Does the Solar System Really Have a Ninth Planet?

Since it is so far from us, Planet Nine, estimated to be 10 times Earth's weight and four times its size, has proven tricky to study. But in the process, our picture of the Solar System has become fuller.

Caltech researchers Konstantin Batygin and Mike Brown, who initially made the announcement about the possible existence of Planet Nine, have published new findings that have helped narrow down the current location of the mysterious giant looming over Earth in the far outer Solar System.

Batygin and Brown initially deduced the existence of Planet Nine by studying the massive clump of icy objects that exist beyond Neptune, in the region called Kuiper Belt. Called Kuiper Belt Objects (KBOs), these are small enough to be substantially affected by the gravity of large bodies. They’re also all inclined at the same plane and orbit on it more or less as a whole.

But six KBOs stood out: these were tilted at a completely different angle to the solar system, as if disrupted from their original plane by a giant gravitational force.

Batygin and Brown inferred that there was indeed a giant planet in this region that was on the same odd plane as the KBOs and was putting them out of whack. They then modelled other effects on nearby KBOs that would be observed if there inference was correct. They found five additional KBOs that appeared to have been flung away – a phenomenon that was in line with their prediction of the size and location of the mysterious ejected planet (the planet used to be near Neptune but was flung out).

This method of deducing the presence and further calculating the details of a planet – by using its gravitational signature as opposed to direct observation – is neither new nor unusual.

In fact, Neptune was discovered in the 19th century by observing slight discrepancies in the expected orbit of Uranus. French mathematician Urbain Le Verrier calculated the actual location and mass of the body that must be tugging at Uranus, and Neptune was thus discovered in 1846. The farther out we go in the solar system, the more difficult visual detection becomes, and the more we rely on mathematics and indirect observation of gravitational effects rather than direct observation of a body.

It’s no secret that a lot of astrophysics these days is more about programming and running numbers through a computer than it is about staring through a telescope. In fact, even giant telescopes are controlled by computers from remote locations and their images and data relayed on screens for scientists to work with.

Planet Nine, the elusive giant

Continuing their research, Batygin and Brown studied more KBOs in multiple orbital radii, to tease out patterns in their paths around the sun that might indicate a perturbation by Planet Nine.

The planet is estimated to weigh 10 times and measure approximately four times as much as Earth. Its orbital period is about 15,000 years, and it orbits on a plane that’s tilted at an angle of approximately 30º to the plane of the solar system, at a distance between 200 AU to 1,200AU (1 AU is the distance between the Earth and the Sun).

As the hypothesised planet moves along this plane, it exerts a gravitational pressure on objects around it, including other planets. While many news reports recently claimed that Planet Nine could possibly be exerting a tug on the Cassini spacecraft currently in orbit around Saturn, NASA has put out a statement declaring that no such effect on Cassini has been observed.

On the other hand, a possible source of information about Planet Nine is Saturn itself. Studying the math of the orbit of Saturn, like Le Verrier did two centuries ago, provides clues about the position of Planet Nine. Had the planet been any less than 400 AU from the sun, its effect on Saturn would be easily observable. If and when the planet is beyond 600 AU, any effect on Saturn can be safely ruled out. In between these distances, however, at two very precise points, the orbital perturbations of Saturn are explained by Planet Nine.

Since the planet is so far out in the Solar System, direct observations are very tricky for several reasons.

A huge hindrance to observing distant objects within our Solar System are the stars at the centre of the Milky Way. The stars that appear casually strewn across our night sky are so bright that they dim out any light – especially reflected light – emanating from this region of the Solar System. Even so, there are still numerous surveys scanning large portions of the sky, meticulously cataloguing every single KBO encountered.

Astronomers and citizen scientists are now using data from these surveys and observations to find, by a process of elimination, the current approximate location of Planet Nine. Saturn’s orbital information that points to the existence of Planet Nine also suggests that the planet is not currently at the previously expected locations. This has eliminated a large portion of the sky.

But observations are further confounded by the heavy, indolent force that is the motion of the planet. Planet Nine is so distant from us that observers have made the same mistake as in the past with Eris and Pluto: its motion is so slow they would think it is a star.

Astronomers have thought that at the point in its orbit where it is closest to the Sun (“perihelion”), Planet Nine would become bright enough to be detected by existing observatories and telescopes (like the Widefield Infrared Survey Explorer), as it crosses the foreground of the Milky Way. So far, however, Planet Nine has not been detected, which leads us to conclude that the planet is not at perihelion. But according to our estimation of its orbital period – 15,000 years – Planet Nine should remain at perihelion for well over a decade, which would have given us ample time to have detected it.

Surveys like the Catalina or PanSTARRS, which scan extremely large patches of sky for very slowly moving objects, have every single KBO they have ever detected meticulously catalogued. As a result, they quickly eliminate large portions of the sky. Large portions of the sky near Planet Nine’s perihelion positions are nearly completely removed from consideration for the moment. According to researchers, the planet should currently be somewhere close to its farthest position from the sun (aphelion), right in the middle of the Milky Way (in the background).

All of that said, direct detection of Planet Nine might not be the primary objective of many astronomers and stargazers at this point. As Batygin and Brown mention in their paper, every time we detect a distant KBO, we get one step closer to detecting the actual planet.

How does Planet Nine fit into our existing picture of the Solar System?

  • It explains the existence of several objects with large perihelions, like the famous dwarf planet Sedna. These objects are on a different plane from the rest of the planets and the sun, and their closest approach to the sun is at an extreme distance from Neptune. Sedna’s perihelion is 76 AU while Neptune is located at 30 AU. These Sedna-like objects (2010 GB174, 2012 VP113, and more) have perihelia at similar locations, appearing close together when they are in these positions, despite very different eccentricities and orbital periods. The origins of such “clustering” have been mysterious until today, but they are perfectly explained by the existence of Planet Nine.
  • Centaurs are objects that are interspersed between the orbits of the four giant planets in the outer solar system. They were previously KBOs existing beyond Neptune but were ejected from their orbits, only to settle themselves in the neighbourhood of the planets. Their origins have been a source of many theories, for instance the theory of what caused them to violently fly away from the Kuiper Belt and enter the planetary orbits. Once again, Planet Nine explains the phenomenon.
  • Planet Nine explains tiny perturbations in the orbit of Saturn that can be detected mathematically at two different locations.
  • Of course, the planet also explains the observations that led to the conclusion of the existence of the planet itself: orbital alignment of extremely distant KBOs on a plane, and the cluster of KBOs that appeared to have been flung away from their original location.

Sandhya Ramesh is a science writer focusing on astronomy and earth science.