Pluto was discovered in March 1930 by astronomer Clyde Tombaugh at Lowell Observatory in Arizona, USA. For the best part of the next 76 years it was known as the 9th planet in the solar system. Indeed for many of us born in the 20th Century, it is hard to let go of that planetary status. To see something that for many years remained a truth, to suddenly be classified as something else, it’s hard. Many of us were left thinking ‘why is Pluto not a planet?’ It is easy to see, that with something more dramatic, like the change from a geocentric to heliocentric solar system, that such a change can be very hard to swallow.
With our advancing understanding of the solar system though, it became harder to justify Pluto’s planetary status. Cosmologists and astronomers started to find more objects out past Neptune, similar to Pluto. As more was learned about the scattered disc and the Kuiper Belt, it turned out that an entirely new classification was required for these objects. Not quite a planet, but not fitting into any other classification at the time.
A New Classification
Pluto, the planet that was? Not anymore. In 2006, the International Astronomical Union (IAU) voted to classify Pluto as a dwarf planet, rather than a planet. This decision was a long time coming.
When Pluto’s moon Charon was discovered in 1978, scientists were able to calculate Pluto’s mass much more accurately than ever before. It became clear that it was much smaller than originally believed. At a tiny fraction of the mass of Mercury, and roughly two thirds the size of Earth’s moon, Pluto was clearly not to the same scale as any other planet. This discovery led some to question whether Pluto was really a planet after all, or some other type of object.
In the 1990s and early 2000s, more objects similar in size to Pluto were discovered in the trans-neptunian region of the outer solar system. Because of that, scientists needed to expand on their definition of a planet.
They also needed to provide a definition for this new type of object. This was needed to separate those types of objects like Pluto into a distinct class. Otherwise, all of the newly found objects would have to be called planets and that just wasn’t a reasonable definition of what they are.
As a result, the International Astronomical Union (IAU), the official governing body for matters concerning naming astronomical objects, came to a definition of the term “planet,” and a separate term “dwarf planet.”
Definition Of A Planet
According to the IAU, a planet is a celestial body that meets the following criteria:
- The object is in orbit around the Sun.
- It has sufficient mass to assume hydrostatic equilibrium (a nearly round shape).
- It has “cleared the neighborhood” around its orbit.
Find out more about the planets of the solar system here.
Definition Of A Dwarf Planet
According to the IAU, a celestial body is classified as a dwarf planet if it exhibits the following characteristics:
- It is in orbit around the Sun;
- It has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape;
- That It has not cleared the neighborhood around its orbit; and
- That It is not a satellite (a moon).
Find out more about the dwarf planets in the solar system here.
Pluto, along with the other trans-neptunian objects of a similar size discovered in the late half of the 20th Century, could not be considered the same as the terrestrial planets. The final nail in this coffin was the discovery of Eris in 2005. They are smaller and have not necessarily cleared the neighborhood around their orbit. Instead, they fit into a classification of their own, as ‘dwarf planets.’ This term was introduced by scientist Alan Stern as part of the new classification introduced in 2006.