It has been nearly a decade since researches at California Institute of Technology (Caltech) noticed irregularities in the orbit of some extreme trans-Neptunian objects (ETNOs) -bodies orbiting the Sun in the outermost region of our Solar System, well beyond Neptune. What was causing the strange behaviour? Have we cracked the mystery?

When Caltech astronomers Konstantin Batygin and Mike Brown first published their research showing evidence of a giant planet in the outer Solar System in 2015, they certainly made the headlines. The announcement came nearly 170 years after the discovery of Neptune, the latest planet to be found orbiting the Sun.

The two researchers had used detailed mathematical modelling and complex computer simulations to explain the odd orbital behaviour of some dwarf planets and other smaller icy bodies in the Kuiper Belt, the outskirts of the Solar System. The explanation, they proposed, was the gravitational pull of an elusive planet, which they dubbed “Planet Nine”.

Read also: The Mysteries of Outer Space.

Other possible explanations were considered, too. In fact, the debate in the scientific community about whether Planet Nine exists is still ongoing. The most sceptical astronomers tend to minimise, calling out some “ghosts” or biases in the data. A more moderate wing believes we should be looking for a primordial black hole instead, or perhaps some new physics. The enthusiast astronomer, on the other hand, is still out there looking for Planet Nine.

But what is it that they’re looking for exactly? According to scientists, the mysterious world has a mass about 5 to 10 times that of Earth, and it could look anything from a rocky super-Earth to a gaseous mini-Neptune, perhaps sharing characteristics with Uranus or Neptune.

One thing we know for sure is that spotting a celestial body in the vastness of space is not exactly a straightforward task. Besides, Planet Nine is believed to be orbiting the Sun in a highly elongated orbit far beyond Pluto, and considering its elusive nature, it may well be at the far edge of its enormous orbit, meaning it would move quite slow (relative to us) and reflect little to no sunlight.

While this most certainly adds difficulty to the quest, astronomers are confident that modern telescopes around the world are powerful enough to at least spot the elusive world, thus the hunt continues. New data is being acquired and paired or compared with old data: in fact, the truth may hide somewhere in there.

Will we find Planet Nine? And if we do find it, will tradition be followed, and the planet be named after a mythological roman god?*

Personally, I would be delighted to know there really is a Planet Nine lurking in the outskirt of the Solar System. This would be a sensational discovery, with a lot of new science to come! At the same time, I would be even more thrilled if we finally found evidence for a primordial black hole, and right in our backyard!

Spotting one, of course, would be way more problematic.

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* Konstantin Batygin and Mike Brown dubbed their predicted object “Planet Nine,” and the name has been used ever since (along “Planet X”). However, it is the person who actually discovers the planet to get the right to name it (pending approval from the International Astronomical Union).