Does Planet Nine exist?
A hypothetical comparison of Planet Nine to Earth (left) and Neptune (right)

In January 2016, two astronomers in the US made an extraordinary claim: we have overlooked a planet on the outskirts of the Solar System, beyond the outermost planet, Neptune. And not just a mere tiddler of a world, which we could be forgiven for having missed, but a hulking great planet as much as ten times the mass of the Earth.

Five years later, Planet Nine, as it was christened, has failed to show up in any telescopic searches. The pair who predicted it – Mike Brown and Konstantin Batygin of the California Institute of Technology in Pasadena – still maintain that the planet is out there in the frigid wastes at the edge of the Solar System. However, others are claiming that the evidence on which the prediction was based is shaky and could in fact be due to “selection bias”.

Brown and Batygin’s smoking gun was an anomaly in a group of Trans-Neptunian bodies, members of a swarm of icy objects around the Sun, further out than Neptune in the so-called Kuiper Belt. The highly elongated orbits of six of these bodies were roughly aligned, like a finger pointing at the same region of the night sky. Brown and Batygin argued that this alignment could be explained if all the bodies were responding to the gravitational tug of an invisible puppet master: an undiscovered planet even further from the Sun than Neptune.

According to their calculations, Planet Nine would be at least five times the mass of the Earth and possibly as much as ten Earth masses. It would be travelling around the Sun in an orbit on average more than ten times farther from the Sun than that of Neptune. Instead of taking 165 years to go around the Sun, as Neptune does, it would take a whopping 15,000 years for a single orbit.

Predicting a new planet from its gravitational effect on other bodies has a precedent. In the first half of the 19th century, the French astronomer Urbain Le Verrier deduced the existence of Neptune from its gravitational tug on Uranus. And Pluto, recognised as a Trans-Neptunian body and demoted from its planetary status by the International Astronomical Union in 2006, was discovered in 1930 after claims – incorrect, as it turned out – that Neptune was also being tugged by an unseen planet.

How a planet could have ended up in such a remote location as that predicted for Planet Nine is a puzzle. One possibility is it was captured from another star – perhaps when the Sun was crowded together with other suns in the stellar nursery where it was born. Another possibility is that the planet was born in the inner Solar System but came off worst in a game of interplanetary billiards, catapulted out into the void by a close encounter with the embryonic giant planet, Jupiter.

At its predicted distance, Planet Nine would reflect very little sunlight, making it extremely difficult to spot. It could explain why searches have turned up no sign of it yet.

But now other astronomers have questioned its very existence. They have looked at Brown and Batygin’s alignment of Trans-Neptunian bodies with fresh eyes and claim that the orbits of their six objects appear aligned because the astronomers who discovered them conducted their searches of the night sky only in certain directions.

Kevin Napier of the University of Michigan in Ann Arbor and his colleagues have carried out a “meta-analysis” of three surveys of the most extreme Trans-Neptunian bodies: a total of 14 objects. They found that their orbits could be explained if they are in fact distributed uniformly across the sky – but we have so far observed only a small fraction of the outer Solar System. Such “selection bias”, not an undiscovered planet, would therefore explain Brown and Batygin’s orbits.

The jury is still out. But it seems that Planet Nine may be destined to go the way of Vulcan. In the mid-19th century, Le Verrier proposed that a peculiarity in the orbit of Mercury, the closest planet to the Sun, could be explained if it was being tugged by the gravity of a planet even closer to the Sun. For half a century, astronomers searched for Vulcan, some claiming to see it, many claiming not to. It turned out, however, to be a mirage. In 1915, Albert Einstein successfully explained the anomaly in the orbit of Mercury with his new theory of gravity: the general theory of relativity.

Vulcan, of course, lives on as the home-world of Mr Spock in the TV and film series Star Trek. Could it be that a similar fate awaits Planet Nine, and that it will survive not in the real world but only in the realm of science fiction?

Marcus Chown’s latest book is “The Magicians” (Faber)

This article is from the New Humanist summer 2021 edition. Subscribe today.