According to the latest IAU resolution, our solar system has 8 planets, a smattering of dwarf planets, satellites, and zillions of SSSB’s. See text of the resolution below.
I am very disappointed in this definition. If it lasts, then our language will have to mutate to get around the difficulties.
Taking the science fiction perspective, ‘planet’ has become an unusable technical term. Consider the following events:
Our resolute star ship captain pops into a new solar system. “Give me a long distance scan.” The science officer scratches his head and says, “I detect several large objects, including gas giants, terrestrial globes, and rocky barren globes.”
The captain askes, “Planets?”
Science officer, “I’m not sure. Give me a few days to calculate their orbits and scan for dust particles before I can make that determination.”
You see, by putting criteria 5A.1.c into the mix, we can’t tell if that earth-sized globe with blue oceans and clouds hanging there before us is a planet or not. In fact, on a technical basis, I’m not sure even the remaining 8 planets will survive the cut.
Pluto was removed from ‘planet’ status to ‘dwarf planet’ because its orbit crossed Neptune’s orbit and therefore ‘has not cleared the neighbourhood around its orbit’. But what about Neptune? Doesn’t the existence of Pluto crossing its orbit make that gas giant a ‘dwarf planet’ as well ( ignoring that it’s larger than all the other terrestrial planets put together)?
And what about trojan bodies? There are clusters of asteroids that ride in the L4 and L5 trojan points of Jupiter’s orbit. (The Trojan asteroids — that’s where the name comes from). Doesn’t that make Jupiter a ‘dwarf planet’ as well?
Add another telescope and I bet we can make all the planets into dwarfs.
No, the best solution was the one that was circulated a few days ago that didn’t include 5A.1.c and gave us 12 planets. It’s definition was based on the the object itself, not the orbit.
In fact, as I suggested earlier, the word planet may have to go away. Make a classification system (like Star Trek’s ‘Class M”) for the size and shape of the object, and another classification system for orbital characteristics (star-centered, doublet, satellite, etc.)
I suspect the IAU definitions won’t survive intact, or will be superceded by a more usable system altogether.
“RESOLUTION 5A
The IAU therefore resolves that “planets” and other bodies in our Solar System be defined into three distinct categories in the following way:
(1) A “planet”1 is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.
(2) A “dwarf planet” is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape2 , (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.
(3) All other objects3 except satellites orbiting the Sun shall be referred to collectively as “Small Solar-System Bodies”.
1The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
2An IAU process will be established to assign borderline objects into either dwarf planet and other categories.
3These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies.”