Dwarf Planets: What Are They?

What are Dwarf Planets?

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

Dwarf planets came to public attention in 2006, when a controversy erupted over the re-classification of Pluto as "not a planet but a dwarf planet". That's when the International Astronomical Union decided that Pluto, long held as the ninth planet of the solar system, was to be demoted. The decision was more of a bookkeeping classification, rather than something based on science. It also prompted a lot of debate, particularly among planetary scientists who is best qualified to decide what a planet is and isn't. The IAU decision did not reflect the planetary science community's opinions and expertise and many suggest that the organization overstepped its bounds completely.

Of course, the decision, which was not made nor heralded by the planetary science community, caused an uproar among scientists. It continues today, even as planetary scientists are finding more of these little worlds in the outer solar system. Thus, it's important to continue refining the definition of dwarf planet because these little worlds hold clues to the solar system's early history.

What is a Dwarf Planet?

In the interests of science and not IAU politics, it's best to look at what a dwarf planet is from the viewpoint of planetary science. In most respects, dwarf planets have the same characteristics as all the other known planets. They are objects in orbit around the Sun and are massive enough that their own self-gravity has formed them into a spherical shape.

The only difference in the definition between dwarf planets and the planets is that so-called "real" planets are said to have "cleared their orbital path of debris". This is an incredibly vague requirement and the primary source of all the controversy. Taken strictly, one could say that any planet with any debris in its path is a dwarf planet. Earth could qualify. So could Jupiter. So, that part of the definition is going to need some extended work.

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. There's some evidence that an even larger world may exist "out there". So, some people decided that astronomers were going to include one of them, Pluto, then they'd need to include them all. Not that this is a problem. A star can have as many planets as it can support, and it doesn't matter what people think is or isn't a planet. What's important is to set an understandable, clear definition of planet that takes into account more than its size and orbit and how much debris is in its orbit.

Factors in Planet Building

It's important to examine the formation histories of worlds. Pluto, for example, started out life as a planetary building block, called a planetesimal. Actually, all the planets did. However, Neptune's gravity likely caused that baby planet to become unstable, ripping it apart into lots of smaller objects. Or, it's possible 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. So, Pluto is not alone in its orbit around the Sun. It didn't have the mass to pull all the material in its orbit together into a single object, although it's not clear why that is a requirement. In any case, astronomers (and not necessarily planetary scientists) classified it differently than the other worlds of our solar system, as a dwarf planet. That's still a planet, but a special class. 

Are There Other Dwarf Planets?

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

Artist view of dwarf planet Eris.
An artist's idea of what dwarf planet might look like. ESO/L. Calcada

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. Time and more observations and studies of smaller worlds may well provide planetary scientists (who ARE the experts in the field) with the knowledge they need to help characterize these worlds as planets in their own right. For now, however, they are still known as "dwarf planets."

Updated and edited by Carolyn Collins Petersen.