If you were in school when Pluto stopped being considered a planet or even you learned in elementary that it was a planet, you still have a lot of questions about how or why that happened. It’s not like Pluto disappeared, it still is in space, nothing about it has changed. But we see things differently.
To explain how things have changed, here are some of the most common questions about the different types of planets:
What is a planet?
The International Astronomical Union adopted the most recent definition of a planet in 2006, where it says that it must do three things:
The International Astronomical Union (IAU) is a worldwide organization of astronomers. It created and adopted this definition after newly found objects (now known as dwarf planets thanks to their definition).
What is a dwarf planet?
The term “dwarf planet” was coined by planetary scientist Alan Stern as a part of a three-way categorization of planetary-mass objects in the Solar System. It was adopted in 2006 by the International Astronomical Union (IAU) as a category of sub-planetary objects.
Dwarf planets are too small to be considered full-fledged planets but too large to fall into smaller categories. This is because this type of planet has not become gravitationally dominant and it shares its orbital space with other bodies of a similar size, meaning it doesn’t orbit the Sun by itself as the Earth does.
There are currently five dwarf planets recognized by the IAU: Pluto, Eris, Haumea, Makemake and Ceres.
What is an exoplanet?
An exoplanet is a type of planet that orbits another star that is not the Sun. They’re also called extrasolar planets and have been found outside our solar system, it is theorized that there are billions of exoplanets in the Milky Way Galaxy alone and that some of them may be habitable (meaning, have conditions favorable to life).
The evolution of the definition
The definition of planet has changed with time because of the scientific process (or method), where it is encouraged to question, hypothesize, discover, and change previous ideas based on what is learned. Nevertheless, this process is not done “just because”, it must have reasons and tests to support it, but also have other scientists doing reviews of another’s work.
This is what changed the idea that the Earth was the center of the known universe, into the model that the Sun is at the center of it. Technological advances also allowed for the discovery of Uranus in 1781, Neptune in 1846.
Pluto was discovered in 1930 and was identified as the ninth planet of our solar system. Nevertheless, technical advances in telescopes led to a better observation of it, by improving the detection of very small and distant objects that led to the creation of a new category (dwarf planet) in 2006.
With this being said, the definition of a planet is not unanimously agreed upon by astronomers and planetary scientists. Some of them argue that it seems very purposely done to limit the number of planets, others that it’s incomplete and unclear.
An active debate about the definition of planet exists, and probably it will continue to happen and deepen with new discoveries and technology.
Outer space is a vast place with a lot to learn about, so things are going to change with time.
