Planet Nine could be hiding in the dark, frozen abyss of the outer solar system. Astronomers are tuning a wide variety of telescopes and instruments to the search for the unseen body.
The Cassini spacecraft, orbiting around Saturn, is one of the myriad of observatories currently searching the darkness for the distant world.
One of the challenges facing astronomers in their search for Planet Nine is the enormous length of a year on the world - between 10,000 and 20,000 times longer than that on Earth. Astronomers often depend on seeing an object move between two images taken over a period of time. In fact, this was how Pluto was discovered in 1930, as a dot of light was seen in two different positions, in a pair of images, relative to background stars.
In January 2016, armchair astronomers were treated to the news that some researchers found evidence that a previously-unseen planet orbits beyond the path of Neptune. These investigators believe the distant world may be seen within five years if a thorough search is undertaken.
The Cassini probe takes precise measurements of the position of Saturn as it races around the ringed planet. This data is utilized within a virtual model tracking the exact locations of the largest planets in the solar system. If a large unseen world lurks among the outer planets, the gravitational effects of that body would affect the motions of the known gas giants.
"For most of the 160 years, astronomers tried to use the positions of the planets themselves to infer the existence of another planet (amusingly: it was the search for this alleged planet that led to the inadvertent discovery of Pluto, which is why the New York Times headline on the day of the Pluto announcement suggests that the planet might be bigger than Jupiter, which it is not)," wrote Mike Brown, a planetary astronomy professor at Caltech who helped propose the idea of Planet Nine.
As the Juno probe arrives at Jupiter in July 2016, data from that spacecraft will be integrated into the search for Planet Nine.
On the ground, the Subaru telescope in Hawaii and Chile's Victor Blanco telescope are also being utilized in the search for the distant planet.
Computer simulations are playing a major role in the search for the mysterious Planet Nine. Already researchers have eliminated over half the possible paths for the world, greatly reducing the amount of sky over which astronomers need to search.
Evidence for the existence of the frozen world, estimated to be 10 times as massive as the Earth, came from the discovery that six objects in the distant Kuiper Belt of interplanetary debris had anomalous orbits. One way of explaining the odd movements would be the presence of a large planet in the depths of the solar system.
If Brown does find the elusive world, he will be the first person in history to discover a world after "losing" one - it was his work that led to Pluto losing its status as a full-fledged planet.
Analysis of how data from the Cassini probe can be used to pinpoint the location of Planet Nine was published in the journal Earth and Planetary Astrophysics.