate."

Caplan said he'd already been considering solar system tests of the atom-sized primordial black hole theory, especially because if these theoretical phenomena exist, they would be so low in mass there'd likely be a whole bunch of them zipping around. You'd expect some to be crossing through our corner of the cosmos at any given time, and in fact, he's written a research paper before about what'd happen if one of these black holes smashed into the moon, or even Earth. 

However, Caplan said he obviously knew it would be insane if the sun captured one.  Moreover, he believes there's a pretty low chance that an atom-size black hole ever got stuck inside any of the stars in our entire Milky Way. "The more massive a galaxy or environment is, the faster everything moves; stronger gravity means things are moving faster," he said. 

This means that, in environments like the Milky Way, which are very massive and known to have a lot of dark matter, the dark matter would be shuffled around super quickly. So, if dark matter is composed of the proposed mini black holes, those black holes would have to be moving around very fast in tandem. This would make it unlikely for them to be captured by objects. In other environments, however, such as in dwarf galaxies or globular clusters that are less massive, dark matter moves far slower on average. 

"You might get lucky enough that a few pieces of dark matter or, in this case, occasionally, a primordial black hole, will get captured by a star-forming cloud, become gravitationally bound to it and sink to the core as the star evolves," he said. And that's precisely what the scientists say they're going to be searching for next: stars in dwarf galaxies that might exhibit evidence of a black hole in their grips. 

He and Bellinger call them "Hawking stars," because Stephen Hawking wrote the seminal paper on primordial black holes. Incredibly, Hawking had suggested that, in its very first second, the early universe would've likely been dense and chaotic enough to spur a ton of these peculiar objects. Plus, importantly for the new theory, Hawking also pointed out that these black holes could have pretty much any mass. 

Hawking even goes as far as to say that an asteroid mass' worth of such black holes could be captured by stars, eating them from the inside. "Because he sort of suggested it in the original paper," Caplan said, "and it's sort of in the footnotes that most people ignore when they're doing this work, we decided to name it after him."

Finally, here's the current blueprint for finding one of these ancient pinprick voids.

Hunting for theoretical giants

Although it'd make for a great "Interstellar: Part 2," no, a star containing a black hole anchor would not look like some sort of time-warping fiery space anomaly. It'd actually look rather normal.

"Your intuition is that the star is gonna get gobbled up like in a CGI sequence," Caplan remarked. "But when you do the science and you put in the proper models for how black holes eat, and how bright black holes are, as they produce all this radiation that pushes back against their surroundings, it can actually take them billions of years to eat the star."

So, if not an unhinged star, what would we be looking for? Well, to give you another short answer, Caplan says the key is to search for puffy, dimmer red giants (in smaller galaxies, of course).

"When a black hole brightens, it can cause the star to have a really strange giant phase," he said, "which is different than standard red giant phases, which can make them stand out if you look for them." 

Black hole feasts and star power

Basically, the anatomy of a black hole (of any size) can be broken down into three main components. First, you have the singularity, which is the unseeable point at the exact center of a black hole into which all matter is compressed. This is how a black hole can be so massive, yet so physically small. Second, you have the event horizon, which is like a ring around the singularity that represents the barrier between our universe and whatever's inside. Light cannot get past the event horizon. It's all very mysterious. 

Third, and key for the new study, there is the accretion disk. That's the loop of matter spinning around the entire body filled with gas and dust. 

Stellar fusion, on the other hand, refers to how stars are powered up. Stars experience an intrinsic nuclear fusion process by which they turn light hydrogen atoms into heavier helium atoms, which generates an outward push to negate the inward pull of their own gravity. This process also contributes to their brightness; it continues on as a star reaches the end of its life and turns into a red giant. Eventually, the fusion reactions stop and the star's gravity wins out. In giant stars, the next step is a supernova explosion, which can result in either a "normal-sized" black hole or a neutron star. (Neutron stars are super-dense and totally fascinating in their own right, but that's for another story.) 

Anyway, we know about normal red giants, but if a star harboring an internal black hole gets to the red giant phase, Caplan argues that something interesting would happen. "Rather than being fusion-powered, the giant phase is accretion-powered," he said. "The fusion actually cuts off, and it becomes this big sort of puffy cloud churning around this really small black hole that is about the mass of the Earth at that stage."

That'd make the trapped black hole about a millionth of the mass of the Hawking star that contains it, but still with about one solar mass' worth of material around it. "This completely changes the internal structure of the star," Caplan said. 

"If the black hole is small enough and has a low enough mass, it will be growing so slowly that the star can live for the entire age of the universe so far without change to it," he added. "If it's a more massive black hole, well, massive black holes have bigger event horizons." 

In other words, those massive black holes would be able to accrete more quickly and grow faster. Larger black holes, the scientists therefore believe, could kill their stars within a few billion years.

As to why an accretion-powered red giant would be dimmer than a normal red giant, this is simply because fusion power leads to more brightness than accretion power does in stars. "This is part of that story about ultra faint dwarf galaxies," he said. "Why are these dwarf galaxies so dim? And why do they seem to have so much dark matter?"

However, that's not to say that accretion power is meaningless. It's arguably more impressive, because an accreting black hole once the size of an atom could bring a star's brightness up high enough to get relatively close to extreme fusion-powered brightness!

"Earl looked through some surveys, and there's about 500 of them that he found that would be good targets for observation," Caplan said of the red dwarf targets. "We have a few hundred candidates that we're hoping to look at in some detail with asteroseismology in the next year."

And if you're wondering how fruitful this upcoming attempt to find red-giant-disguised stars roaming the universe with atom-sized black holes in their cores could be, well, I'd consider two things.

For one, the theory that dark matter can get captured by cosmic bodies in general isn't really that controversial. As an example, scientists working with the IceCube facility located at the South Pole are searching for evidence of dark matter signals coming from within Earth. For another, searching for dark matter is quite literally a shot in the dark.

It's surely possible the truth is simply a swarm of voids dotting the universe and getting stuck inside stars.

"I wouldn't be surprised if they don't exist," Caplan said, "but I also wouldn't be surprised if they did."

Update 1/31: Earl Bellinger's name and title have been updated.