Planet definition, Tethys/Enceladus, etc.

http://news.yahoo.com/s/ap/20060813/...sc/planet_spat

We'll get an official ruling from the IAU soon. My guess:

"Planet' will be replaced/subdivided into

* Gas Giants (Jupiter, Saturn, Uranus, Neptune)

* Terrestrial worlds (Mercuty Venus Earth Mars)

* Ice Dwarf planets (Pluto, UB313, Sedna, etc.)

[well, it is hardly *my* guess! Many people think this & I agree.]

The only real points of debate are

1) Does distance from the Sun matter at all? [mho: yes, but I'm not
sure how!]

2) How do we decide the lower bound between Ice Dwarfs and Asteroids?
The only obvious demarcation is the 'roundness' criteria - and I'm not
a fan! The problem can be seen in two of Saturn's moons, Tethys and
Enceladus. (I know, Moons can't be planets, but they come in convenient
sizes to frame the discussion)

http://www.solarviews.com/cap/sat/tethys.htm

Tethys is nice and round, and Enceladus is the next-smallest moon in
the Solar System. It is under half the diameter of Tethys. If the moons
of the Solar System are ordered by size, the Tethys-Enceladus gap is by
far the largest in the Solar System. And the size is about where we
would want some sort of dividing line. Sadly, Nature doesn't conform to
our wishes:

http://www.spacedaily.com/images/sat...celadus-bg.jpg
Enceladus is round too! And yet its gravity is under 1% of Earth's.
I'm starting to think it is more of a function of the likely fact it
formed slushy.