We tend to think of the solar system as our little corner of the universe, but there are still many things we don’t know about it. For example, what is disturbing the orbits of small space rocks beyond Neptune’s orbit? Some scientists believe that there is another planet out there, often called Planet Nine. What if it’s not a planet? Researchers from Harvard University have published an article exploring the possibility that our solar system is home to a small, ancient black hole.
For decades, the solar system had nine official planets, but Pluto was kicked out of the planet club and downgraded to the dwarf planet state. That was the only logical conclusion astronomers felt after learning more about other Pluto-like objects in the Kuiper Belt, a ring of icy rocks beyond Neptune. That also shows you how much remains to be learned about this region of space and how a planet, or even a black hole, could be hidden in a vacuum.
Scientists began taking Planet Nine seriously several years ago when Caltech researchers published compelling evidence that something had pushed the orbits of many Kuiper Belt Objects (KBO) to align. The team suggested that a small gas giant of about ten times Earth’s mass could do the trick, but no one has been able to confirm that such a planet still exists.
Harvard professor Avi Loeb and university student Amir Siraj argue that a small black hole would also explain the effect on KBOs. However, this would not be your common stellar mass black hole but a hypothetical primordial dungeon. Today, black holes only form when stars run out of nuclear fuel and collapse into a singularity, but in the early universe, scientists speculate that regions of high-density space might have formed smaller singularities without starting out as a star.
A small black hole hanging in our solar system would be perfectly dark, at least plus weather. Loeb and Siraj say it should be possible to detect such an object by observing “accretion flares” as small bodies approach the black hole and are shattered by gravitational forces. The team believes there would be at least some of these each year as the rocks from the Kuiper Belt and the Oort Cloud spiral down into the invisible mass. We might even see these flashes in the next 10 years as the Vera C. Rubin Observatory runs the Legacy Survey of Space and Time (LSST) project. The LSST will scan the entire southern sky every three nights with a wide angle lens that should be sensitive enough to detect accretion flashes.
The primordial black hole hypothesis is certainly less plausible than a planet or group of small objects causing orbital anomalies in our solar system. Still, it is worth investigating. Confirming such an object in our own backyard would be a watershed moment for science.
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