[extropy-chat] SPACE: How many planets?

[Mike Lorrey]

--- Alan Eliasen <eliasen at mindspring.com> wrote:
> 
> Mike Lorrey wrote:
> > Actually, roundness was the primary criteria I had stated here not
> so
> > long ago...
> 
>    As I recall, that discussion dwindled off just as it started to
> get
> interesting.  Nobody ever defined what "roundness" meant and how it
> was to be
> defined.
> 
>    After all, Sedna is probably a lot "rounder" than Jupiter, which
> has an equatorial radius of 71492 km, and a polar radius of 66854 
> km.  It's all squished.

Atmospheric squishing from angular velocity or tidal influence is not
an issue here. It is assumed that any planet of any significant size is
going to be tidally influenced by other bodies in a similar way, as the
Earth is influenced by the Moon and vice versa.

Roundness is a matter of distinguishing the fact that some bodies have
enough gravity and resulting internal pressure and heat to cause their
material to have enough fluidity so as to become round, as opposed to
an oblong or an otherwise accreted misshapen pile of rubble.

> 
>    So what is the equation that quantifies "roundness," and what is
> the line that differentiates planets from non-planets?

Take its average internal temperature and pressure at different points
over the life and compare against the average elastic strength of the
material the body is made up of. If it is hot and high pressured enough
to cause some significant majority of the material to flow into a ball,
at some point in its evolution, then it is 'round'.

=====
Mike Lorrey
Chairman, Free Town Land Development
"Live Free or Die, Death is not the Worst of Evils."
                                       - Gen. John Stark
Sado-Mikeyism: http://mikeysoft.zblogger.com

__________________________________
Do you Yahoo!?
Yahoo! Finance Tax Center - File online. File on time.
http://taxes.yahoo.com/filing.html