Back in My Day, We Had 9 planets

tee shirt snorgtees funny planets pluto 3

Well the Solar System I knew back then had 9 planets (Pluto was still a planet then) and of the moons Earth had one, Mars two, Jupiter 12, Saturn 9, Uranus 5 and Neptune 2 , totaling 31 (Jupiter and Saturn alone now have almost that total number each). Then in 2006 this changed, Pluto got demoted and now, as this tee shirt suggest: there are only eight planets.

To understand why Pluto isn’t a planet, perhaps it helps if we define what the term planet means. The expression goes back to ancient times and comes from the Greek word for “wandering star.” Because the planets are bodies that move in orbit around the sun, they do not stay in a fixed location in the sky in relation to the stars. 

Ancient peoples, including the Greeks, noticed that over a course of months these special stars seemed to move along a particular path across the sky. Usually it was in one direction, but occasionally they would move backwards, then again forwards. The constellations (or star groups) that they passed through were given special significance, and today we know these as the zodiac.

In his Timaeus (360 BC) Plato mentions, “The Sun and Moon and five other stars, which are called the planets”. His student Aristotle makes a similar distinction in his On the Heavens: “The movements of the sun and moon are fewer than those of some of the planets”. And Hyginus explicitly mentions “the five stars which many have called wandering, and which the Greeks call Planeta.”

By the end of the 19th century the word planet had become a working term applied only to a small set of objects in the Solar System. After 1992, however, astronomers began to discover many additional objects beyond the orbit of Neptune, as well as hundreds of objects orbiting other stars. These discoveries not only increased the number of potential planets, but also expanded their variety and peculiarity. Some were nearly large enough to be stars, while others were smaller than Earth’s moon. These discoveries challenged long-perceived notions of what a planet could be. But after the discovery of Eris, it forced International Astronomical Union to act on a definition. In October 2005, a group of 19 IAU members narrowed their choices to a shortlist of three, using approval voting. The definitions were:

A planet is any object in orbit around the Sun with a diameter greater than 2000 km.
A planet is any object in orbit around the Sun whose shape is stable due to its own gravity.
A planet is any object in orbit around the Sun that is dominant in its immediate neighborhood.

It essentially created a medial classification between planet and rock (or small Solar System body), called dwarf planet and placed Pluto in it, along with Ceres and Eris.

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