The Planet Quiz Show

Astronomers had never officially defined what a planet was. In August of 2006 the International Astronomical Union, the world governing body for astronomers, finally tackled the problem, and Pluto did not fit the final definition. Astronomers said that a planet in our solar system

    a.  had to be round.
    b.  had to go around the sun.
    c.  could not go around another planet.
    d.  had to clear its orbital space.

It was the last part of the definition that caused Pluto to be dropped from the listing as an official planet of our solar system. Although Pluto meets the first three parts of the definition of a planet, it doesn’t have enough mass (matter) to possess a sufficient amount of gravity to pull the debris found in its orbital path into itself. Although today, there are nearly 1000 objects that have the potential of striking Earth, this is a small number compared to the billions of objects that lay in the path of the Earth when it was a young planet. During the history of the past five billion years, Earth’s gravity has pulled virtually all of the rocks, boulders, and asteroids that lay in Earth’s path onto itself, as have the other seven planets of our solar system. Objects that meet the first three parts of the definition are now called Dwarf Planets, while even smaller bodies are called by the general name of Small Solar System Bodies. The term comets, asteroids, meteoroids still exist to understand what type of small solar system body is being discussed.

Even when Pluto was a planet of our solar system, it was by far the oddest member. Pluto goes around the sun in the most oval-shaped path (1) that takes it high above and below the other planets of our solar system (2). It is equally composed of rock and ice, which makes it different from any of the other planets (3). Pluto was by far the tiniest (4) of all the planets, much smaller than Earth’s moon. The strangeness of Pluto led some astronomers to believe that Pluto should be reclassified as a Kuiper Belt object because the characteristics of Kuiper Belt objects fit Pluto’s characteristics very well. Kuiper Belt members represent objects composed of rock and ice that never made it to planetary status. Their bits and pieces were too far away from each other to collide and form a larger planet. They are found orbiting the sun at distances between 30 and 100 times the Earth-sun distance and as of 2008, there were over 1000 of them.

Two new moons were discovered by the Hubble Space Telescope to be orbiting the dwarf planet Pluto. Pluto now has three moons.

Only Eris (formerly 2003UB313) and Pluto, along with the asteroid Ceres, have been designated dwarf planets by the International Astronomical Union. Here is a size comparison of the largest Trans-Neptunian Object (TNO's). Notice the size of the Earth at the bottom of the picture.