On Thu, Jun 16, 2016 at 07:24:34PM +0000, darby eckles (via tml list) wrote:
> Question for the astrally informed:Assuming a Sol-like system, say a
> planet like earth had been hollowed out, or had it's magma and core
> removed leaving just a shell.  Would the remainder's gravity be
> reduced on the surface?  Would it's gravitational pull be reduced?

Very greatly.  The mass of Earth's crust is less than 1% of the mass
of the planet, with a corresponding reduction in gravity on the
surface if that's all that was left.  The interior gravity would be
close to zero. Despite that, the planet would still need substantial
internal support to avoid collapsing inward.

It could no longer retain an atmosphere, either.  The interior could
probably be sealed and filled with air, but that would have to come
from elsewhere.  The mass of air inside the nearly zero gravity hollow
would need to be hundreds of times greater than the original
atmosphere above the surface.  Oxygen could be extracted from rock
during the mining process, but there's nowhere near enough nitrogen in
an Earthlike planet to source it locally.

> Would the overall reduction of mass of the planet distort the orbits
> of other planets?

Not noticeably.  In the specific case of Earth, the Moon would leave
orbit unless engineered to remain.

- Tim