Is Pluto a planet?

Staetements from

  1. The American Museum of Natural History
  2. Phil Plait, the Bad Astronomer
  3. Nine Planets and Counting

A Geologist Surveys the Solar System

I received the following letter in the wake of the IAU’s ill-fated redefinition of the word “planet”. The north wall of Troster now has little signs explaining which orbit belongs to which planet or dwarf object. Pollux has a planet.

August 17, 2006. Rather than counting how many planets are dancing on the head of this particular stellar pin, let’s start here.

There are several families of objects in the Solar System.

  1. Sun. Category of one. A star, and the lynchpin of the whole kit and kaboodle. Without the gravity of the Sun, there is no Solar System.
  2. Terrestrial planets. Four of these. Two are nearly identical in size. These larger two have been able to retain heat from their formation and produce some heat due to radioactive decay in their cores, thus exhibiting active volcanism. The other two are smaller; the slightly bigger one with very little geological activity, the smallest with no geological activity that we know of.
  3. Asteroids. Amalgams of rock, dust, and ices. The ices are stable. A few of these are round, but most are small enough that thay take on all kinds of bizarre shapes—dog bones, peanuts, shoes, half-eaten wedges of cheesecake. These are divided into several families. One type of family describes their present orbit/position; like the Trojan asteroids, two “camps” of which which follow and preceed Jupiter around the Sun. (Earth merely has “Trojan dust.”) Saturn, and probably Uranus and Neptune, also seen to have these asteroidal hangers-on. Another type of family is connected by their purported origin; by analyzing the composition and orbital parameters of many asteroids, it seems that there may have originally been several larger bodies which were shattered in collisions.
  4. Giant planets. Serious-a** planets, yo. Four of these. Two are similar in composition to the Sun, and considerably larger than anything else in the Solar System, both ruling over complex systems of moons and rings, with powerful magnetic fields. Slightly smaller are the “Ice Giants,” with substantial amounts of methane, water, and ammonia, cores which are proportionally much larger, and weird, off-center magnetic fields.
  5. Kuiper Belt. Many of these objects are larger than most asteroids, and contain more ices, but otherwise may be similar in origin and structure to asteroids. The “Kuiper Belt” is the area which starts a little ways inside Neptune’s orbit and extends out to about 50 times the distance of the Earth from the Sun. In this formulation, Sedna is a KBO, I guess.
  6. Comets. Comets are similar in structure and size to asteroids and in composition to Kuiper belt objects, but they have orbits which bring them close enough to the Sun that their ices “boil off” and form a tenuous cloud. If they get close enough to the sun, they can even form tails, as so much material boils off that it is swept away into space in a long stream. Most comets are thought to come from the Oort cloud, a swarm of comets residing as much as half a light-year from the Sun.