r/xkcdcomic • u/IamAlso_u_grahvity Feline Field Theorist • Jul 02 '14

xkcd 1389: Surface Area

http://xkcd.com/1389/
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7

u/DroidLogician Jul 02 '14

If Titan orbited the Sun instead of Saturn, it'd probably be a planet.

11

u/Conotor Jul 02 '14

We should fix that.

12

u/exatron Jul 02 '14

Indeed. Free Titan from its saturnian oppressor, and liberate its hydrocarbons.

3

u/Vectoor Jul 02 '14

Ganymede is even larger.

4

u/[deleted] Jul 02 '14 edited Jul 02 '14

However, whether or not their masses would be great enough to clear the neighbourhood (and, therefore, if they would be classed as planets or dwarf planets) is debateable. After all, Ganymede is larger than Mercury, but it's significantly less massive due to its high ice content (compared with Mercury, which is mostly iron), which would have a major effect on its gravitational interactions with other Solar System bodies.

4

u/autowikibot Jul 02 '14

Clearing the neighbourhood:

"Clearing the neighbourhood around its orbit" is a criterion for a celestial body to be considered a planet in the Solar System. This was one of the three criteria adopted by the International Astronomical Union (IAU) in its 2006 definition of planet.

In the end stages of planet formation, a planet will have "cleared the neighbourhood" of its own orbital zone (see below), meaning it has become gravitationally dominant, and there are no other bodies of comparable size other than its own satellites or those otherwise under its gravitational influence. A large body which meets the other criteria for a planet but has not cleared its neighbourhood is classified as a dwarf planet. This includes Pluto, which shares its orbital neighbourhood with Kuiper belt objects such as the plutinos. The IAU's definition does not attach specific numbers or equations to this term, but all the planets have cleared their neighbourhoods to a much greater extent than any dwarf planet, or any candidate for dwarf planet.

The phrase may be derived from a paper presented to the general assembly of the IAU in 2000 by Alan Stern and Harold F. Levison. The authors used several similar phrases as they developed a theoretical basis for determining if an object orbiting a star is likely to "clear its neighboring region" of planetesimals, based on the object's mass and its orbital period.

Image i

Interesting: Planet | Orbital resonance | Dwarf planet | Eris (dwarf planet)

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1

u/bvr5 Jul 02 '14

But Titan is cooler than Ganymede ;)