Over the last decade, more and more astronomers have become convinced that there is something still undiscovered out at the edge of the Solar System. This thing, this “Planet Nine” or “Planet X” could be lurking far away, many times more distant than even Neptune. The movement of some comets and distant “Trans-Neptunian Objects” suggests that their paths are being perturbed by a large mass whose nature we simply do not understand. It might be a dense, stony body. It might be something larger, a “dark giant” so far from the sun that it swings through perpetual night. Such an object could play a significant role in directing comets and asteroids down the Sun’s gravity well to smash into the inner planets. It might have, in a very indirect and impersonal way, have been the real “dinosaur killer.”
That doesn’t make this mysterious Planet X a threat, but it does mean that its role in the history of the Solar System could be greater than its fantastic distance might suggest. So understanding it could be very important.
The movement of those disturbed objects has given some broad ideas of how big a Planet X could be. It can’t be a Jupiter-sized object. That’s something that would both make itself clearly evident, and clearly visible, even at such a distance. But Planet X could be several times as big as Earth. Most likely 5 to 15 times more massive, putting this unseen object into the “Super Earth / Mini-Neptune” range of planets that seem to be quite common when we look at other stars.
But a paper described this month in Science, has a more exotic suggestion: What if Planet X isn’t a planet at all. What if it’s a black hole? In fact, what if it is several black holes? A whole collection of them, practically on our doorstep.
That’s right, there could be light-eating, mater-compacting, singularity-generating black holes right here in our own Solar System, swinging around our own sun within their own discs of gravitationally-demolished comets and surrounded by halos of profoundly weird dark matter. And these aren’t just any black holes but primordial black holes. Fragments of the universe’s origin.
Primordial black holes are not the collapsed remains of former stars. Instead they are a group of still hypothetical objects that emerge from the math of many studies on conditions shortly after the Big Bang. In those first microseconds of the universe’s explosive growth, the density of matter/energy at some points would have been so great that, as Steven Hawking detailed extensively in the 1970s, black holes of any size might have formed. The tiniest of these objects would have long ego “evaporated” through a process that lies on the borders of of relativity and quantum mechanics. But larger versions, versions with masses the size of planets, would have lives easily long enough to still be around today.
Because they are much smaller than the normal stellar-mass black holes, they’re even more difficult to detect. In fact, some past theories had even suggested that the “missing matter” of the universe might simply be many, many, many primordial black holes (that theory is no longer considered a serious contender).
But in the new article, physicists Jakub Scholtz the U.K. and James Unwin in the U.S., don’t just suggest that the mystery mass within our own system could be from several primordial black holes. They also have a way to find them.
If Planet X is actually Primordial Black Holes X, Y and Z then, unlike all the trillion-trillion other suspected such objects out there, these light-sucking gravity traps are relatively close. Close enough that we might see the “crumbs” of energy that result when matter goes spiraling down the steep gravity well into the bottomless gullet of the black holes.
As evidence for their claims, Scholtz and Unwin point to a series of images picked up in the extensive dataset of OGLE. They suggest that several of these images, which others have selected as potential evidence of Planet X, are at least as well explained by primordial black holes with a mass greater than the Earth’s. And, somewhat perversely, even though black holes are just that, black, and they’re many times smaller than a planet of the same mass would be, they could be actually easier to spot. So long as we know what to look for.
Interactions in the matter, and dark matter, surrounding these small black holes should generate small flashes of gamma ray energy. Gamma rays certainly aren’t unknown in space, but unlike most cosmic rays these flashes would be coming from a source much closer to hand. By skimming through data from the Fermi Gamma-ray Space Telescope, Scholtz and Unwin believe they could find these flashes, if they exist, and determine if our Solar System really is home to a set of genuinely exotic dark companions to the planets, moons, dwarf planets, asteroids and comets that we know.