Remember hearing about Planet 9 a while back? That’s astronomer Mike Brown’s designation for a planet that he thinks is out there in the outer solar system. So far, it remains elusive, despite everybody’s best observational efforts. (Of course, we already have a planet 9, called Pluto, so to avoid confusion, it sometimes gets referred to as Planet X.)

So, why does Mike thinks he’s got gravitational evidence of a massive world somewhere out in the Kuiper Belt? Essentially, he has observed the changes in orbital motions of bodies “out there”, and imputes those changes to the gravitational effect of a planet hiding somewhere. If it exists as a planet (and not, say, a loosely-bound clump of large planetoids or something), then eventually someone should be able to catch a glimpse of it as it reflects light, or emits a little heat (since it would be warmer than surrounding space). But, so far, nobody’s seen this possible planet.

Black Holes in the Outer Solar System

What if it’s not a planet? What if it’s something massive, but not another world? Say, like a black hole? That’s the conjecture behind a new method that two astronomers at Harvard have devised. They suggest that a small, planet-mass black hole could also exert a gravitational tug on objects in the outer solar system, perturbing their orbits. The method, developed by Dr. Avi Loeb and graduate student Amir Siraj, suggests searching for emissions as the black hole swallows up material.

How would that work? Well, if there’s really a black hole out there, it’s probably continually accreting matter. Its “food” would come in the form of gas and dust from interplanetary space, and the breakup of bodies in the Oort Cloud or Kuiper Belt. That accretion action creates higher temperatures in the region around the black hole. Any frozen bodies, such as cometary nuclei, would start to melt as they passed by. Eventually, they’d be too weak to keep their structural integrity, and they’d break apart. Once that happens, their material swirls into the black hole. That process emits radiation.

Look for the Hungry Black Hole.

So, the idea is to search for flares of this radiation when “stuff” near the black hole is superheated in the accretion disk. It’s about the only way that a black hole gets “lit up”. Otherwise, it’s invisible to the eye. But, the radiation emitted from the accretion disk is detectable. And, if a hungry black hole is gobbling up comet nuclei bits and pieces, or material from rocky bodies that collide “out there”, then the flares from that feast would show up in our detectors.

So, Why Go After Planet 9?

So, why pick Planet 9 as a possible study subject for this method? For one thing, it’s relatively close by, as such things go. That makes it a good subject to try out the method before using it on more distant objects. Whatever it is has a measurable gravitational effect on bodies beyond the orbit of Neptune (that’s how Mike discovered it).

Interestingly, a black hole at the same distance would do the same thing. And, right now, nobody has detected any light or other radiation from a possible planetary body in the supposed “regime” where the suggested Planet 9 exists. But, Brown laid some groundwork for testing the idea, since he’s found “something” that remains unknown. Loeb has suggested that a grapefruit-sized black hole with a mass five or ten times that of Earth could have the same effect as a massive planetary body (or dwarf planet body) like the proposed Planet 9.

Modeling a Black Hole Instead of Planet 9

So far, this is all a theoretical model. And, it’s not the first time that astronomers have suggested the idea of Planet 9 as a black hole. There are a lot of riddles to solve.

For one thing, planet-mass black holes are a relatively new concept. Yet, astronomers study stellar-mass and supermassive black holes, as well as intermediate-mass ones. So, they can extrapolate some of what they know about the big black holes to understand planet-mass ones. As far as I know, none have actually been discovered, so that’s another problem to solve.

But, the idea of a black hole with a few times the mass of Earth affecting orbits of distant solar system objects is intriguing. And, it can be studied. The LSST telescope at the Vera Rubin Observatory in Chile covers large areas of the sky very quickly. The idea is to point it at a broad region where the supposed Planet 9/Black Hole candidate may exist. The large field of view of the VRO telescope would be very useful since nobody knows exactly where the object lies. it’s kind of like aiming at the broad side of a bar to look for a nail in one board.

The Outer Solar System as a Treasury

We all know that the Kuiper Belt and Oort Cloud regions teem with remnants of solar system formation. There’s a storehouse out there of the ices that existed in our protosolar nebula. Now, they exist safe in the coldest regions of the planetary system, well away from the Sun. That makes those objects treasuries of material that predate the Sun and planets.

I think it would be pretty cool if those regions also harbored black holes. The smallest black holes (the micros and the minis) are largely thought to be theoretical. Finding a planet-mass black hole in our backyard would actually be a major discovery. It would be even cooler than a theoretical Pluto-killer planet that, so far, hasn’t been seen.

Want to know more? Check out this story from Harvard Center for Astrophysics and the Black Hole Institute for more details.