What Is a Dwarf Planet?

What is a dwarf planet?

When the final vote on the definition of "planet" was made, and the eight dominant bodies in the solar system were declared (quite rationally) a class separate from the others, a new class of objects was defined. The "dwarf planets" are all of those objects which are not one of the eight dominant bodies (Mercury through Neptune) yet still, at least in one way, resemble a planet. The best definition heard so far is that a dwarf planet is something that looks like a planet, but is not a planet. The dwarf planets are bodies in the solar system which are large enough to become round due to their own gravitational attraction. Why do astronomers care about round? If you place a boulder in space it will just stay whatever irregular shape it is. If you add more boulders to it you can still have an irregular pile. But if you add enough boulders to the pile they will eventually pull themselves into a round shape. This transition from irregularly shaped to round objects is important in the solar system, and, in some ways, marks the transition from an object without and with interesting geological and planetary processes occurring (there are many many other transitions that are equally important, however, a fact that tends to be ignored in these discussions).

How many dwarf planets are there?

There are two ways to look at this question. First is to ask: according to the official list kept by the IAU, how many dwarf planets are there? The current answer (as of 1 April 2007) is only three. The asteroid Ceres is the only asteroid known to be round. Pluto is also considered a dwarf planet. And Eris, a little larger than Pluto, is a dwarf planet also. Three is all the IAU will currently deal with.

Reality, however, does not pay much attention to official lists kept by the IAU or by anyone else. A more interesting question to ask is: how many round objects are there in the solar system that are not planets? These are, by the definition, dwarf planets, whether or not they ever make it to any officially sanctioned list. If the category of dwarf planet is important, then it is the reality that is important, not the official list.

So how many dwarf planets are there? Ceres is still the only asteroid that is known to be round. After that it gets complicated. All of the rest of the new dwarf planets are in the distant region of the Kuiper belt, where we can't actually see them well enough to know for sure if they are round or not.

While we can't see most of the objects in the Kuiper belt well enough to determine whether they are round or not, we can estimate how big an object has to be before it becomes round and therefore how many objects in the Kuiper belt are likely round. In the asteroid belt Ceres, with a diameter of 900 km, is the only object large enough to be round, so somewhere around 900 km is a good cutoff for rocky bodies like asteroids. Kuiper belt objects have a lot of ice in their interiors, though. Ice is not as hard as rock, so it less easily withstands the force of gravity, and it takes less force to make an ice ball round. The best estimate for how big an icy body needs to be to become round comes from looking at icy satellites of the giant planets. The smallest body that is generally round is Saturn's satellite Mimas, which has a diameter of about 400 km. Several satellites which have diameters around 200 km are not round. So somewhere between 200 and 400 km an icy body becomes round. Objects with more ice will become round at smaller sizes while those with less rock might be bigger. We will take 400 km as a reasonable lower limit and assume that anything larger than 400 km in the Kuiper belt is round, and thus a dwarf planet.

How many objects larger than 400 km are there in the Kuiper belt? We can't answer this question precisely, because we don't know the sizes of more than a handful of Kuiper belt objects (for an explanation why, see the discussion on the size of Eris), but, again, we can make a reasonable guess. If we assume that the typical small Kuiper belt object reflects 10% of the sunlight that hits its surface we know how bright a 400 km object would be in the Kuiper belt. As of late August 2006,  44 objects this size or larger in the Kuiper belt (including, of course, Eris and Pluto), and one (Sedna) in the region beyond the Kuiper belt. In addition our large ongoing Palomar survey has detected approximately 30 more objects of this size which are currently undergoing detailed study.

We have not yet completed our survey of the Kuiper belt. Our best estimate is that a complete survey of the Kuiper belt would double this number.

For now, the number of known objects in the solar system which are likely to be round is 53, with the number jumping to 80 when the objects from our survey are announced, and to ~200 when the Kuiper belt is fully surveyed.

Beyond the Kuiper belt there may be even more dwarf planets than in the Kuiper belt. Our best guess is that the region where Sedna resides could contain another ~2000 dwarf planets.

The  new dwarf planets in the solar system are very different from the previous 8 planets. Most are so small that they are smaller across than the distance from Los Angeles to San Francisco. They are so small that about 30,000 of them could fit inside the earth.

What does the new solar system look like?

Here is the before (black circles with nine planets) and after (add red ellipses to show the dwarf planets) views of the solar system:

(images may be freely used; credit: Mike Brown, Caltech)

In order from closest to furthest, the dwarf planets are shown below (with the planets, shown for comparison, in blue). A few of them have been studied well enough that we know interesting details. Click the links to learn more.......

Who has discovered dwarf planets?

Many people have found dwarf planets, starting with the discovery of Ceres by Piazzo in 1801, continuing with Pluto by Tombaugh in 1930, and the moving to the discovery of the many many objects in the Kuiper belt beginning in 1992. For the likely dwarf planets listed above, here is the list of discoverers (some planets were discovered by teams which change a bit, in general the team name or the team leader is listed. Sometimes no discoverer is recorded):

• Brown et al.:  15 dwarf planets
• Deep Ecliptic Survey: 8 dwarf planets
• Near Earth Asteroid Tracking Survey: 4 dwarf planets
• Spacewatch: 4 dwarf planets
• Jewitt et al.: 3 dwarf planets
• Rupenstein, Ferrin, Danzl, Roe, Luu, Piazzo, Tombaugh:  1 dwarf planet each