Dear Pluto: We all still love you and you are still our favorite planet, despite what the astronomical union thinks!
Pluto’s journey through space has always captured our imagination. Since its discovery in 1930, the dwarf planet has been on an epic orbital trek that won’t be completed until Monday, March 23, 2178. This celestial milestone highlights Pluto’s unique position within our Solar System—a world with an orbit so long and unusual that it hasn’t even completed one cycle around the Sun since we first laid eyes on it.
The Hunt for a Hidden World
The story of Pluto begins with peculiar movements observed in the orbit of Uranus. In the 19th century, astronomers noticed that Uranus wasn’t moving quite as predicted by Newtonian gravity. While this led to the discovery of Neptune in 1846, questions about other influences on Uranus’s orbit persisted. American astronomer Percival Lowell hypothesized that an additional planet, beyond Neptune, was exerting a gravitational pull.
Although Lowell passed away before his theory was proven, his calculations guided Clyde Tombaugh to search the skies. Using a blink comparator, Tombaugh meticulously compared images of the night sky, looking for any movement among the stars. His efforts paid off in 1930, when he identified a moving object—Pluto.
Named by an 11-year-old girl, Venetia Burney, after the Roman god of the underworld, Pluto quickly joined the ranks of the Solar System’s planets. For decades, it was celebrated as the ninth planet, but its reign was short-lived. Before Pluto could even complete a single orbit, the International Astronomical Union (IAU) redefined the criteria for what constitutes a planet.
In 2006, Pluto was demoted to “dwarf planet” status because it didn’t dominate its orbit. Unlike Earth, Mars, or Jupiter, which have cleared their orbital paths of debris, Pluto shares its space with other Kuiper Belt objects.
Pluto’s Unusual Orbit
Pluto’s orbit sets it apart. Not only is it elongated and tilted compared to the eight classical planets, but it also takes a staggering 248 Earth years to complete one circuit around the Sun. At its closest, Pluto is 30 astronomical units (AU) from the Sun, and at its farthest, 49.3 AU—over 7.4 billion kilometers away. For 20 years of its orbit, from 1979 to 1999, Pluto was closer to the Sun than Neptune.
Despite its smaller status, Pluto hosts a family of moons. Charon, its largest, is so massive relative to Pluto that they are often referred to as a “double planet.” According to NASA, “Charon is about half the size of Pluto itself, making it the largest satellite relative to the planet it orbits in our Solar System.”
Pluto also has four smaller moons—Styx, Nix, Kerberos, and Hydra—each adding to the complexity of this distant world.
Pluto’s first full orbit since its discovery will occur in 2178. While this momentous event is beyond our lifetimes, it’s a reminder of the vast scales of time and space that define our universe. Pluto’s journey, much like its story, is a testament to the unyielding curiosity of humanity and our desire to explore the unknown.
