December: Pluto – the Erstwhile Planet

December, 2008

Pluto – the Erstwhile Planet

By Calvin L. Chrisman

Everyone knows what a planet is, right? Well – maybe not. Fans of the former planet, Pluto, will remember its demotion to dwarf planet status by the International Astronomical Union (IAU) in 2006. This decision has resulted in a passionate debate that has continued up to the present. In a rapidly changing environment where more and more Pluto sized objects are being discovered, it is likely to continue for some time.

Pluto Source: NASA
Source: NASA

Pluto was discovered in 1930 by Clyde Tombaugh in a highly tedious process involving visual reviews of numerous photographic plates, searching for any small point of light that changed position from exposure to exposure. His search was inspired by a prediction by the astronomer, Percival Lowell. Lowell had calculated that the orbits of Neptune and Uranus were being disturbed by the gravitational effects of another planet beyond the orbit of Neptune. He had calculated where the unknown planet should be found and Tombaugh was searching that area. As it turned out, Lowell’s calculations showed disturbances in the orbits of Neptune and Uranus because of an incorrect estimate of the mass of those planets. Pluto was much too small to have any noticeable effect and Tombaugh had discovered it by accident.

When it was discovered, Pluto was thought to be approximately the size of Earth. Today, we know it is smaller than the Moon and is only about one percent of the size of Earth. Further complicating matters is the 1978 discovery of Pluto’s moon, Charon. Charon is about half the size of Pluto and, technically, does not orbit Pluto. They both orbit a point in space between them. Pluto, along with Ceres (a large round object in the asteroid belt), Eris (slightly bigger than Pluto and farther out from the Sun) and Makemake (yes, Makemake – beyond the orbit of Pluto and named after the Easter Island god of creation) have been defined as dwarf planets. Should Charon be a dwarf planet instead of a moon or should it be both? And you thought this would all be easy.

The most illogical part of the new IAU definition is that dwarf planets are not considered planets. Without getting into diagramming sentences, it looks like dwarf is an adjective modifying the noun planet. Doesn’t that make a dwarf planet a planet? Astronomers talk about dwarf stars as stars. Our sun is a dwarf star (bet you didn’t know that). Also, dwarf galaxies are galaxies. A meeting was held to discuss this issue as recently as this August with no resolution. The discussion is far from over.

Last month, we talked about the conjunction of Jupiter, Venus and the crescent moon at the end of November and the beginning of December. If you have been watching, you will have noticed how rapidly Jupiter is moving to the west. At the beginning of November, it was to the left (east) of Venus by about the distance of your hand spread out and held at arms length. In the beginning of December it is to the right (west) by a little more than the distance of your finger held at arms length. By the end of December, it will be twice the distance (two open hands at arms length) to the west of Venus that it was to the east two months before. Keep an eye on these planets as the month progresses. On December 21st (the beginning of winter), Jupiter will be joined by the planet Mercury in the west just before sunset. Initially, Mercury will be difficult to see because of the Sun’s glow, but by the 28th, it will be visible and close to Jupiter. The gap between them will narrow over the next few days as the crescent Moon passes through the three planets. It will be a beautiful sight.

Clear Skies

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