Talk:Definition of planet/Definition of planet unsourced archive
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[edit] Removed paragraph
"Nonetheless they could be considered as such since, though the Moon orbits the Earth, the timing of its orbit is in tandem with the Earth's own orbit around the Sun — looking down on the ecliptic, the Moon never actually loops back on itself, and in essence it orbits the Sun in its own right. This is true of any moon sufficiently far enough away from its parent body that its orbital speed round the planet is slower than the planet's speed round the Sun. The required distance from the planet to the moon depends on the mass of the planet, and the distance from the planet to the Sun, but not the mass of the moon. If the distance from the Sun to the planet increases, or the planet's mass decreases, then the required distance between the planet and moon increases. Consequently, the same argument could be used that Jupiter and Callisto or Saturn and Iapetus form double planets. "
[edit] Sphericity table
However, deciding which objects in the Solar System are spheroid is more complicated than it seems. In mathematical terms, spheroids consist of an ellipse rotated around one axis. Consequently they have two axes of equal length and one that is either longer or shorter; they resemble spheres that have been deformed (by stretching or squashing) in one dimension. A section through one axis will produce a circle, and a section through the other two axes will produce an ellipse.[1]
All spheroids, however, have the points on their surfaces joined by smooth curves (which form the elliptical or circular sections). On a topographically irregular body this can only be an approximation; however, taking such irregularity into account, a definite contrast exists between bodies, such as Enceladus, which are essentially spheroidal, and irregular bodies, like Neptune's moon Proteus, whose limbs do not show smooth curvature.[2]
If one uses this mathematical basis to define a spheroid, then the boundary between spheroidal and irregular objects within the Solar System frays noticeably, as this table illustrates:
| Object | Dimensions (km) | Mass (1019 kg) | Density (g/cm³)[d] | Shape |
|---|---|---|---|---|
| Ceres | 975 × 909 | 95 | 2.08 | Spheroid |
| 4 Vesta | 578 × 560 × 478 | 27 | 3.4 | Spheroid |
| 2 Pallas | 570 × 525 × 500 | 22 | 2.8 | Irregular |
| Enceladus | 513.2 × 502.8 × 496.6 | 10.8 | 1.61 | Spheroid |
| 10 Hygiea | 500 × 385 × 350 | 10 | 2.76 | Irregular |
| Miranda | 480 × 468.4 × 465.8 | 6.59 | 1.20 | Spheroid |
| Proteus | 436 × 416 × 402 | 5.0 | 1.3 | Irregular |
| Mimas | 414.8 × 394.4 × 381.4 | 3.84 | 1.17 | Spheroid |
| 511 Davida | 326.1 | 3.6 | 2.0 | Irregular |
| 704 Interamnia | 316.6 | 3.3 | 2.0? | Irregular |
| Nereid | 340 | 3.1 | ? | Irregular |
| 3 Juno | 290 × 240 × 190 | 3.0 | 3.4 | Irregular |
Plainly, there is no clear mass or size boundary dividing those objects in the Solar System which could be considered "spheroids" and those which are obviously irregular. The irregular objects Pallas, Hygeia and Proteus are all larger than regular objects, such as Miranda and Mimas. Also, as demonstrated by the dimensions listed in the table, the term "spheroid" is, in any case, fairly loose. Vesta, by the above formulation, could be considered either a spheroid or irregular. (see image)
[edit] The new diagram
The new diagram listing the changing numbers of planets is quite nice, but it does contradict the article slightly. Galileo considered his four moons "planets", though I don't know yet whether this term was widely used for them, or for how long. Until we sort this out it might be a good idea to take the diagram down for now. I'll place it here until I can get some better information.
Serendipodous 09:30, 22 February 2007 (UTC)
[edit] Size comparison with Jupiter
The picture comparing the sizes of Pluto with the Earth, the Moon and other KBOs suggests a false qualitative difference between Earth and the other objects. It is important to counter this with the observation that planets come in a huge range of sizes. The differences between Earth and Jupiter are much greater then the differences between Earth and Pluto, and that is important to deciding what a planet is. Algr 04:29, 15 July 2006 (UTC)
- Hmm. I think the main problem is that the listed picture doesn't reflect the topic of the section anymore either. It's been part of this article for a long time and was specially made for it, so I am rather attatched to it, but the fact is that the issue raised in that section is not the small size of the KBOs/asteroids, but their large sizes relative to each other. I may swap it out with a more appropriate image, but I would like to find a way to keep it in, so I'll repost it here until I can figure out how.
Serendipodous 07:43, 15 July 2006 (UTC)
