Sunday, March 15, 2009


DM-i-L asked what's the real difference between a dwarf planet and a planet.

The IAU specification's only real difference is that the dwarf planet has not cleared its orbit. Clearing an orbit refers to removal of all small debris and most large debris from the region through which the object passes on its orbit around the star. This removal involves incorporating all of that debris into the main body or gravitationally accelerating it out of the region.

Earth has cleared its orbit of all but a few large objects. First, of course is Luna. Second, there is at least one and probably several small objects in a horseshoe orbit around Earth. Other than that, the path that Earth takes around the sun is clear. The same is true for the other seven planets:
Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

The named dwarf planets, Ceres, Pluto, Eris (2003 UB313), Haumea, and Makemake (not pronounced as "make, make" but as "mocky mocky") all have other debris in their orbits.

But, what about Saturn and its dust and rings? Well, Saturn is gravitationally in control of all that junk; there's basically nothing in its orbit that isn't strongly affected by Saturn.

There are currently only eight planets in our solar system. There are hundreds of known extrasolar (out of our system) planets.


Grumpator said...

Poor Pluto.

deborah said...

Thanks for the answer. I now understand what a clear orbit means although I don't think it has anththing to do with the planet itself in clearing the debris? Right? The debris either goes out in space or is attracted to the planet itself and becomes part of the planet?