It wasn’t long ago that there were nine planets in the Milky Way: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. However, in the early 2000s, other celestial bodies were found orbiting in our solar system that were similar to Pluto in size and orbit. As a result, these bodies would either have to become planets or Pluto would have to be re-categorized. Therefore, in 2006, the International Astronomical Union (IAU) established three criteria for the definition of a planet : 1) it orbits around the Sun, 2) it has enough mass that its own gravity makes the planet round, and 3) its gravity clears other objects out of its orbit. With these criteria, Pluto was deemed too small to be a planet and was demoted to a ‘dwarf planet.’ And then there were eight.
For many years, astronomers have speculated that there may be additional, unseen planets in our solar system. Earlier this year, Batygin and Brown published an article in The Astronomical Journal giving evidence for “Planet 9” . The major clue in the search for Planet 9 was the disruptions observed in the orbits of the dwarf planets. While some astronomers dismissed these disruptions as an observation error, Batygin and Brown concluded that these perturbations must be due to the gravitational pull of a planet. Based on their calculations, Planet 9 is more than 10x the mass of Earth (by comparison, Uranus is 14x the mass of Earth). Due to its orbit, the closest Planet 9 ever gets to the Sun is approximately 250x the distance between the Earth and the Sun. Remarkably, this planet orbits around the sun once every 10,000 to 20,000 years! While this planet is able to fit computer simulations, no one has ever seen it and NASA requires more evidence before accepting it as a bona fide planet.
The main problem with observing Planet 9 is that it could be anywhere and astronomers would have to look in every direction. The Gastineau group from France has attempted to limit the possible location of planet 9 . Using information collected by the Cassini spacecraft since 2003, the team has been able to track the location of Saturn with high accuracy. Fitting the gravitational influence of Planet 9, they postulate that planet 9 orbits between 108° and 129° of galactic longitude, with the highest probability at 117.8°. With this information, astronomers can focus their telescopes on a particular subsection of the sky while looking for this silent giant that has left a trail of clues in our cosmos.