What do you mean “if Planet 9 exists?” We know it exists; Pluto is still within our solar system! Well yes, this is true. Though, as of 2006, Pluto was redefined as a dwarf planet instead of a planet, which is how we classify the Earth, Mars, and others within our solar system.
Speed Running the History Books
Let’s travel back in time before the invention of most common household items, before the invention of the automobile, before the Revolutionary War, all the way back to Ancient Greece, 700-480 B.C.. Ancient Greeks first detailed the Earth’s moon, the Sun, Mercury, Venus, Mars, Jupiter, and Saturn as planets since they were the only objects that “wandered†across the night sky. At this time, Earth was considered to be at the center, where all other objects in space revolved around it.
Aristarchus of Samos placed the Sun at the center of the solar system in the 3rd century with his Sun Centre Theory, but the idea was not widely accepted until Nicolaus Copernicus reinvigorated the idea in the 16th century. This idea was confirmed once the telescope was invented on October 2, 1608 (existing in early records but credited to Hans Lippershey as he was the first to obtain a patent for its invention). In addition, the telescope proved that the moon was not a planet, since it orbits the Earth, not the Sun.
Uranus was discovered in 1781, Neptune in 1846, and Pluto in 1930. Ceres, discovered in 1801 between Mars and Jupiter, was initially listed as a planet, but due to its size was the first to be deemed as not a moon, nor a dwarf planet, but as an asteroid. A massive asteroid field beyond Neptune was discovered in 1992. Known as the Kuiper Belt, it contains many dwarf plants such as Quaroar, Sedna, Eris, Biden, Goblin, and FarOut (discovered between 2002 and 2018).
So, where did Pluto go wrong?
But we still haven’t answered the question “How is Pluto not the 9th planet?” Well, a planet, from the International Astronomical Union, is defined as a celestial body that is in orbit around the Sun, has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round shape), and has cleared the neighborhood around its orbit. Meanwhile, a dwarf planet is a celestial body that is in orbit around the Sun, has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, has not cleared the neighborhood around its orbit, and is not a satellite. A satellite, as an aside, is just like the satellites we send up to space, smaller objects that orbit larger objects.
Pluto orbits the Sun, is closely spherical, and is not a satellite, but it has not cleared the neighborhood around its orbit. So when it travels around the Sun, it shares part of its path with other objects, such as other dwarf planets. So, for now, Pluto remains as it is in the Kuiper Belt.
Well… why look for another planet?
The answer to this question actually goes back to the early 1900s. Scientists observed a slight hiccup in the orbits of Uranus and Neptune. They believed this hiccup was attributed to the existence of a larger planet beyond our reaches, with such a large gravitational field that it caused Neptune and Uranus to stray, ever so slightly. Think of it like how a gust of wind will cause you to be pushed just a bit to the side as you walk to your destination. When Pluto was discovered in 1930, scientists thought they found the answer they were searching for and closed the book. Fast forward to 2006, when Pluto was redefined as the dwarf planet and the realization settled that its gravitational force is not strong enough to influence Uranus’ and Neptune’s orbits, the book was reopened.
To further pique the curiosity of astronomers, some of the dwarf planets in the Kuiper Belt have extremely elongated, non-spherical orbits around the Sun. For example, the dwarf planet Sedna has an off-set, ovular orbit that takes roughly 11,000 years to complete one rotation around the Sun. In addition, at its peak, Sedna is over 900 astronomical units (AU) away from the Sun. For some perspective, the Earth is 1 AU, or 93 million miles, from the Sun. The presence of a massive object, and its gravitational pull, could support the reasoning behind the oddly shaped orbital paths like Sedna’s.
There is a massive list of scientists eager to answer the question of Planet 9s existence. A cohort of astronomers under Dr. Scott Sheppard at the Carnegie Institute for Science discovered many of the dwarf planets within the Kuiper Belt through their search for Planet 9. Dr. Mike Brown and Dr. Knstantin Batygin from the California Institute of Technology have a plethora of peer-reviewed journal articles detailing their journey and findings pointing toward the existence of Planet 9. Dr. Brown and Dr. Batygin predict that Planet 9, if out there, would be 3-5 times larger than Earth.
Dr. Michael Rowan-Robinson from the Imperial College London started sifting through unidentified objects from the Infrared Astronomical Satellite project in 1983. Out of over 250,000 unidentified objects, Dr. Rowan-Robinson narrowed the search to one object, due to its location within the galaxy, and slight travel across the sky over three months in 1983. However, it is significantly smaller than what Dr. Brown and Dr. Batygin predicted, leading to the possibility that these are two two different objects. Thus, the question of the existence of Planet 9 still remains.
Since the last planet discovery was almost 200 years ago, this discovery would be a turning point in our lives. With the launch of the next biggest and technologically advanced SPHEREx telescope projected to be in 2023, keep your eyes and ears open for new and exciting discoveries. There are so many things we do not know about our own galaxy, but with each discovery, we are getting one step closer to understanding the glory of our Milky Way home.
Featured Image: Milky Way Galaxy by Pablo Carlos Budassi is licensed under CC-BY-SA-4.0.
About the Author
Hello, my name is Justine Smith and I am a Master's student in the Warnell School of Forestry and Natural Resources at UGA. My study focuses on the use of passive camera surveys on private agricultural properties to observe the impact wild pigs have on native plants and animals, and quantify damage to crops associated with the species. I am originally from New Jersey and am slowly but surely acclimating to the intense Georgia heat. When I'm not looking at photos from my cameras, I can be found reading books on the back porch, playing video games, or exploring the woods for some discarded treasures to take home, like deer antlers. Contact me at: justine.smith@uga.edu
