Black holes are generally imagined as terrifying star-sized planet eaters or more terrifying supermassive star eaters – things to rightfully be afraid of. Would you be terrified of a black hole the size of a grapefruit? An apple? A golf ball? A team of astronomers from Harvard University and the Black Hole Initiative (who knew they had their own initiative?) have put for the idea that the long-rumored Planet Nine may not be a planet at all but instead one of this miniature black holes with enough gravitational pull to disrupt the outer edges of our solar system. If we can’t find a planetary Planet Nine, how will we find an apple-sized Black Hole Nine?
“We find that if Planet Nine is a BH, its existence can be discovered by LSST due to brief accretion flares powered by small bodies from the Oort cloud, which would be detected at a rate of at least a few per year.”
Well, that explains it … if you’re a Harvard science professor like the famous Abraham ‘Avi’ Loeb and his partner, undergraduate student Amir Siraj. In their new paper, accepted recently by The Astrophysical Journal Letters, Loeb steps out of his normal comet/asteroid/alien spaceship mode (you may remember his name from his various theories on `Oumuamua) and into a black (BH) hole just big enough for his foot. He proposes that a small primordial black hole (one created by the Big Bang and not due to the collapse of a dead star) with the same estimated mass as Planet Nine (about five times that of Earth) could be lurking somewhere in the Oort cloud amongst its icy planetesimals. As those tiny ice bodies melt, they could be sucked into this tiny black hole. When that happens, a visible flare of gas would be emitted from the black hole, which could possibly be seen by the Legacy Survey of Space and Time (LSST) scheduled to begin once the Vera C. Rubin Observatory in Chile is completed later this year.
“If multiple bursts are observed over the course of a year, the proper motion of the source can be used to identify the orbital parameters of the BH.”
Loeb isn’t the first to propose that the theoretical Planet Nine is actually a theoretical Black Hole Nine, but he and Siraj are the first to propose a simple method of finding it. Of course, this method requires the patience of waiting until the LSST is completed and at least part of the way through its 10-year mission of surveying the southern sky. However, it’s much more elegant and less needle-in-the-haystackish than other proposals to send tiny laser-sail spacecrafts to search for it.
“Additionally, if Planet Nine is a black hole with a magnetic charge, then the synchrotron emission from the accretion flow around it could make its flares much brighter and more easily detectable.”
Would you be excited or disappointed if it turns out that Planet Nine is a tiny black hole, not a mysterious giant planet or worse, a mysterious giant Nibiru heading towards a cataclysmic collision with Earth? Would you fear a grapefruit-zed black hole that eats ice particles like kids catching snowflakes on their tongues in a snowstorm?
For now, we wait.