Astronomers are on the trail of something big. Their target is between 5 and 15 times the mass of Earth and orbits the sun beyond Neptune. This is Planet 9, the last undiscovered orbiting body in the solar system. And its discovery is expected in the not too distant future.

The reason for the excitement is the growing evidence that Planet 9 must be out there. Astronomers can see that other trans-Neptunian bodies—asteroids, comets, and the like—seem to cluster together in patterns that cannot easily be explained unless a large planet is shepherding them in some way.

This evidence hints at the planet’s mass but also suggests it must be a long way off—perhaps 250 times the distance from Earth to the sun, which is why it is so hard to spot.

But today astronomers say there may be another reason nobody has seen Planet 9: because it may not be a planet at all. Instead, they say, our solar system may be orbited by a primordial black hole—a superdense lump of matter about the size of a tennis ball. And if that’s the case, then we need to search for it in an entirely different way.

First some background. Cosmologists have long supposed that the early universe was filled with quantum fluctuations that caused matter to become concentrated in some regions and absent in others.

Some of these regions would have been vast, seeding the formation of entire galaxies. But most would have been tiny, with many containing enough mass to trap light—in other words, to form black holes.

These so-called primordial black holes are entirely different from those formed by the collapse of large stars or the supermassive ones that rage at the center of galaxies (and that have recently been imaged for the first time).

Instead, primordial black holes are tiny and numerous but difficult to spot. Indeed, there is little evidence for their existence.

A five-Earth mass primordial black hole (actual size 5cm)

At least, that was the case until earlier this year, when astronomers reported a number of puzzling observations pointing to the possibility that primordial black holes may be common after all.

These observations come from an experiment called the Optical Gravitational Lensing Experiment, or OGLE, which looks for changes in the brightness of distant stars and galaxies caused by gravitational lensing. This is the relatively rare phenomenon in which a large mass focuses the light from an object behind it, acting like a lens. If these objects align in a way that places Earth at the focal point, astronomers get a magnified view of the more distant object for free.

Most gravitational lenses are huge—entire galaxies, for example, that focus light from even more distant galaxies behind them. But OGLE has spotted a number of lenses that seem to be much smaller and closer, sitting within our own galaxy. These objects are highly compact and about five times the mass of Earth.

Nobody knows what they are but one possibility is that they are primordial black holes. If so, then our universe must be filled with them.

This possibility has attracted the attention of Jakub Scholtz at Durham University in the UK and James Unwin at the University of Illinois at Chicago. “If the OGLE events are due to a population of primordial black holes then it is possible that the orbital anomalies of trans-Neptunian objects are also due to one of these primordial black holes that was captured by the Solar System,” they say. If that’s the case, we are much closer to a primordial black hole than we ever imagined.

Scholtz and Unwin explore this idea today. They say that Planet 9 can have reached its present position in only one of three ways. The first is that it formed in this distant location. However, this is unlikely because there has not been enough time since the formation of the solar system for the necessary accretion to have occurred at that distance.

The second possibility is that the planet formed closer to the sun and was then somehow catapulted out to its current location. This too is unlikely, because it would have required a catastrophic event like the passing of a nearby star. But there is no evidence that this has happened in the lifetime of the solar system.

The final possibility is that Planet 9 was a free-floating planet captured by the sun’s gravitational field. Little is known about free-floating planets and their numbers in the galaxy.

But Scholtz and Unwin make the point that if this kind of capture is possible, so is the capture of a primordial black hole. “We argue that while there is a low probability of capturing an Earth mass primordial black hole, it is no more improbable than capturing a free floating planet of similar mass,” they say.

They go on to calculate the capture probability based on the number of nearby primordial black holes that the OGLE observations suggest.

One consequence of this theory is that Planet 9 will be impossible to spot with visible-light and infrared telescopes. That means astronomers’ current searches for the planet are doomed to failure.

A primordial black hole would have a very different signature, say Scholtz and Unwin. They hypothesize that it would be surrounded by a halo of dark matter and that annihilation of dark-matter particles would generate gamma rays.

This signal might even be strong enough to be observed by the Fermi Gamma Ray Space Telescope. Scholtz and Unwin say they plan to look for this signal in the Fermi data at some point in the future.

That’s fascinating work that provides an entirely new perspective on Planet 9 and how astronomers should look for it. It also raises the prospect that one of our neighbors is more exotic than anyone imagined. A primordial black hole on our doorstep? Fancy that!

Ref: arxiv.org/abs/1909.11090 : What if Planet 9 is a Primordial Black Hole?