If we think back to 2005, Pluto was the 9th planet from the Sun and we knew very little about it. It was a dark distant dot far out in the Solar System. Now, Pluto has been reclassified as a Dwarf Planet. Until 2015, the best we could do is generate computer models of the surface brightness on Pluto using data from the Hubble Space Telescope. Even to Hubble, Pluto only appeared as a dot.
Pluto’s story begins after the discovery of Neptune. Astronomers thought that there could be a another planet beyond Neptune which was pulling on Uranus and Neptune. In 1906 Percival Lowell started the search for a 9th planet in the Lowell Observatory, Flagstaff Arizona. Percival died in 1916 having not discovered anything, or so he thought. Unknown to him, he had taken images of Pluto twice in 1915 but didn’t spot it. The search didn’t resume until 1929 when Clyde Tombaugh was given the job. His task was to take images of the sky from night to night and compare them to look for moving stars. On February 18th 1930, he found an object moving and the discovery was confirmed by subsequent observations.
The public were then asked for suggestions for the name of the new object. Pluto, who was the Roman god of the underworld, was suggested by Venetia Burney, an eleven year old girl at the time. Around the same time, Walt Disney introduced a character of the same name. Whether they were inspired by it or not, nobody is sure.
Initially Pluto was thought to be size of the Earth, it was later realised that Pluto was actually much smaller. In the early 1990s, more small bodies were discovered in the same region as Pluto and this began to call Pluto’s planetary status into question. In 2005, another object was discovered beyond Pluto which turned out to be more massive than Pluto.
The International Astronomical Union (IAU) is the body that decides these matters. In August 2006, the matter was voted on and a proposal to redefine what made a planet was agreed. There are three conditions a body must meet to be defined a planet;
- It is in orbit around the Sun.
- It has sufficient mass to assume hydro static equilibrium (a nearly round shape).
- It has “cleared the neighborhood” around its orbit.
Under this definition, Pluto meets the first two criterion but not the third and therefore isn’t a planet. Many people disagreed with this at the time. At the same time the IAU also decided that if a body met the first two criteria but not the the third, it would be defined as a Dwarf Planet.
The first spacecraft to visit Pluto was called New Horizons. It was launched in 2006 and flew by Pluto in 2015. Before New Horizons we had limited knowledge about Pluto. We now know that Pluto is much more interesting than first thought. It has 5 moons, the closest and largest of which is called Charon. They both orbit each other which is unusual in our Solar System.
The internal structure of Pluto is thought to contain a rocky core surrounded by a mantle of liquid water which is covered by the frozen surface. The surface is made from frozen water, and some other frozen gases. The surface shows huge contrasts in colours which is due to large areas of very dark material and large areas of very bright ice. One such bright region is called Sputnik Planitia. Pluto’s surface is covered in plains, mountains and craters. Pluto’s orbit is more elongated that the other planets’. At times in its orbit, it is much further away from the Sun than at others. When it is at the closest part of the orbit, the surface ice sublimates to form a thin atmosphere. As it approaches the furthest part of the orbit, this atmosphere freezes back onto the surface, possibly falling as snow.
Pluto is very far from the Sun, on average around 6 billion kilometres. It takes 246 Earth years to orbit the Sun once. This means it hasn’t completed an orbit since it was discovered. At this distance it is very dark and cold. The temperature on the surface is an average of -232 degrees Celsius. You can experience how dark Pluto is for yourself. At a certain time during dawn and dusk, the light we experience here on Earth is equivalent to mid-day on Pluto. NASA have an online tool which allows one to calculate the next time you can experience this. I have included the link below, so be sure to give it a try for yourself!