Defining a Planet
What’s the definition of a planet? The International Astronomical Union is meeting to answer that question.
Aug 16, 2006
The International Astronomical Union, currently meeting in Prague, came out on Wednesday with a proposed definition of the word planet. The definition reads: "A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet." Basically, if it's round and orbits the sun, it's a planet.
This definition would save Pluto from being outcast to the "small solar system bodies" category. But not only would it leave Pluto as a planet, it would incorporate its moon Charon (pronounced "Karen"). Because Pluto and Charon orbit a common center, they would be considered a double planet. There are already double stars, systems also known as binary stars that orbit a common center of mass.
So with the addition of Charon, we are up to 10 planets. But there are more. The asteroid-belt member known as Ceres is round and orbits the sun. Therefore, it would be considered a planet also. It is the largest asteroid and was the first to be discovered.
And beyond Pluto at least one more planet is waiting, possibly a hundred more. 2003 UB313, with the more friendly nickname of Xena, is round and orbits the sun. It is one of hundreds of rocky bodies in the Kuiper belt, a ring of objects that stretches from just beyond Neptune, at 30 AU, out to 50 AU from the sun.
Having such a large number of possible planets and seeing that there are vast differences between these planets would necessitate subcategories within "planet". And there are two which already exist. The four planets closest to the sun are smaller rocky bodies called "terrestrial" planets. Terrestrial comes from Terra, which relates to the Earth. So we have a class of planets that are Earth-like.
The next class of planets is the gas giant planets and consists of Jupiter, Saturn, Uranus, and Neptune. They all have similar characteristics in location, size, composition, and appearance, which make it especially easy for the public to grasp.
But then there is Pluto. Which never really fit in in the first place. If the new definition is approved, it would be the best known representative of a subcategory of Plutons. Plutons would be those objects, such as Xena, which fit the definition of planet but reside far out in the icy realm of the Kuiper Belt.
Pluto would also be considered a dwarf planet. Dwarf planets are yet another subset of planet to accommodate objects smaller than terrestrial planets such as the asteroid Ceres, and possibly other asteroid-belt objects which may prove to be round.
And let's not even start with trying to explain how these definitions have to fit not only our solar system but the many extrasolar systems being discovered across the galaxy. Save that for another day when the definition is official.
Sound messy? It is. But it is the nature of astronomy. Astronomical objects don't want to be put in boxes and labeled. They are part of a continuum. Stars range from massive to tiny, sometimes orbiting each other while orbiting the center of their galaxy. Galaxies themselves can be dwarfs or giant spirals or ellipticals, orbiting each other heading toward a collision or free-floating in space. And therefore planets range the gamut from large to small, orbiting each other and then sun, all playing their own roles in the dance of the universe.
Read about the IAU's proposed definition of planet.
