How many dwarf planets are there in the outer solar system? (updates daily)

(As of 1 Nov 2013 also includes latest thermal and occultation results)

As of Mon Feb 24 2020there are:10 objects which are nearly certainly dwarf planets,27 objects which are highly likely to be dwarf planets,68 objects which are likely to be dwarf planets,130 objects which are probably dwarf planets, and741 objects which are possibly dwarf planets.

In 2006, when the 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. In other words, a dwarf planet is something that looks like a planet, but is not a planet. Specifically this means that 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 occuring (there are many many other transitions that are equally important, however, a fact that tends to be overlooked in these discussions).

How many dwarf planets are there? Ceres is 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. Most 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 do we know in the Kuiper belt that are 400 km or larger? That question is harder to answer, because we don't actually know how big most of the objects in the Kuiper belt are. While we can see how bright there are, we don't know if they are bright because they are larger or are highly reflective. In the past, we had to just throw our hands up in the air and say we don't know enough to even make reasonable guesses. But in the past few years, systematic measurements of the sizes of objects from the Spitzer Space Telescope and now the Herschel Space Telescope have taught us enought that we can make some reasonable estimates of how reflective objects are. (It's complicated: read the details here ) These reasonable estimates, combined with all available actually measurements, give us the list of the largest Kuiper belt objects, sorted by diameter, below. Carefully note the lack of any error bars. Every single measurement or estimate below is uncertain to some extent or another. I don't include the individual uncertainties in the table, but instead use the ensemble uncertainties to inform classification below. In other words: take the sizes of specific objects with bigger or smaller grains of salt.

I subjectively divide this list into a few categories, taking into account both the uncertainties in the sizes and the uncertainties in the size where an object becomes round.

Near certainty: We are confident enough in the size estimate to know that each one of these must be a dwarf planet even if predominantly rocky.

Highly likely Anything larger than 600 km is all but certainly round. Even objects significantly smaller are likely round. The predicted and/or measured size of an object in this category would have to be grossly in error or the composition would have to be primarily rocky in order for it not to be a dwarf planet.

Likely: Anything icy larger than 500 km is highly likely to be round. But the size uncertainties are large enough that some of these objects could, in reality, be small enough to be less certain.

Probably: All icy satellite larger than 400 km are round, so we expect these objects to be round if the size estimate is correct.

Possibly: We don't know where the transition from non-round to round occurs, but in icy satellites it is between 200 and 400 km. Objects this size in the Kuiper belt could thus possible be round, but we don't know. Probably not: Below 200 km no icy satellite are round. We expect the same in the Kuiper belt. A few of these object could be bigger than expected, however, and could turn out to be large enough to round themselves.



The table also lists the estimated albedo used to determine the size or the calculated albedo from the measured size. Also listed is the absolute magnitude, which in this case refers to how bright the object would be if you were looking at it while you were standing on the surface of the sun and the object were at the distance of the earth. As in the rest of astronomy, smaller magnitudes are brighter and every 5 magnitudes represents a factor of 100. In the comments section I list the source of the size or albedo as described here and add subjective comments when required.

As of Mon Feb 24 2020

there are:

10 objects which are nearly certainly dwarf planets,

27 objects which are highly likely to be dwarf planets,

68 objects which are likely to be dwarf planets,

130 objects which are probably dwarf planets, and

741 objects which are possibly dwarf planets.



