Shedding Light on Dark Matter

A chat about primordial black holes and more with James Unwin

Assistant Professor James Unwin, along with collaborator Dr. Jakub Scholtz of the University of Durham, recently put forth an exciting hypothesis about a possible black hole on the edge of our solar system. While the science has been widely reported in several media outlets, we wondered about Professor Unwin’s personal perspective on the project. The interview that follows was conducted over email and has been edited for clarity.

The original paper, What if Planet 9 is a Primordial Black Hole?, is available on Cornell University's High Energy Physics open access platform.

The paper has also been covered by Science, NBC, the MIT Technology Review, Business Insider, and UIC News.

What drew your interest to exploring the possibility that Planet 9 could be a black hole? 

I actually first learned about the evidence for Planet 9 during a visit to Chicago's famous Adler Planetarium, in a fantastic show about current astronomy and astrophysics topics. It sparked my interest, as you might imagine, and a couple of days later I called my long-time collaborator, Dr. Jakub Scholtz. We started running through various ideas about other astrophysical objects that might be lurking at the edge of our solar system, rather than a mundane planet. We came up with two important points. First, the edge of our solar system would be a very strange place to find a large planet, since the evidence suggests Planet 9 is roughly ten times further from the Sun than Neptune. This made us think it might not be a planet. Second, if it's not a planet but a more exotic object, then the experimental signals could be very different, and we'd need to use different telescope to look for it. This is essentially the fundamental point of our paper.

After thinking about the idea of an exotic object at the edge of our solar system for a couple of months and exploring various possibilities, we stumbled across a very intriguing paper. A reanalysis of data from the OGLE experiment, which looks for instances of microlensing, had identified a number of candidate events that could be indicative of primordial black holes. Surprisingly, the mass range identified in the OGLE data was precisely the same as inferred from the evidence for Planet 9. The coincidence of these two observational anomalies was remarkable, and convinced Dr. Scholtz and me that primordial black holes were the scenario to focus our attention on.

Do you secretly hope it is one or the other?

Either would be amazing! The discovery of a new large planet this far from the sun would be extraordinary, unexpected, and change the history of the solar system as we know it. But the discovery of a primordial black hole in orbit of the sun would fundamentally advance our understanding of early universe cosmology and high energy physics.

What ideas do you have for further study of this possibility?

Our paper presents the first study of this scenario. We argued that dedicated studies are necessary to tease out whether there is something out there. Currently, only the case of a planet is being actively considered. While this is still the more likely possibility, it is definitely worth looking for more exotic alternatives—especially since there is a great deal of relevant data already available. All that is really needed is a dedicated reanalysis. Jakub and I intend to develop new tools to go through this data and look for smoking-gun signals of this primordial black hole hypothesis.

How rare (or common) are free planets? Black holes?

If it's an earth mass primordial black hole and accounts for around one percent of dark matter (in line with the population inferred from the OGLE events) then there would be roughly 30 primordial black holes per star in the vicinity of our solar system. The chance of a primordial black hole being gravitationally captured by the solar system is small. But, that said, even though it is probabilistically unlikely, the chance of capture is actually quite comparable to the probability of a planet falling to this distance and unusual orbit.

As someone who has studied a potential black hole, what is your take on why they capture people’s imaginations? Were any aspects of the study fun for you?

Thinking about these ideas was very fun and invigorating, and we also learned a lot of interesting physics. But also, it's quite serious, since this is a testable scenario that could be confirmed with dedicated experimental searches.

Black holes are fascinating since these systems can be on the scale of everyday experience, but at the same time, they embody the collision of quantum mechanics and general relativity. A black hole with a mass ten times that of earth is roughly the size of a bowling ball. Moreover, if there were a black hole in our solar system, we could quite conceivable send a probe to study the object, which would be an incredible opportunity for science.