Essay on Planets, Moons, Satellites, Asteroids, and Comets
John L. Hubisz, Ph.D., Hubisz@unity.ncsu.edu

I taught astronomy for over 20 years and, of course, included astronomical examples in my physics courses. In the light of recent stories, I would like to present my criteria for distinguishing the various astronomical objects in the title of this note for the introductory student. Keep in mind that word usage does change with new knowledge and that sometimes the original word becomes inappropriate.

Matter aggregates in two fundamental ways: chemically (electromagnetic force) and gravitationally (Newton’s inverse square force law of attraction.) As matter bumps, scrapes, collides, etc. with other pieces of matter, they become charged and oppositely charged bodies attract each other tending to make larger and larger chunks. As the mass (not the size or volume!) gets larger, the gravitational force of attraction tends to dominate and as long as there is matter around, it will accumulate on the more massive bodies. Lower massive bodies take on random shapes, held together by the chemical forces of attraction and to an extent, gravitational forces. The more massive the resulting body, the tendency is to take on the most efficient shape, a sphere.

For this reason, planets and moons are close to spherical. There are other effects that alter that a little bit. A planet is a body that revolves around or orbits a central star (or, perhaps, a cluster of stars.) With a large mass one would expect that there would be very little debris in the vicinity of the body. A moon is a body that revolves around or orbits a central planet. Now, there are other bodies revolving about planets and stars. These could all be called satellites, even the artificial ones that we have launched. Mars is often said to have two moons, but we now know that they are, simply, small irregular-shaped rocks. There are many, many of these irregular-shaped rocks wandering about the orbiting planets and the central star, some getting captured by planets (Mars has at least two and the outer planets have many.) Their masses are too small to take on a spherical shape or to even maintain any semblance of an atmosphere. We label most of these as asteroids. There is a large cluster of these asteroids orbiting our Sun in an approximate ellipse between Mars and Jupiter (the Asteroid Belt) and then there are tens of thousands of these, and probably more, orbiting our Sun.

Comets fit into the category of asteroids, but have frozen gases on their surfaces, which sublimate as they approach the Sun. Eventually, they lose all their gaseous material and become irregular-shaped rocks maintaining their orbits about the Sun.

What about Pluto? If it turns out not to be spherical, then it would not be a planet even though it has a satellite. (Its environment has not been “cleaned.”) Apparently an international body that considers these questions has determined that it is not massive enough, so they want to call it a “dwarf planet” along with asteroids like Ceres. Our Sun is a dwarf star and Earth is a “dwarf” when compared to the very massive Jupiter, Saturn, Uranus, and Neptune.

Is it worth discarding textbooks because Pluto is listed as a “planet?” No, don’t bother. The discussion of the situation with your children or students would be invaluable.