Ask Dr. SETI ®
by Dr. John Boardman
from the November, 2015 issue of Dagon,
used by permission
About a decade ago, the International Astronomical Union re-classified the planets and smaller objects which orbit the Sun. A new category was established, "dwarf planets", apparently objects which revolve around the Sun, and are smaller than planets and larger than asteroids. The dividing lines among the categories "planet", "dwarf planet", "asteroid", and other such items can be established by comparing their mean radii. Mercury, the smallest planet, has a radius of 1,516 miles. (The current World Almanac is the most useful reference book I own, but it has not yet caught up with the metric system.) Eris, the largest and most distant dwarf planet thus far discovered, has a radius of 925 miles. Ceres, the first discovered and smallest known dwarf planet, has a radius of 294 miles. (For, comparison, Earth's Moon has a mean radius of 1,079 miles.)
I am not certain that Ceres should be classified with the dwarf planets. All the other known dwarf planets, including Pluto, the first of them to be discovered, are out at the fringes of the Solar System, beyond the orbit of Neptune. But Ceres is in the Asteroid Belt, between the orbits of Mars and Jupiter. These rocky objects probably never collected into a single planet because of tides raised in their material by the gravitational effect of Jupiter, which is more massive than all the other planets combined. But the dwarf planets out at the edge of the Solar System are composed of much lighter material, mainly frozen light gases. (The mean temperature. of Pluto is about 50° K, which means that the "snow banks" detected by the New Horizons probe could probably be made of methane or ammonia. The temperature 50° K is the same as -223° C or -370° F.) Since it has the same origin and composition as the other members of the Asteroid Belt, I would prefer to see Ceres regarded not as the smallest dwarf planet but as the largest asteroid.
When the International Astronomical Union "demoted" Pluto from a planet to a dwarf planet, a lot of people seemed to be upset by it. But its great differences from the planets, and its similarity to the other dwarf planets on the outer fringes of the Solar System, compelled the change. I suspect that these objections might be based on the notorious pseudo-science of astrology. The casters of horoscopes have had 85 years to work out the alleged influences of Pluto on their clients, and now they must either abandon it, or decide what are the influences of the other dwarf planets Haumea, Makemake, and Eris.
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