In this section we will respond to questions we are often asked either in person or by e-mail. If you have a question about asteroids, comets, or the work we do at the MPC, please contact us and ask us!
Is Pluto a minor planet?
Questions concerning Pluto are undoubtedly the most asked when I’m talking to non-astronomers out in the Real World. For a piece of ice and rock smaller than the Moon, it garners a lot of interest!
Allow me a preamble: Pluto was discovered in 1930 and quickly became the 9th planet, as very little was known about it. In the 1960s and 70s advances in technology permitted Pluto to be studied in greater detail thus shedding light on its true nature: It wasn’t a large planet after all, but actually smaller than our Moon and with a density slightly higher than ice—it was a dirty snowball!
Despite the discovery of a moon, Charon, it became clear that Pluto was not a “major” planet; it was too small, it’s orbit was very different to the other planets’, and other similar objects were being discovered (Chiron, in 1977) or were already known (Ceres, discovered in 1801)—most astronomers were ready to remove it from the list of major planets. The nail in the coffin came in 2005 with the announcement of Eris, an object beyond the orbit of Pluto that could be slightly larger in size and was some 25% heavier. In 2006, the IAU (International Astronomical Union) defined the term planet for the first time, with the consequence that Pluto was left out of the planet club. However, in a somewhat controversial decision, the IAU coined the term dwarf planet to designate objects too small to be planets, but which were nevertheless spherical in shape.
So finally we come to answer the question of whether Pluto is a minor planet or not, and that answer is yes, it is, and its minor planet number is 134340. Any object in the Solar System that is not the Sun or one of the 8 known major planets is a minor planet by exclusion. Furthermore, due to its spherical shape, Pluto is also a dwarf planet, like Ceres or Eris. But there’s more! Pluto is the prototype for a class of objects called plutoids, which are simply dwarf planets located beyond the orbit of Neptune.
As you can see, Pluto is a small object that’s caused a lot of trouble for its size. If you want to know even more about Pluto’s demotion from planethood, and what makes a planet, a planet, read this. The video below is a short piece from Discovery News worth watching.