Emily Sims

In January, Caltech researchers published the paper “Evidence for a Distant Giant Planet in the Solar System” in the Astronomical Journal which detailed evidence for a giant planet with a highly elongated orbit in the outer solar system. This planet has been dubbed ‘Planet Nine’, and is thought to be ten times bigger than Earth, orbits 76 times farther than the sun than Earth, and has a theoretical orbit of 10,000-20,000 years around the sun.


Image Credit: The New Yorker

After absorbing these large statistics, why do scientists believe in the existence of Planet Nine? It has long been thought by scientists that the early solar system started with four planetary cores that pulled the gas around them to form the four gas planets—Jupiter, Saturn, Uranus, and Neptune. Over time, collisions and ejections led to them changing shape and moving to different locations. However, with this theoretical discovery, the possibility of there being five cores rather than four may gain some traction. Planet Nine could represent the fifth core whose potential proximity to Jupiter or Saturn may have led to the distant, eccentric orbit predicted of Planet Nine.

Within the Kuiper Belt, a region of small icy objects in the outer solar system, there are six objects that all line up in a single direction. This orbital alignment could only be maintained by an outside force, such as a planet. The researchers, Konstantin Batygin and Mike Brown, have also noted that Planet Nine is sufficiently large enough to be classified as a planet. Not only is it thought to be a staggering 5000 times the mass of Pluto, evidence appears to suggest that Planet Nine gravitationally dominates a region of the solar system larger than any of the known planets.

“This would be a real ninth planet,” says Mike Brown, “there have only been two true planets discovered since ancient times, and this would be a third. It’s a pretty substantial chunk of our solar system that’s still out there to be found, which is pretty exciting.”

The path to the theoretical discovery was, as is the case with a number of scientific discovery, the product of piqued curiosity and happy accidents. In 2014, a former postdoc of Mike Browns published a paper relating distant objects in the Kuiper Belt with obscure orbital features and the potential presence of a small planet. This kick-started Brown’s curiosity, and together with Batygin, spent a year and a half investigating these distant objects, and quickly found that the six most distant objects followed elliptical orbits that point in the same direction in space (30 degrees downwards to the plane of the eight known planets.) After computer simulations and mathematical modelling, the researchers noticed that if they ran their simulations with a planet in an anti-aligned orbit—the distant Kuiper Belt objects in the simulation assumed the alignment observed in reality. This evidence all points to a new planet- right?

Well, it isn’t actually that simple. Whilst this evidence is exciting, phantom planets are nothing new. “There are dozens of examples where researchers have said there must be another planet to explain some orbital anomaly,” says Mike Brown. For example, Pluto was the original Planet Nine, but its planetary status was disputed after a number of objects of similar size were discovered in the Kuiper Belt. When the International Astronomical Union formally defined the term ‘planet’, Pluto was subsequently reclassified as a dwarf planet.

The evidence is also still theoretical.  Jim Green, NASA’s director of planetary science, cautions that it is still too early to claim that the solar system again has nine planets. Also, I am a little sceptical about the fact that we can image objects outside our solar system, but have not yet found this. The Subaru telescope and the twin 10 metre telescopes in Hawaii are our best chance to directly observe the planet after a few months. Scientists have begun to scan the sky for Planet Nine, using information about its rough orbit.

Planet Nine is an extremely exciting concept and could lead to a surge of interest in astronomy and the potential of our outer solar system, but until the planet is directly observed it cannot be classed as a planet. For now, the sky is the limit.