The Ninth Planet in Our Solar System.

The recent discovery that the orbits of some Kuiper belt objects (KBOs) share properties has proved puzzling. A pair of scientists have now proposed a bold explanation: there may be a planet-sized object yet undetected in our solar system.

Mysterious Clustering

KBOs, the population of mainly small objects beyond Neptune, have proven an especially interesting subject of study in the last decade as many small, distant bodies (such as Eris, the object that led to the demotion of Pluto to dwarf planet) have been discovered.

Previous studies have recently discovered that some especially distant KBOs — those that orbit with semimajor axes of a > 150 AU, nearly four times that of Pluto — all cross the ecliptic at a similar phase in their elliptical trajectories. This is unexpected, since gravitational tugs from the giant planets should have randomized this parameter over our solar system’s multi-billion-year lifespan.

Physical alignment of the orbits of Kuiper belt objects with a > 250 AU (and two objects with a > 150 AU that are dynamically stable). [Batygin & Brown 2016]

Physical alignment of the orbits of Kuiper belt objects with a > 250 AU (and two objects with a > 150 AU that are dynamically stable). [Batygin & Brown 2016]

Two scientists at California Institute of Technology, Konstantin Batygin and Michael Brown (you might recognize Brown as the man who “killed Pluto”) have now increased the mystery. In a recently published a study, they demonstrate that for KBOs that have orbits with a > 250 AU, the orbits are actually physically aligned.

To explain this unexpected alignment — which Batygin and Brown calculate has only a 0.007% probability of having occurred by chance — the authors ask an exciting question: could this be caused by the presence of an unseen, large, perturbing body further out in the solar system?

Simulating a Ninth Planet

The authors test this hypothesis by carrying out both analytical calculations and numerical N-body simulations designed to determine if the gravitational influence of a distant, planetary-mass companion can explain the behavior we observe from the large-orbit KBOs.

Simulation of Planet Nine's effects

Simulation of the effect of a distant planet (M = 10 M, a = 700 AU, and e = 0.6) on KBOs; click for a better look! The perihelion position of KBOs with a > 250 AU clusters around 180° from the perihelion position of the perturbing planet. More-transparent points are less observable. [Batygin & Brown 2016]

The result? It turns out that such a distant planet can cause the orbits of KBOs with a > 250 AU to all align in the opposite direction of the orbit of the planet. What’s more, the gravitational pull of this planet can also explain other unresolved puzzles about the Kuiper belt, such as the presence of high-perihelion Sedna-like objects, as well as a population of KBOs we’ve observed that have misaligned orbits.

Unfortunately, Batygin and Brown found it isn’t possible to exactly determine the properties of the possible planet, since multiple combinations of its mass, eccentricity, and semimajor axis can create the same observational results. That said, they believe the distant perturber’s orbit is highly eccentric, its orbital inclination is low, and it’s fairly massive (since anything less than an Earth-mass won’t create the observed clustering of KBO orbits within the age of the solar system).

As an example, one possible set of parameters that approximately reproduces the observed KBO orbits is the following:

  • planet mass of 10 Earth-masses
  • semi-major axis of a = 700 AU
  • eccentricity of e = 0.6

This would correspond to a perihelion distance of 280 AU and an aphelion distance of 1,120 AU.

The authors speculate such a planet might have been formed closer in to the Sun, but it was ejected later on during our solar system’s evolution. Interactions with the Sun’s birth cluster could have then caused the planet to be retained in a bound orbit.

Future Tests

Solar system

Our solar system on a logarithmic scale (click for the full view). KBOs with a semimajor axis of a > 250 AU may be being aligned by a planetary-mass body with an even more distant orbit. [NASA]

How can we test this hypothesis of a ninth planet? Obviously, directly observing the planet would confirm its presence. But the authors’ model has an additional testable hypothesis: if it’s correct, there should be a population of high-perihelion Kuiper belt objects that don’t exhibit the same alignment of their orbits as the KBOs we know about, but instead have opposite-aligned orbits. If we discover such a collection of objects, that would be an excellent confirmation of this model.

The authors caution that their work is preliminary, and additional investigation will be required to better understand the possibilities presented here. But with any luck, future theoretical work, as well as observational tests of this model’s predictions, will help us determine whether there might be a distant ninth planet in our solar system!


Check out this video (created with WWT!), which walks us first through a view of the six aligned KBO orbits, then shows a possible orbit for the hypothesized planet, and then shows an additional population of already-discovered objects (also predicted by the model) that have orbits perpendicular both to the plane of the solar system and to the planet’s orbit. [Caltech/Robert Hurt]

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