The elusive planet X may be real after all

Caltech researchers have found evidence suggesting there may be a ‘Planet X’ lurking deep in our Solar System. This hypothetical Neptune-sized planet would orbit our sun on a highly elongated trajectory far beyond Pluto. The object, which the researchers have nicknamed ‘Planet Nine’, could have a mass about 10 times that of Earth, orbit about 20 times farther from the Sun on average than Neptune and take up to 20,000 Earth years to make one full orbit around the Sun.

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Following the discovery of the planet Neptune in 1846, there was considerable speculation that another planet might exist beyond its orbit. The search for this, so-called Planet X began in the mid-19th century and culminated at the start of the 20th with Percival Lowell's hypothesis that tried to explain apparent discrepancies in the orbits of the giant planets, particularly Uranus and Neptune, speculating that the gravity of a large unseen ninth planet could account for these irregularities.

Clyde Tombaugh’s discovery of a large trans-Neptunian object in 1930 appeared to validate Lowell’s hypothesis, and Pluto was officially named the ninth planet. However, by 1978, Pluto was conclusively determined to be far too small for its gravity to affect the gas giants, resulting in a renewed search for a tenth planet. The search was all but abandoned in the early 1990s, when a study of observations made by the Voyager 2 space probe found that the irregularities observed in Uranus’s orbit were due to a slight overestimation of Neptune’s mass.

After 1992, the discovery of numerous Kuiper Belt objects with sizes and masses similar to that of Pluto (particularly Sedna), led to a debate over whether Pluto should remain a planet, or whether it, and its many similarly-sized neighbours, should be given their own separate classification. Although the controversy on what constitutes a planet continues, the matter was formally closed (at least for now) in 2006 when the International Astronomical Union (IAU) reclassified Pluto and its largest neighbours as ‘dwarf planets’, leaving the eighth planet, Neptune, as the outermost planet in our Solar System.

The as yet undiscovered ‘Planet Nine’, as imagined by a NASA artist, would have two-to-four times the diameter and ten times the mass of Earth and would take anything between 10,000 and 20,000 years to complete one full orbit around the Sun. Its atmosphere will likely consist of a thick layer of hydrogen and helium.

While today the astronomers widely agree that Planet X, as originally envisioned, does not exist, the concept of an as yet unobserved major planet has been revived by a number of astronomers to try and explain other anomalies observed in the outer reaches of our Solar System.

I went from trying very hard to be sceptical about this ‘Planet Nine’, to suddenly thinking: ‘this might actually be true’ - said Dr Brown.

The possibility of Planet X came into prominence again in 2014, when, after the discovery of VP113 (a large Kuiper Belt object with a Sedna-like 4200-year orbit), two astronomers Chad Trujillo and Scott Sheppard noticed a strange orbital clustering of this, and several other large trans-Neptunian objects (TNOs). They argued that the trajectorial grouping of these TNOs suggests the existence of a large trans-Neptunian planet of between 2 and 15 Earth masses orbiting beyond 200 AU with possibly a high inclined orbit at some 1500 AU. Trujillo and Sheppard predicted the orbit of this super-Earth to be anti-aligned to the clustered trans-Neptunian objects. Because Pluto is no longer considered a planet by the IAU, this new hypothetical object has become known as ‘Planet Nine’.

On 20th January 2016, Mike Brown, the discoverer of Sedna, and Konstantin Batygin published an article further corroborating Trujillo and Sheppard’s initial findings and proposing an unknown super-Earth-sized planet in our Solar System based on a statistical correlation in the orbits of six distant trans-Neptunian objects.

Brown and Batygin, who originally ran computer simulations trying to disprove Trujillo and Sheppard’s theory, analysed six extreme TNOs, including Sedna, in a stable configuration of orbits mostly outside the Kuiper Belt, and found that these six objects have orbits that are aligned in approximately the same direction in physical space and lie in roughly the same plane. They calculated that the probability of this occurring by mere chance alone is 0.007%. “I went from trying very hard to be sceptical that what we were talking about was true, to suddenly thinking, ‘this might actually be true’.” said Dr Brown.

The possibility of as-yet-unknown planet in our Solar System came into prominence thanks to a strange orbital clustering of six distant trans-Neptunian objects.

What makes this claim interesting is the fact that Dr Brown is an expert on finding distant objects in our Solar System – it was his discovery of 2,326 km-wide dwarf planet Eris (that was at the time thought to be slightly bigger than Pluto, although now we know that it is actually slightly smaller than Pluto, which is 2,372 km-wide) in the Kuiper Belt in 2005 that led to the demotion of Pluto from full planet status a year later.

The probability of such orbital clustering of six trans-Neptunian objects occurring by mere chance alone is barely 0.007%.

Brown and Batygin estimated this, as yet undiscovered, planet to be two-to-four times the diameter and ten times the mass of Earth (about 60% the mass of Neptune). Unlike the near-circular paths traced by the known planets of our Solar System, this super-Earth would be orbiting in a highly eccentric elliptical trajectory, that takes it no closer than about 200 AU and no further than about 1500 AU from the Sun, taking between 10,000 and 20,000 years to complete one full orbit. It is hypothesised that this ‘Planet Nine’ will likely have a thick atmosphere of hydrogen and helium.

Mission data from the Kepler Space Telescope, which so far discovered over 1000 confirmed exoplanets (along with many thousands as yet unconfirmed planet candidates), ‘super-Earths’ seem to be very common in extra-solar planetary systems throughout the Milky Way. Therefore finding one in our own Solar System wouldn’t be very surprising.

“The possibility of a new planet is certainly an exciting one for me as a planetary scientist and for all of us” said Jim Green, director of NASA’s Planetary Science Division. “This is not, however, the detection or discovery of a new planet. It’s too early to say with certainty there’s a so-called ‘Planet X’. What we’re seeing is an early prediction based on modelling from limited observations. It’s the start of a process that could lead to an exciting result.” he concluded.