The conventional wisdom is the mysterious planet at the edge of our Solar System called Planet Nine (or Planet X, to others) -- if it exists at all -- is likely an ice giant five to 15 times the mass of Earth.

Planet Nine is the hypothetical planet in the far outer Solar System whose gravitational pull probably explains the improbable orbital configuration of a group of trans-Neptunian objects (TNOs) mostly orbiting beyond the Kuiper Belt. The Kuiper Belt is a circumstellar disk extending from the orbit of Neptune (which is some 30 astronomical units from the Sun) to 50 AU from the Sun.

The existence of Planet Nine was first posited in 2014 by astronomers Scott Sheppard and Chad Trujillo. These men claim there might be a "massive trans-Neptunian planet" on the outskirts of our solar system because of the "similarities in the orbits" of distant objects circling Neptune.

In January 2016, Konstantin Batygin and Mike Brown of Caltech said they'd found signs of Planet Nine using modeling and computer simulations. In October of the same year, both scientists claimed the existence of Planet Nine adds "wobble" to our Solar System, tilting it in relation to the Sun.

The hypothesis was put forward in a paper titled "Solar Obliquity Induced by Planet Nine" by Batygin and Brown, along with Caltech graduate student Elizabeth Bailey.

"Because Planet Nine is so massive and has an orbit tilted compared to the other planets, the solar system has no choice but to slowly twist out of alignment," said lead author Bailey.

This is roughly where things have stood since 2016 concerning Planet Nine, which remains undiscovered despite an intensive, ongoing search for it. Planet Nine remains a tough planet to spot because it's probably located 250 times the distance from Earth to the Sun.

On Sept. 29, however, a team of astronomers put forward their incredible theory Planet Nine might not be a planet but a "primordial black hole."

Primordial black holes are a theoretical type of black hole said to have formed in the earliest Universe, shortly after the Big Bang some 13.8 billion years ago. This type of black hole wasn't formed by the collapse of stars but was formed by a process still being debated.

Quite tiny, a primordial black hole might weigh as little as one times the mass of the Earth. It's worth noting a black hole five times the mass of the Earth could fit in the palm of your hand.

If this hypothesis is true, Planet Nine might actually be a superdense lump of matter (or a black hole) about the size of a tennis ball.

Planet Nine (illustration)
Planet Nine (illustration) NASA

The case for Planet Nine being a primordial black hole is being made by Jakub Scholtz at Durham University in the United Kingdom and James Unwin at the University of Illinois at Chicago.

“If the OGLE (the Optical Gravitational Lensing Experiment) 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,” argued Scholtz and Unwin.

Scholtz and Unwin claim Planet Nine (or the primordial black hole, in their case) can have reached its present position in only one of three ways. The first is the primordial black hole formed in this distant location, which they say is unlikely given the known facts.

The second possibility is that primordial black hole formed closer to our Sun and was then somehow catapulted out to its current location. This, too, is unlikely. The final possibility is the primordial black hole was a free-floating planet captured by the Sun’s gravitational field.

This, too, appears impossible but not improbable.

“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,” said the duo.