Here There Be Dwarves

I’ve been looking into this ‘Dwarf Planet’ issue. I was not happy to have Pluto no longer be a ‘real planet’, but decided maybe it made sense to have another classification for those items in our system that did not quite fit the definition of ‘planet’. Of course, they devised that definition so that it would exclude Pluto, because another body they did not want to consider as a planet was roughly the same size as Pluto. Or so it seems.

Still, it works, as long as they use the definition of ‘dwarf planet’ consistently, right? But, are they?

Mercury is a planet (now called a Classic Planet). It’s about twice the size of Pluto, so... They had to put the dividing line somewhere, right?

Eris is further out than Pluto, and may or may not be as large. The diameters for these two said Pluto was slightly larger, but each had a margin of error. So, if Eris is larger than they think, and Pluto is smaller than they think, then they could be the same size, or Pluto could be slightly smaller. Ergo, if one is a dwarf planet, it makes sense that the other is also.

I had never heard of Makemake or Haumea until I started this research, but they have both been named dwarf planets. Their diameters are each a bit more than half of Pluto’s.

One rule of the definition of a dwarf planet is that it must have enough mass to pull itself into a roughly spheroid shape. It has been postulated that these bodies would need a diameter of at least 400 km before that was likely. I was surprised to read, then, that Haumea was not a sphere - its diameter through the equator is much longer than its diameter through the poles. The explanation (excuse?) was that Haumea rotates so fast, it has warped into a flattened shape.

So far, Ceres is the only asteroid to be named a dwarf planet. Its diameter is less than 1,000 km, but it is the largest asteroid. There are at least 3 other asteroids that are over that minimum diameter of 400 km, and so far as I have been able to discover, they are at least roughly spherical.

There are also 10 or more heavily bodies beyond the orbit of Pluto that are currently under consideration for being Dwarf Planets.

And that brings us to Charon. Long known as - and even now considered - Pluto’s moon, I suggest it be named a Dwarf Planet. Charon’s diameter is slightly less than half of Pluto’s, considerably larger than Ceres’. Yes, there are plenty of big moons in our system, but none of them are so large in relation to the planet they circle. And, technically, Charon does NOT revolve around Pluto. With this division of mass between them, both Charon and Pluto revolve around a point that is between them.

At least one astronomer has suggested that Pluto and Charon be considered a double Dwarf Planet. I definitely agree.

Name	Diameter (km)	# Moons
Mercury	4,879	0
Pluto	2,368	5
Eris	2,326	1
Makemake	1,430	0
Haumea*	1,300	2
2007 OR10	1,280
Charon	1,207
Sedna	1,000
Orcus	946
Ceres	942	0
2002 MS4	934
2005 UQ513	920
2007 UK126	880
Salacia	854
Quaoar	844
Ixion	804
Varuna	700
Pallas	544
Vesta	525
Hygiea	431

* NOT spherical