Over 50 dwarf planets and counting...

The fateful decision

In 2006, the International Astronomical Union (IAU) defined a planet as any object that (1) orbits the Sun directly, (2) is round, and (3) is gravitationally dominant within its orbit. Currently, the only objects in the solar system that make the cut are the eight classical planets—Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. For objects that satisfy the first two conditions, but not the third, the IAU established a new class of objects: dwarf planets. Objects that only satisfy the first requirement, such as asteroids and comets, were given the blanket classification of “small solar system body”—“small body” for short.

The new dwarf planet classification originally included the once-planets Pluto and Ceres, as well the newly discovered Eris, which we now know to be slightly smaller but more massive than Pluto. Within the next few years, two more dwarf planets were added to the mix: Haumea and Makemake.

One of the arguments for creating a separate class for dwarf planets was a practical one: If Pluto was a planet, then similarly-sized Eris would have to be as well. If Eris was a planet, what about an object that was eighty percent the size of Pluto? What about seventy percent? Sixty? Fifty? One solution—and one which was even briefly adopted—was to say that any object that was round and orbited the sun was a planet. The problem with this approach, according to some astronomers, was that this led to a solar system with dozens of planets—at least fifty, by estimates at the time, and possibly hundreds.

Ultimately, the IAU decided to distinguish between gravitationally dominant planets and dwarf planets, in part to allow the number of planets in the solar system to stay constant, while the number of dwarf planets could increase as they were discovered, much as the number of asteroids and other small bodies has done for hundreds of years.

A half-executed plan

The only problem—apart from public outrage at Pluto's “demotion”—was that there were some issues with how to implement the new dwarf planet classification. Because the IAU went ahead and started declaring official dwarf planets, they set themselves up as the official gatekeepers of dwarf planet status. But after starting off with three dwarf planets and later adding two more, something went wrong: They stopped adding new ones. It has now been the better part of a decade since Haumea and Makemake were given official dwarf planet status, and no new dwarf planets have been added since then. Bewilderingly, this isn't because there haven't been more dwarf planets discovered; it almost seems as if they've just moved on to other things.

A reasonable proposal

At the very least, it makes sense to assume that since Ceres is round and therefore a dwarf planet, anything larger than Ceres will be a dwarf planet as well. Simply using this criterion, Quaoar and Sedna, which were both discovered before 2006, and 2007 OR10, which was discovered in 2007—and is likely bigger than both Haumea and Makemake—should all be classified as dwarf planets.

To take things a step further, the best information we have tells us that icy objects in the solar system—like moons of the giant planets and objects out past Neptune—can be assumed to be round if they are larger than about 400 km across. The smallest known round object in the solar system is Saturn's moon Mimas, at 396 km across, but a few larger rocky bodies such as the asteroid Vesta (525 km) and Neptune's moon Proteus (420 km) are known not to be round. (Since the moons mentioned do not orbit the Sun directly, they cannot be considered dwarf planets, and are used here for comparison only.)

To determine the number of objects that currently should be classified as dwarf planets, we need to know how many of them are round. We can assume that Vesta is not round only because it is a rocky body, and rocky bodies require more gravitational force to be made round than icy bodies. (In fact, the best estimates put the lower roundness threshold for rocky bodies at about 800 km—twice the threshold of icy bodies.) Knowing this, we can use a very conservative lower threshold of 500 km to determine which icy objects (i.e., objects beyond Neptune) are likely to be round, and therefore likely to be dwarf planets. This threshold gives us the following results, with currently accepted dwarf planets starred:

Likely and official dwarf planets

From largest to smallest, with official dwarf planets marked with an asterisk (*)

Pluto*	2,374 km
Eris*	2,328 km
2007 OR10	1,535 km
Makemake*	1,447 km
Haumea*	1,403 km
Quaoar	1,046 km
Sedna	1,032 km
Ceres*	946 km
Salacia	883 km
2002 MS4	865 km
Orcus	828 km
2013 FY27	796 km
v774104	750 km
2005 UQ513	728 km
Varuna	721 km
2002 AW197	714 km
2014 UZ224	710 km
2015 RR245	708 km
Varda	694 km
2004 GV9	692 km
2003 AZ84	691 km
2005 RN43	685 km
2006 QH181	683 km
2002 UX25	683 km
Ixion	657 km
2007 JJ43	654 km
2010 KZ39	637 km
2001 UR163	636 km
2007 UK126	608 km
2010 RF43	607 km
Chaos	606 km
2003 UZ413	597 km
2008 ST291	591 km
2012 VP113	588 km
2002 TC302	588 km
2002 XW93	584 km
2005 RM43	580 km
2013 FZ27	575 km
2010 FX86	560 km
2010 RE64	560 km
2002 XV93	555 km
2014 UM33	540 km
2004 XR190	535 km
2003 VS2	522 km
2004 TY364	520 km
2010 VK201	520 km
2008 OG19	512 km
2014 FC69	504 km
2007 JH43	503 km
2003 QX113	503 km
2005 UQ513	500 km
2014 FT71	500 km

Even using this fairly conservative estimate gives us 52 known dwarf planets orbiting beyond Neptune, plus the asteroid dwarf planet Ceres, bringing the count of known dwarf planets up to 53. In addition to these, more dwarf planets are discovered all the time, including a new one announced just a few days ago. (It's 2014 UZ₂₂₄, which at 710 km across is well above our already-conservative threshold.)