The Search is on for Planet Nine

Then felt I like some watcher of the skies
When a new planet swims into his ken.

John Keats: On First Looking into Chapman’s Homer

Pluto, discovered in 1930, orbits in the Kuiper Belt, a ring of asteroids and icy debris beyond Neptune. About ten years ago, it was reclassified as a “dwarf planet”, so now we have eight planets. Is there a ninth anywhere? A recent paper in the Astronomical Journal presents evidence of a large planet in an orbit far beyond Neptune. If confirmed, this would be a dramatic confirmation of our understanding of solar system dynamics [TM088, or search for “thatsmaths” at].


Orbits of six trans-Neptunian objects aligned in one direction, suggesting a perturbing body, Planet Nine (?). Caltech/Robert Hurt.

What is the basis for such a bold claim? In the mid-nineteenth century, astronomers observed irregularities in the orbit of Uranus, the then-most-distant planet. Again and again, they calculated the orbit, but Uranus always drifted away from the predicted path. Was our understanding of planetary motions defective, or was there some unknown influence causing the observed anomalies?

In 1846, two scientists, one French and one British, calculated that the disturbances could be caused by a new planet orbiting further from the Sun. John Couch Adams produced an estimate of where observers should point their telescopes to find the new planet, and notified George Biddell Airy, the Astronomer Royal. Independently, Urbain Le Verrier calculated a position and persuaded the Berlin Observatory to search for it. Almost immediately, a planet was found within one degree of Le Verrier’s position and about 12 degrees from Adams’ estimate.

The name Neptune was bestowed by Le Verrier on the new planet. The discovery was a powerful vindication of Newtonian dynamics and of the predictive power of mathematics. Physicist François Arago described Le Verrier as “the man who discovered a planet with the point of his pen.”

Is there a Ninth Planet?

The analysis of orbital irregularities to calculate the nature and location of a possible perturbing influence has been done again very recently by two astronomers at the California Institute of Technology, Konstantin Batygin, a theorist, and Michael Brown, an observer. They studied the movements of several objects in the Kuiper Belt.

Six of these objects have elliptical orbits all pointing in the same direction. The orbits are also tilted about 30 degrees from the ecliptic, the Earth’s orbital plane. It is as if the bodies are bound together in some way, but they are much too small for their gravity to shape the orbits in this way. The likelihood of the observed arrangement occurring by chance is remote, so there must be some external force shepherding them into alignment. The idea that a new planet is responsible is analogous to the reasoning that led to the discovery of Neptune.


Perihelia of minor bodies: Clustering is evident for the six TNOs (red). Image from Batygin & Brown (2016). The Astronomical Journal, 151:22 (12pp).

Batygin and Brown ran an extensive series of computer simulations over periods of four billion years, comparable to the age of the solar system. By including a ninth planet, they were able to generate phase-locked orbits very like those observed. They also found stable orbits perpendicular to the ecliptic plane. Objects on such orbits have been observed in the past few years, suggesting that Batygin and Brown are on the right track.

They explored a range of planet masses and orbits for the proposed planet. The evidence suggests that “Planet Nine” is on an elongated orbit 20 times more distant from the Sun than Neptune and is ten times more massive than Earth. The search is now on for Planet Nine and we must wait to see if it is found. If so, the solar system is much larger than we imagined.


Batygin, Konstantin and Michael E. Brown, 2016: Evidence for a distant giant planet in the Solar System. Astron. J., 151:22 (12pp), 2016 February. doi:10.3847/0004-6256/151/2/22

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