Re: Fast-rotating planets and gravity effects on the surface

If a planet rotates at its critical speed,
or escape velocity, the centrifugal force will balance
the gravitational force.  For Earth, the critical speed is
slightly greater than 11 km per second.  Earth rotates at
about 0.5 km per second;  planets which rotate at less than
about one-tenth of their critical speed can retain atmospheres
for billions of years.  If a planet were rotating at critical
speed, it would lose its atmosphere eventually.  Still, this
does NOT mean that the solid planet would break up.  Solids
are held together by electomagnetic forces, in addition
to their gravity.  Tidal forces wouldn't directly affect a
planet rotating at critical speed, as long as the planet
were roughly spherical and its crust and core roughly
evenly layered.  The electromagnetic force is almost forty
orders of magnitude stronger than gravity.

We see examples in our own solar system, of this general
principle at work.  The planet Saturn is quite oblate -
that is, its equatorial diameter is considerably larger
than its polar diameter - due to its relatively rapid
rotation.  Faster rotating planets would have equatorial
bulges which would gradually break away and form gaseous
disks and rings.

An astronaut on the surface of a planet gradually increasing
in its speed of rotation would feel lighter and lighter.  As
the rotation passed the critical mark, she would no longer
be able to stand on the surface, and would begin to float
away.  Landing a spaceship on such a planet would therefore
be difficult.  The spaceship could fire its rockets away from
the surface to propel it downward, and could implant long
arms into the surface which would resist shears and strains
caused by the rotation of the planet.

In the real universe, planets like this would not likely
form, nor become supercritically rotating.  A collision with
another massive body might pass enough angular momentum
to a planet to make it rotate supercritically, but such
a collision would probably shatter the planet as well!