What If Planet 9 is a Primordial Black Hole? | Unveiled

What If Planet 9 is a Primordial Black Hole?

Our understanding of the planets in the solar system has changed over time. For more than 70 years the general consensus was that there were nine orbiting the sun… but then, in 2006, when Pluto was demoted to “dwarf planet”, that number was revised to just eight. The search for Planet 9 is far from over, though... And theories abound for what and where it could be. Perhaps this seemingly massive (and definitely elusive) object isn’t actually a planet at all.

This is Unveiled and today we’re answering the extraordinary question; What if Planet 9 is a primordial black hole?

Planet 9 is a mysterious cosmic body theorised to orbit the sun way past Neptune and even Pluto - with most theories resting on the belief that this unknown something exerts a gravitational influence on the icy objects that make up the Kuiper Belt. According to predictions made by the California Institute of Technology in 2015, Planet 9 could be ten times more massive than Earth is… and twenty times farther from the sun than Neptune, with an orbit that takes up to 20,000 Earth years to complete. In reality, though, that’s all we have… a few predictions based on some perceived gravitational anomalies. But Planet 9 doesn’t necessarily need to be a planet to solve the mystery; it could be a black hole.

Though we often think of black holes as extraordinarily massive objects - far more massive than a planet - and some certainly are, there are actually four categories of black hole that vary in size and mass: Primordial, Stellar, Intermediate, and Supermassive. Stellar black holes are usually between 5 to 100 times more massive than stars; Supermassive black holes are millions or even billions of times more massive than that; and Intermediate black holes generally cover everything in between Stellar and Supermassive. Primordial black holes, though, are currently hypothetical structures, but it’s thought their mass could be more on par with a planet’s - and, according to a 2019 study by physicists Jakub Scholtz and James Unwin, the probability of the sun capturing a primordial black hole (if they exist) “is comparable” to the chances of it attracting a rogue “ninth” planet.

Whatever Planet 9 is, it’s probably between 50 billion and 150 billion kilometres away from us. Even if we knew it was there, observing something so far away would be incredibly difficult... But if it was in fact a black hole, then we’d be keeping our eyes peeled not for a dim and distant orb, but for a halo-like accretion disc. More than changing the appearance of what we’re even looking for, though, were Planet 9 shown to be as a black hole it’d have wide-ranging implications for the solar system in general.

Primarily, if it were proven primordial, then its discovery would one hundred percent confirm that primordial black holes exist. Right now, they’re theorised structures thought to date back to just shortly after the Big Bang… but the evidence for them is mounting. A 2019 study using an OGLE telescope, or Optical Gravitational Lensing Experiment telescope, found potential evidence of smaller gravitational lenses - that is, when a massive enough object bends light under its own gravity - within our own galaxy. The suggestion is that these smaller lenses could well be primordial black holes. So, if it were to transpire that Planet 9 were one such site, then it would suddenly become a place of major scientific interest. It would mean that, for all our previous study and contemplation of black holes, we’d now have a real-life example on our cosmological doorstep - ready and waiting for our fastest and most advanced probes to pay it a visit.

Black holes wouldn’t be the only mystery of space that we’d now have new insight into, though. If Planet 9 was found to be a black hole it could also lead to a breakthrough in understanding dark matter. Very little is currently known about dark matter, except that it contains significantly more matter than there is in the visible universe. But, one theory says that primordial black holes are actually a type of dark matter themselves, called a MACHO - or a Massive Compact Halo Object. Having a primordial black hole so close to us, then, could revolutionize the way we go about researching dark matter and dark energy - opening a new and exciting window onto the 95% of the universe that remains a relative mystery. Suddenly, Planet 9 finds itself promoted from being an unknown quantity in the solar system… to becoming the single-greatest research opportunity Earth has arguably ever had, promising potential answers to important questions about galactic evolution, the earliest stages of the universe and high energy physics.

Discovering a black hole in our solar system might not be all good news, however. A primordial black hole would be small in size, but it could still prove dangerous over time - and in a variety of ways. For one, there’s Hawking Radiation and how it could develop into a problem. The debate rages on about whether black holes ever lose so much radiation that they evaporate completely or if they stop at a certain point… but if they do keep shrinking until they disappear then that moment could release a massive amount of energy. As such, if Planet 9 is a black hole and it’s in decay, then when it finally disappears it could release a blast of energy many millions of times more powerful than a hydrogen bomb, probably as a gamma ray. Given it’s also thought that a strong enough gamma ray could tear apart the Earth’s atmosphere and even adversely affect human DNA… Setting one off from within our own star system could be a disaster to surpass anything else we’ve ever seen.

But, of course, this hypothetically local black hole could go the opposite way and grow in size and mass. Again, scientists aren’t exactly sure how supermassive black holes form… but one theory is that primordial black holes act as the very first seeds which eventually - over billions of years - accrue enough matter to become supermassive. So, if Planet 9 really was a black hole and it was growing, then the devouring of our solar system would simply be one of its first steps toward becoming bigger. And there’d be really nothing we could do to prevent that particular outcome - though there is some comfort because of those “billions of years” before it could happen.

If (or when) the mystery of Planet 9 is finally solved, it will be a historic moment in space, science and astronomy. But, if this mysterious, theorised mass turned out not to be a “planet” at all, but instead an early, ancient black hole… then it’d mean us rethinking everything we thought we knew about our solar system. On the plus side; it would offer an incredible research opportunity… but it could also spell the beginning of the end for our particular corner of the cosmos. And that’s what would happen if Planet 9 was a Primordial black hole.