Dwarf Planets

What are Dwarf Planets?

Dwarf planet Pluto as seen by New Horizons on July 14, 2015. JHU/APL/New Horizons mission/NASA

You've probably heard all about the big kerfuffle in planetary science circles about the definition of "planet". Here's what happened: in 2006, there was quite a controversy when the International Astronomical Union decided that Pluto, long held as the ninth planet of the solar system, was to be demoted to be merely a "dwarf planet". As you might imagine, that decision has been the object of a lot of debate, particularly among planetary scientists who are best qualified to decide what a planet is and isnt.

The IAU decision did not reflect the planetary science community's opinions and expertise. 

What is a Dwarf Planet?

In most respects, dwarf planets have the same characteristics as all the other known planets. They are objects in orbit around the Sun that are massive enough that gravity has formed them into a spherical shape.

The primary different between dwarf planets and regular planets is that planets are said to have "cleared their orbital path of debris". This is an incredibly vague term and the primary source of all the controversy. However, under closer examination it becomes clear what the spirit of the condition is to impart.

Take the case of Pluto: it is actually one of many small bodies orbiting in the Kuiper Belt region of the outer solar system. At least a few of these objects are of similar size to Pluto. So, some scientists reasoned that if you are going to include one of them, Pluto, in the category of planet, then you would need to include them all.

Beyond that, you really have to examine the formation of these objects. Pluto, for example, started out life as a planetary building block. However, Neptune's gravity likely caused the planet to become unstable, ripping it apart into lots of smaller objects. Or, it's very likely that infant Pluto suffered a collision with another planetary building block, which led to the formation of its largest moon, Charon.

Other objects in the Kuiper Belt may well have gone through similar processes in the early solar system. 

They're all orbiting out beyond Pluto in the Kuiper Belt.  That is to say, Pluto is not alone in its orbit around the Sun, and since it has not the mass to pull the rest of that material together into a single object it is classified differently than the other worlds of our solar system, as a dwarf planet. That's still a planet, but a special class. 

Personally, I agree that objects like Pluto should be classified separately from the other eight planets. However, I don't much like the term dwarf planet; I think planetary remnant is more descriptive. It conveys the reality of Pluto's existence, that it was a planetary building block. But, that's my opinion, and not necessarily shared by planetary scientists.

Are There Other Dwarf Planets, Besides Pluto, in our Solar System?

There are several objects listed as dwarf planets in our solar system. Among them are : Ceres, Pluto, Haumea, Makemake, and Eris.

Eris was once believed to be larger than Pluto, which is what sparked the discussion of planet definitions in the first place, but was recently determined to be smaller by a tiny amount.

Charon, officially considered a moon of Pluto, is sometimes mentioned as a dwarf planet since it is of similar size to Pluto. This makes some sense because Charon is of similar size (though still noticeably smaller) than Pluto. Therefore, they both orbit a point between them, rather than Charon orbiting Pluto in the traditional planet-moon configuration.

For now, however, Charon is generally left out of the discussion of dwarf planets.

Updated and edited by Carolyn Collins Petersen.