Is Sedna a planet?
No, at least not by our definition. Astronomers have been unable to agree on a precise definition of planet, but we have a suggestion for a definition below. By our definition, Sedna is not a planet. Nor is Pluto. What is the definition of a planet? It is difficult for scientists to have to define a word that everybody thought they already knew the meaning of. But discoveries such as Sedna, Quaoar, 2004 DW are blurring the line between planets, asteroids, and comets. These objects are all big, so what are they? We prefer to call them planetoids. To us, a planetoid is any round object in the solar system that is not big enough to be considered a planet (actually we don’t know that any of these objects are round, but it is a reasonable assumption). So what is a planet? We define a planet to be any body in the solar system that is more massive than the total mass of all of the other bodies in a similar orbit. For example, many asteroids cross the orbit of the Earth. Yet the Earth is more
