Saturday, August 19, 2006

Planetary diversity

In our solar system...
1. There are rocky planets and gas planets... The difference between a gas planet and a rocky planet is huge, yet they are all called "planets".
2. The difference between a gas planet and a rocky planet is "scientific" (not arbitrary) just like the difference between a solid and a gas is scientific.
2. "Historic planets" have been found to orbit the Sun following a plan called the ecliptic.
3. Kuiper Belt Objects have been defined as such for having a non-regular orbit around the sun... for being relatively small (compared to the historic planets) and for being composed of ice and rocks (like comets?).
3. There are satellites that are bigger than Mercury.
4. There is at least one "Kuiper Belt Object" that is bigger than Pluto.
5. There are some big "asteroids" large enough to be round-shaped like a planet.
6. There are quite a few "Kuiper Belt Objects" that are much more massive than Ceres but they are still less massive than Pluto.
7. There are no reasons to believe that new larger KBOs will not be discovered. The possibility that an unknown KBO is even larger than Mercury does exist.

Among all the extrasolar planets that we know...
1. There are no evidence that they orbit their star in a regular way.
2. There is at least one planet discovered without orbiting a star.
3. There are no evidence that a satellite of an extrasolar (gas) planet is not as big as the Earth.

Among the stars...
1. There many classes of stars based on their size and their longevity but they are all called stars.
2. Brown dwarfs are actually not big enough for thermonuclear fusion. There are no such clear-cut distinction to be made between planets bigger than Mercury and the smaller... objects.

Conclusion: If we want to set up a long-term standard, there are no other solutions than giving up the previous arbitrary definitions set up in terms of orbit and the size of "historic" planets. We need a gravity-based definition. I understand the frustrations about the issue of planets not being satellites, and satellites being smaller than planets. If we are serious about sorting out the controversy, then we should call all round satellites "planets".
Roundness can be defined in terms of a necessary mass and density for gravity to shape a round object. Excluding all new round objects from joining the club is not a solution.



Post a Comment

<< Home