07 February 2005
Moons and Planets
Planets, moons, asteroids. What determines our definition of each? With the discovery of Pluto in 1930, this question began to emerge in the planetary science field of astronomy. As the image displays, Pluto is much smaller than Earths moon, which itself is much larger than Saturns largest moon Titan (current research at the ESA Cassini website. Further, Pluto has a moon of similar proportions, Charon with a equatorial diameter about half of what it has.
Do we define a moon as something that orbits a body that orbits a star? Does this apply to irregularly shaped satellites such as the 2 minor moons of mars (each is only a couple km across). And though we have no record of them currently what about when we find something orbiting a moon? will there be a new classification for those celestial bodies? Does phrasing things in a question answer anything?
Perhaps an enhanced definition is in order here. As anything with sufficient mass will be round, perhaps this series of bodies is best known as Planetoids, with seperate classifications for composition and what it orbits. So as to not close the loop, so to speak, lets include larger and smaller objects in the following sequence (note the names and classifications are rough drafts):
Clusters and Superclusters of Galaxies (groupings of galaxies)
Galaxies (huge collections of star systems)
Stars (white dwarves to red giants and all main sequence stars on the HR Diagram)
'brown dwarves' (sort of like a combination between Jupiter and a weak, small star), Jovian (jupiter, saturn, etc., massive gaseous planets),
Terrestrial planets (earth, venus, etc., smaller, hard crusted planets),
Moons (round, terrestrial bodies that orbit anything of a higher order),
Comets (dirty snowballs in essence)
asteroids (whether they orbit a planet, moon, or the sun),
'gravel sized astroids' (something from a grain of sand to a golf ball size)
Dust (anything smaller than a grain of sand)
Gasses (simple molecular or atomic gasses)
"energy" (subatomic particles like x-rays, photons, EM radiation, solar wind, etc.)
So what to do with Quaoar and Sedna (in the picture above). Quaoar (pronounced kway-o-are) has an orbit less circular than the other planets (just as Pluto does). So is it a simple matter of mass that limits it classification? perhaps. Sedna on the other hand, which spends most of its time in the distant 'Oort Cloud' (what a fucking cool name!), has an orbit that is highly elliptical, more closely resembling a comet. Yet, its shape appears to be round, and its composition would suggest we group this as a planet as well. One day the revision will happen in the astronomical science, but it will take the discovery of dozens of more objects like this before it happens.
Posted by novae at 4:21 PM