Re: What is the term for a nearly spherical celestial body?



canopus56 wrote:

John Freck wrote:
What is the proper term for a celestial body that is nearly spherical
body? Is there any term, a word or noun phrase, that describes and
takes altogether our moon, Pluto, the spherical moons of Jupiter, and
Saturn, Ceres, and other dwarf planents?

The language used in IAU Resolution 5A is: "A 'planet' is a celestial
body that . . . (b) has sufficient mass for its self-gravity to
overcome rigid body forces so that it assumes a hydrostatic equilibrium
(nearly round) shape. . . ."

As JMB notes in this thread, "nearly round" or "nearly spherical" means
"hydrostatic equilibrium." The Stern paper cited in another thread in
this newsgroup -

Stern, S.A., & Levison, H.F. 2002.
Regarding the criteria for planethood and
proposed planetary classification schemes.
Status: To appear in IAU Proceedings 2000.
http://www.boulder.swri.edu/~hal/planet_def.html
http://www.boulder.swri.edu/~hal/PDF/planet_def.pdf

- discusses hydrostatic equilibrium for planets, KBOs, TNO's and
asteriod belt members. Whether a planet, drawf planet or pluton is at
hydrostatic equilibrium depends on its density.

Stern's Figure 1 is instructive and easy to follow.

For a rocky planet like the Earth with a density of 5.7gm/cm^3,
hydrostatic equilibrium can be achieved by objects as small as 100km in
radius (200km dia.). The Earth of course is many times this minimum
size.

For a lower density body like Titan at about 2.2gm/cm^3, hydrostatic
equilibrium, the minimum body radius is larger - over 300km (600km
dia.) Titan is much larger than this minimum.

Note that asteriod Vesta falls below this minimum requirement.

I haven't looked them up, but there are undoubtedly many Jupiter and
Saturn moons that are larger than this minimum diameter.

The result is that spheriosity alone is not a sufficient criteria to
distinguish between the many populations of known objects in the solar
system. Some of those populations include -

Planets
Asteriod Belt objects
Trans-Neptunian objects
Kuiper Belt objects
Oort Cloud objects
Moons of Jupiter, Saturn and Earth

That is why IAU Resolution 5A contains the other two criteria: "(1) A
'planet' is a celestial body that (a) is in orbit around the Sun, . . .
and (c) has cleared the neighbourhood around its orbit."

Subcriteria (a) (orbit) excludes the moons of Jupiter, Saturn and Earth
as planets.

Subcriteria (b) (spheriosity) excludes a plethora of objects, like
asteriods, Centaurs, short and long period comets

Subcriteria (c) (% of mass in oribtal shell) excludes everything else

- leaving only "planets".

- Canopus56

so... if Mercury had a piece of it knocked off it would no longer be a
planet?


.

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