The Dwarf Planets

One might think when hearing the term “dwarf planet”, that the object in question is simply a small planet, a subset of the planet category. This, in fact, is not the case, as the term dwarf planet, recently coined, is a separate category all together. While there are many objects that are good candidates for dwarf planet status there are in fact only three such classified objects: Ceres, Eris, and former planet Pluto.

It’s All in a Name

In 2006 Pluto stopped being a planet, ending a 76 year run as the ninth planet in the solar system. The reason for this change started with the discovery of Eris, a planet-like object that was larger than Pluto and was at first thought to be the tenth planet of the solar system. This lead the IAU (International Astronomical Union, the body responsible for naming astronomical objects) to do what had yet to be done in hundreds of years- lay out the ground work and define the term “Planet.”

As they defined planet and discovered that Eris did not fit the criteria, they also realised that neither did Pluto. This required them to come up with a separate category for objects like Eris, Pluto, and the soon after discovered Ceres. A dwarf planet category was created, and to qualify for dwarf planet status, an object must meet the following four criteria:

• The object must be in orbit around the Sun
• The object must have enough mass to attain hydrostatic equilibrium
• The object must have “cleared the neighbourhood”
• The object must not be a satellite

These criteria mean that the term dwarf planet can only apply to objects in our own solar system (#1, orbiting the Sun), must have enough gravity to attain a roughly spherical shape (#2, hydrostatic equilibrium) must have enough gravity and influence to have cleared it’s own orbit of similar sized objects (#3, meaning that if it shares orbit with objects larger than an asteroid it can’t be a planet) and can’t be another planet’s moon (#4).

There are a lot of objects in the solar system that might, one day, be classified as dwarf planets once the proper information is gathered. For example, three of the largest objects in the asteroid belt, Vesta, Pallas and Hygiea, might achieve that classification once it can be determined if they have achieved hydrostatic equilibrium, as they meet the other three criteria.

What Dwarf Planet Does Not Define?

The four criteria from the IAU for dwarf planets leaves quite a lot of room for variance. The size and mass, for example, of a dwarf planet is completely undefined (interestingly enough for something labelled “dwarf”). This means that an object discovered with a size equal to or greater than Earth one day that has not cleared its orbit of smaller objects would achieve the dwarf planet distinction.

Final Thoughts

Dwarf planet—and planet, for that matter—in the current definition leave out all the objects outside of the solar system. Currently astronomers are using the distinctions of dwarf planet, planet, and “solar system object” (which meet the dwarf planet criteria 1, 3 and 4 but have not achieved hydrostatic equilibrium) to apply to extra solar objects, but that may change as the IAU continues to meet and discus astronomical nomenclature.