Many of us remember Pluto as the used-to-be 9th planet in our solar system that was demoted to a dwarf planet. So what makes Pluto a dwarf planet and not a planet? What is interesting about Pluto, and, if it was so small, how exactly was it discovered earlier than other dwarf planets?
In 1930, American astronomer Clyde Tombaugh discovered Pluto after astronomers found small discrepancies between observational data and predictions for Uranus’ orbit. Astronomers believed this indicated a 9th planet and set out to look for this new world. Tombaugh found Pluto pretty close to where astronomers predicted it would be. However, Pluto was much smaller than expected, too small to affect Uranus’ orbit. In addition, there may have been erroneous calculations when determining the discrepancies of Uranus’ orbit. It now seems that Pluto’s discovery was a coincidence, and if the errors had not been made, Pluto might have not been discovered until much later. A fun fact about Pluto – while Tombaugh discovered this “9th planet”, later that year an 11-year-old girl from England named Venetia Burney named it Pluto.
Pluto became a dwarf planet after the discovery of other objects about the same size, like Ceres, Eris, Makemake, and Haumea. These planetary objects orbit around the Sun just like planets, but they are too small to be considered planets. Instead, they are classified as dwarf planets. Pluto is actually only half as wide as the United States, much smaller than our own Moon. It is mostly rock (about 2/3), with the rest of the mass being ice. Since Pluto is so far from the Sun and only has a temporary atmosphere, the temperature on Pluto is freezing, with an average of 40K (-387.67 °F).
In 2015, the New Horizons spacecraft did a flyby study of Pluto and provided a lot more information about Pluto and its five moons. Pluto’s largest and closest moon, Charon, is like most other moons in the inner solar system. Charon keeps one face pointed toward its central planet all throughout its orbit. However, the other four moons, Styx, Nix, Kerberos, and Hydra, behave like spinning tops, showing all faces to Pluto multiple times throughout their orbits. See here for the video. New Horizons also provided us with details about Pluto’s surface. There is an area called Tombaugh Regio composed mostly of solid nitrogen ice. Vast regions on Pluto lack craters, which means some activity has erased them. There are also mountains that are several kilometers tall, which is about the height of the Rocky Mountains.
We are still learning a great deal about Pluto, but it is a fascinating dwarf planet that has provided us with many mysteries about how the different features were formed. The New Horizons spacecraft’s data has given us a much better insight into Pluto, but there is still much to be discovered.
The Cosmic Perspective: The Solar System by Bennett, Donahue, Schneider, and Voit