The Search for Planet X

Astronomers at Caltech announced this week that evidence suggests a ninth planet could exist in the outer reaches of the Solar System. Unexpected alignments in the orbits of several Kuiper belt objects point to the existence of the planet, which would move on an incredibly elongated orbit that would take it up to 93 billion miles from the Sun (or 75 times the distance of Pluto). They estimate that it could be up to 10 times more massive than Earth, putting into a category of objects that are between Earth and Neptune in mass, and are fairly common in other star systems. Although the astronomers remain open to other explanations, the search for Planet Nine has already begun.

This is an exciting possibility for the astronomical community. The existence of such a planet would challenge our understanding of the evolution of the solar system. How could such a massive planet exist so far from the Sun? This is certainly not the first time that a huge trans-Neptunian object has been proposed. It occurs to me that the possibility of a ninth planet could revive one of the most interesting chapters in the history of astronomy – the search for Planet X.

It’s worth having a brief reflection on the history of the idea of Planet X, and how this week’s news potentially fits into the wider picture.

A false colour picture of Pluto. The hypothetical Planet Nine may be 75 times the distance from the Sun as Pluto, meaning light from the Sun would take up to a week to travel the distance.
A false colour picture of Pluto. The hypothetical Planet Nine may be 75 times the distance from the Sun as Pluto, meaning light from the Sun would take up to a week to travel the distance.

The search for Planet X began in the mid-nineteenth century. In 1846, the mathematician Urbain Le Verrier had hypothesised the existence of Neptune through his analysis of perturbations in the orbit of Uranus. He wrote to German astronomer Johann Gottfried Galle, who discovered Neptune the very next day in the precise position that Le Verrier had predicted. Although this was a decisive confirmation of Newton’s Law of gravitation, there remained irregularities in the orbits of both planets that were still unexplainable through Newtonian physics. It was suspected that another planet orbited beyond Neptune. Throughout the rest of the nineteenth century, various astronomers postulated theories about the existence of such a planet.

In 1906, astronomer Percival Lowell began an extensive search for a trans-Neptunian object, which he called Planet X. Lowell had a reputation for an overactive imagination – he had been the astronomer who popularised the idea that canals on Mars were a sign of intelligent life – and he hoped that the discovery of Planet X would restore his scientific credibility. He wrote extensively about the planet, predicting its size and orbital characteristics. But he passed away in 1916 without confirming his theory, and the search for the ninth planet was temporarily halted for over a decade due to his family’s legal battles with the observatory.

Lowell observing Venus in the Lowell Observatory, 1914.
Lowell observing Venus in the Lowell Observatory, 1914.

When astronomer Clyde Tombaugh discovered the planet Pluto from the Lowell Observatory in 1930, it seemed as though the search was over. It was almost in the exact position predicted by Lowell, leading many astronomers to believe that this was indeed Lowell’s Planet X.

However, the planet’s dimness and orbit eccentricity led other astronomers to question the discovery. It was significantly dimmer than Lowell had predicted, suggesting that it was either very dark, or very small. Additionally, its highly elliptical orbit meant that it was unlikely to have caused the irregularities in the orbits of Uranus and Neptune. Pluto was unlikely to be Lowell’s hypothesised Planet X.

Pluto’s estimated size was continuously revised during the next few decades. Its size and mass were eventually confirmed in 1978, with the discovery of its moon Charon. Pluto was in fact very small, but with a high albedo (due to a surface covering of methane ice). With confirmation that Pluto was too small for its gravity to affect giant planets, it was concluded that Lowell’s prediction of a planet in an orbit close to Pluto’s actual position had been entirely coincidental.

Furthermore, the irregularities in the outer planets’ orbits were explained in the early 1990s, with measurements from the Voyager 2 spacecraft suggesting that astronomers had overestimated Neptune’s mass. Coupled with the fact that numerous distant space probes had showed no signs of irregularities in their trajectories that might be indicative of a large undiscovered body, the evidence for Planet X ceased to exist. The idea faded from the public imagination. Pluto and other known trans-Neptunian objects were downgraded to dwarf planets. Today, the name Planet X is commonly used for any hypothetical large planet beyond Neptune.

The ninth planet proposed by Caltech this week bears little resemblance to the one predicted by Lowell. Its existence is suggested not by irregularities in the orbits of the outer planets, but by an orbital alignment of small bodies in the Kuiper belt. It is also significantly more distant from the Sun than Lowell predicted. If the planet is confirmed to be real, it will be unlike any planetary body that currently exists within our Solar System. We may have to look further afield, to the super-Earths or mini-Neptunes that orbit other stars, to help us understand the complex relationship this planet has with the rest of our Solar System.

Of course, it’s also good news for science fiction writers. What sort of world could be lurking at the edge of the Solar System? Is it some sort of Nemesis planet, occasionally nudging the orbits of comets and sending them plunging inwards towards the Earth? Or will it just be a dead husk of a world, so distant that even Pluto may seem like a close neighbour? Either way, the possibilities are exciting. It’s amazing to think that even as our telescopes are uncovering thousands of worlds around other stars, and our most distant probes are crossing into interstellar space, we are still discovering new things about our Solar System, our home in the cosmos.

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