I’m not an astronomy aficionado. In fact, I can’t identify any constellation. I find the subject interesting as much as it is overwhelming, and that’s probably why I never really took the time to familiarize myself with all the wonders in the sky. That said, even if I’m not invested in the space news, I heard about the Pluto confusion (I’m into science fiction, after all, space is in my life in some way). At one point, I just asked myself what it was all about and that led me on the path to learning about the discovery of Pluto and everything else. So, let’s start at the beginning:
Who’s Responsible For The Discovery of Pluto?
Everything began in 1905 when Percival Lawrence Lowell, an American astronomer whose work was already major and recognized in his field, realized that there was a problem with the positions of Neptune and Uranus. They were displaced. The gravity of another planet was probably responsible for the fact that the two already-known planets didn’t follow their predicted paths.
The team led by “human computer” Elizabeth Williams started performing mathematical calculations based on the differences in the orbits of Neptune and Uranus to help Lowell know where to search for the unknown object. Sadly, Lowell died in 1916 and the search stopped there.
A few years later, a young researcher working for the Lowell Observatory based in Arizona, Clyde William Tombaugh was given the task to take over the search for what was called Planet X at that time. Using Lowell’s observations, Elizabeth Williams’ calculations, and the predictions made by astronomer William Henry Pickering, Tombaugh started to photograph a specific section of the sky in April 1929. What he found then led him to discover the existence of the “ninth planet.” Few mythological names were proposed to name it, Pluto was officially adopted on May 1, 1930—it was suggested by Venetia Burney, an 11-year-old English schoolgirl.
The Pluto Controversy, what is it about?
And then, the problems began. Some astronomers doubted that Pluto was really a planet. Was it big enough? The question of Pluto’s mass would become a recurring topic in the community. At first, its mass was determined by the gravitational effect observed on Neptune and Uranus. There was not a way to observe the “planet,” yet. At first, it was estimated that Pluto was approximately the same mass as Earth. By 1948, estimates were lower, closer to the mass of Mars.
In 1978, everything changed when James Walter Christy discovered Charon, the largest of the five moons of Pluto—the others are Styx, Nix, Kerberos, and Hydra. With this discovery, it was finally possible to really measure Pluto’s mass for the first time. The result was approximately 0.2% of Earth’s mass, certainly not enough to affect the orbit of Uranus the way Percival Lowell observed all those years ago. That particular problem was solved when new data was collected in 1989, allowing a new estimate of Uranus’s mass. Like with Pluto, Uranus was not as big as it was thought. This proved that the mysterious Planet X never existed. The theory that led to the discovery of Pluto was false from the start. But Pluto was found anyway.
As years went by, new more precise observations of Pluto—and the area it was found in—led to more discoveries. In 1992, it was determined that Pluto is not alone, but part of a population of objects called the Kuiper Belt— a shadowy zone beyond the orbit of Neptune thought to be populated by hundreds of thousands of rocky, icy bodies each larger than 62 miles (100 kilometers) across, along with 1 trillion or more comets. Naturally, its classification as a planet was then again discussed. It became a real controversy when some officials working for museums and planetariums started to omit Pluto in the list of the planets in the Solar System.
In 2005, when Eris, a new trans-Neptunian object bigger than Pluto was discovered, it was decided that the fate of Pluto was to be decided for good. In 2006, the International Astronomical Union (IAU) revealed its official definition of a planet in the Solar System:
A planet is a celestial body that is in orbit around the Sun, has sufficient mass to assume hydrostatic equilibrium (a nearly round shape), and has “cleared the neighborhood” around its orbit.
Fulfilling only the first two of these criteria, Pluto was no longer a planet and was therefore classified as a dwarf planet. In fact, Pluto is the largest known dwarf planet in the solar system.
In 2015, a load of new images coming from NASA’s New Horizons mission Long Range Reconnaissance Imager (LORRI)—and other instruments—helped determine Pluto’s diameter. After some revisions, it’s 2,376.6 km.
Even if Pluto is not a planet anymore, there is so much we don’t know that we will probably never stop talking about it.