| 1) In ancient times, Greek astronomers
noticed that there were certain
lights in the night sky that seemed to move across the sky in relation
to the stars. The Greeks called these moving lights ‘planetes’ a Greek
term meaning a wanderer. From this comes our word planet. As the
science of astronomy became more sophisticated it was noticed that
there were nine such planets in our solar system. They were named
Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and
Pluto. 2) Historically there had been no formal definition of a planet and the discovery of other large celestial bodies in our solar system, most notably Ceres, Sedna and Eris, led to a debate over what exactly a planet is. 3) In 2006, the International Astronomical Union decided on a three-point definition of a planet within our solar system. It must be a celestial body that is, firstly, in orbit around the Sun. Secondly, it must have sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium. In other words, it must be big enough and have a strong enough gravity to give it a round shape. Finally, it must have cleared the neighborhood around its orbit. This means that its size and gravitational pull must be great enough to have pulled in any smaller objects that it encounters on its orbit around the Sun. This does not include any of its own satellites such as Earth’s moon. 4) A new category, named a dwarf planet, was introduced to describe those large celestial bodies that satisfied the first two points of the definition decided in 2006, but not the third. 5) Back in 1930, a new planet had been discovered at the edge of our solar system. It was named Pluto, after the Roman god of the underworld. Pluto is small, in fact smaller than our own Earth’s moon, and has a diameter of only 2,300 km. 6) For many years, Pluto was thought of as the farthest known planet from the Sun and the ninth planet in our solar system. However, as many other celestial bodies of a similar size were discovered, scientists had to decide whether to include these newly discovered celestial bodies as new planets. In 2006, when scientists came up with their three point definition of a planet, it was recognized that Pluto fulfilled the first two points, but did not fulfill the third. The third stated that a planet had to dominate and clear the neighborhood of its orbit. Scientists identified two areas in which Pluto did not do this. Firstly, Pluto’s main moon, Charon is about half the size of Pluto and true planets are always much, much bigger than their moons. Also, celestial bodies that dominate and clear their neighborhoods ‘sweep up’ asteroids, comets and other pieces of debris that lie in the path of their orbit. Pluto doesn’t do this and has a rather untidy orbital path. It was, therefore, decided that Pluto was not a planet and was then demoted to the status of a dwarf planet. 7) It seems that the heavens are changing not only above our heads but in the minds of men. The 2006 decision of the International Astronomical Union has reduced the number of planets in our solar system from the long accepted total of nine to the currently accepted figure of eight. It is hard not to think of Pluto as a planet, as most of us have all our lives, but the experts have decreed it is not. |