Planet X or a mirage?

On September 23, 1846, Johann Galle of Berlin Observatory received a message from Paris-based astronomer Urbain Le Verrier: “Direct your telescope to a point on the ecliptic in the constellation of Aquarius, in longitude 326 degrees, and you will find within a degree of that place a new planet...” Just in an hour, aided by the mathematical prediction, Galle discovered Neptune, within a degree of the spot indicated by Le Verrier.

Neptune’s discovery was one of the greatest achievements of mathematical astronomy: Le Verrier and several other astronomers, including British mathematician John Couch Adams (who had made a prediction similar to Le Verrier’s), knew there must be a planet beyond Uranus of a similar size whose gravitational influence could explain discrepancies in Uranus’ orbital pattern.

Along similar lines, researchers at California Institute of Technology (Caltech) have been claiming mathematical evidence for “Planet X or Planet 9” deep in the solar system — a rocky sphere 5-10 times the mass of Earth having an orbit about 20 times farther from the Sun on average than Neptune. This theoretical planet could be taking between 10,000 and 20,000 Earth years for one full revolution around the Sun.

Besides, the existence of Planet 9 and its gravitational influence could explain a curious pattern in perihelia (plural of perihelion — the point in the orbit of a planet, asteroid, or comet at which it is closest to the sun) and the direction of the orbit of certain eTNOs, extreme trans-Neptunian objects like Sedna and Biden, named after Joe Biden — that whether they were in alignment, or clustered (Evidence for a Distant Giant Planet in the Solar System, by Konstantin Batygin and Michael E Brown).

A recent analysis of 14 of those bodies — by Kevin Napier, physicist at the University of Michigan, and his colleagues — showed no evidence for such clustering. Since then, believers and doubters have been arguing on the expected lines: While one side suggests the primary reason for believing in Planet 9 has been rubbished, the other points out the paper doesn’t actually disprove what the title claims — No Evidence for Orbital Clustering in the Extreme Trans-Neptunian Objects.

So, does the paper disprove orbital clustering of eTNOs? No. The author in the conclusion said: “This work also does not analyse whether some form of clustering could be consistent with the 14 eTNOs we consider.” Suggesting “further studies”, Napier et al stated: “It is important to note our work does not explicitly rule out Planet X/Planet 9; its dynamical effects are not yet well enough defined to falsify its existence with current data.”

I’m a firm believer in the existence of Planet 9. I was among those who were deeply saddened by the International Astronomical Union’s 2006 decision of reclassifying Pluto as a dwarf planet. This solar system with eight planets appears incomplete; Earth must have a rocky big brother. I’m not alone; millions like me are desperate for its discovery.  

To many, this extreme belief in Planet 9 is a reason for worry. According to David Clifford Jewitt, the first (along with Jane Luu) to see a body beyond Pluto in the Kuiper belt (the region of the solar system beyond Neptune’s orbit), “Particularly scary thing about the ninth planet is that a lot of people believe there is a ninth planet and that we all know is a huge psychological bias, to the effect that if you want to believe something is true, you will find evidence, real or not, that is true (as told to YouTube channel Veritasium).”

We are aware of the Pluto fiasco. Many believed there must be a planet beyond Neptune and when Pluto was discovered in 1930, it was soon declared the Sun’s ninth planet, despite little evidence in support (from 1992 onward, many bodies were discovered orbiting in the same volume as Pluto, showing it’s merely one of the objects in the Kuiper belt).

Interestingly, “Planet X” was an extensive project, launched in 1906 by Percival Lowell — who had founded Lowell Observatory in Arizona in 1894 — in search of a possible ninth planet.

Is it foolhardy to claim the existence of Planet 9/X? No. “One of the most startling discoveries about other planetary systems has been that the most common type of planet out there has a mass between that of Earth and Neptune,” according to Batygin, associate professor of planetary sciences at Caltech (as published in caltech.edu). “Until now, we’ve thought the solar system was lacking in this most common type of planet. Maybe we’re more normal after all.”

Computer simulations, too, back the presence of a ninth planet. And if there is Planet 9, from where did it come? Scientists have been holding the view that the early solar system began with four planetary cores that grabbed all the gas around them, forming Jupiter, Saturn, Uranus, and Neptune. However, according to Brown, Planet 9/X could represent the fifth core, which got too close to Jupiter or Saturn that it was ejected into its “distant, eccentric orbit”.

So, until Planet 9 is either found or outright disproven, discussions around its potential existence will gravitate towards depending on our biases. I’m keeping my fingers crossed for Planet X!