Planetary gravity question.

Given the knowledge that many listmembers seems to posess, I hope you might be
able to help me with something...:)

Would a planet the size of our moon be able to attain a level of gravity that
Earth posseses? If so, could it be habitable? (Possibilities for fertile soil
etc...) Could it be a fully fledged eartlike planet?

I'm fluff writing, hence the question.:)

> "Johan Böjeryd" wrote:

The only way a planet the size of our moon would be able to have the gravity
equivalent to that of Earth is if it were made out of much denser material
than the moon is currently composed of. It's a question of mass and density
more than size, really.

Mk

> Would a planet the size of our moon be able to attain a level of

Yes, if it were a lot denser than the moon is. The moon's diameter is 0.27 of
Earth, mass is.012E. That means the moon's gravity is
g=M/r^2 = .012/.27^2 = .1646 of Earth.
The moon's density is.61E; Earth is the most dense of the planets. Double the
moon's density to 1.22E and you get.33gee. Make it six times as dense as Earth
and you get.99gee at the surface. If you took an Earthlike planet and knocked
most of the lighter layers off the top, that would be about right,
more or less, approximately.   I think the atmospheric density would
drop off a lot faster than it would on Earth, though--you'd have have
mountain peaks in semi-vacuum.

> Yes, if it were a lot denser than the moon is. The

Wow. That was math. Help.:)

I knew that it depended on mass but now thats confirmed...:) Would the
inclusion of denser materials and metals make the soil toxic in any way? I
suppose that a planet which is this dense would a lott more profitable (with
richer finds) to mine?

This stuff is falling into place nicely!:)

> On Sun, Aug 04, 2002 at 11:34:17AM -0700, Johan B?jeryd wrote:

I fear not. Earth's density is about 5.515 g/cm^3 - about the same as
iron, because it has an iron core. Fair enough. This hypothetical planet
would have a density of about 20.2 g/cm^3 (radius x density is constant
for constant surface gravity, radius of moon is about 1737.5km, radius of
Earth is about 6371.01km).

For reference, pure gold has a density of 19.32 g/cm^3; platinum is
21.3; iridium is 22.16. Within reasonable error tolerances, we're looking at a
solid lump of precious metals 2,000 miles across.

> I think the

This is a non-trivial calculation. :-)

> I knew that it depended on mass but now thats

Very much so! (At least to humans and things in the human food chain.)

> I suppose that a planet which is this dense would a

Yup. Mining colony, almost certainly with most of its food shipped in from
elsewhere.

> Wow. That was math. Help. :)

> I knew that it depended on mass but now that's confirmed... :)

The way it works with the Earth is that a lot of the heavy elements are in the
core, which is dense. You could have a world with a comparatively large core
and thin mantle and have it fairly
Earth-like.  It wouldn't necessarily affect the soil.

If you want to do some realistic planetbuilding, you ought to get
Stephen Gillett's book called World-Building,  ISBN 0-89879-707-1.
If you just want to make it up as you go along, you could say that it has
toxic amounts of thorium, uranium, whatever. Or the local plants look a little
plastic, because they incorporate chlorine into their makeup. Whatever you
like.

> At 5:29 PM -0400 8/4/02, Laserlight wrote:

Too much of the heavy stuff and it gets hard expecting that there will be
enough silicates in the mantle for conventional vegetation to take hold.

> If you want to do some realistic planetbuilding, you ought to get

I've seen references to the fact that a moderate amount of isotopes is
necessary to have enough mutation in order to have evolution working well. Too
much and nothing lives very long due to radiation exposure.

G'day,

> Too much of the heavy stuff and it gets hard expecting that there

In a terrestrial sense... something could look Earth-like without it
relying on conventional vegetation. Though Johan probably does mean
conventional with regard to being close to Terran analogs.

> I've seen references to the fact that a moderate amount of isotopes

There's still some arguments about that with regard to what we're finding some
groups can survive now that we've stopped assuming they couldn't;)

Cheers

> I fear not. Earth's density is about 5.515 g/cm^3 - about the same

Or a more normal planet with a small neutronium core. Or put a black hole in
the center. Depends, as always, on how much Science you want in your Pseudo
Scientific Blarney.

On or about Sun, Aug 04, 2002 at 10:40:10PM -0400, Laserlight typed:

> At 8:31 AM +0100 8/5/02, Roger Burton West wrote:

Well, then your planet is a target for Cyberman invasion all the time...

> Roger Burton West wrote:

You and the cybermen both, apparently.