[meteorite-list] Other Earths -- WAS: Re: Habitable Planet, etc.

From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Thu, 30 Sep 2010 00:27:11 -0500
Message-ID: <3C91582D37754660BA1822F41BAC31BF_at_ATARIENGINE2>

Hi, Carl, and Other Inquiring Minds,

> How much is "slightly"?

Well, I just answered that one, Carl, but the principle
involved is this: a bigger planet means that when you're
on the surface of it, you're much further away from the
center of its mass, hence gravity does not go up directly
as the diameter increases.

Imagine that we could build a spherical shell that com-
pletely enclosed the Earth, but was erected 4000 miles
above the Earth's surface. Massive as it would be, it would
be a minimal increase of the Earth's mass.

If you walked around on the outside of that shell, you
would be 8000 miles from the Earth's center instead of
the 4000 miles from the center we are "down here." The
gravity would be only 25% of "normal" gravity on the
outside of that (imaginary) shell.

Imagine a planetary system where an "Earth-like" world
condenses from a heavy-metal-and-element-poor nebula,
like one that forms a calcium-rich or light-metal-rich star.
(They exist, BTW.)

Without an iron core of any great size, they would be made
of rock only, rich in silicon, calcium, magnesium, aluminum...
A lithophile planet, not a siderophile planet, is perfectly
possible. Basalt would be a rare deep-mantle stone and
iron a precious gem-metal, both almost never seen.

It would be a low-density world. It would not compress
as easily as iron-rich worlds. It would be much bigger for
its mass than a world like Earth, hence it would have a
much greater diameter but a LOWER surface gravity.

Imagine a two-Earth-mass world with a density of 3.26,
like a heavy rock; that's only 60% of the density of our Earth.
It would have 3.33 times the volume of Earth, 2.19 times
the surface area of Earth, and 1.49 times the diameter of
the Earth (11,840 miles in diameter).

Its surface gravity would only be 88% of the Earth's, despite
having twice the mass! It would have a deep siliac crust and
very high mountains, higher rates of erosion, no plate tectonics,
no volcanoes. The only vertical movements in the crust would
be isotasy; continents would be buoyant plutons of less dense
rock. There would be twice the water of the Earth spread out over
2.2 times the area --- the oceans should be similar to Earth's.

With twice the atmosphere of Earth and lower gravity, the air
would have a much greater scale height, meaning air pressure
would not fall off as rapidly with altitude as the Earth's. The
air would be denser and breathable (if it contain oxygen) at
much greater altitudes -- you could breath on top of a
52,000-foot-high mountain and fly a piston-engine aircraft
to 135,000 feet or more.

Sounds like an interesting place, doesn't it? (I've always
thought so.) There are millions of possible "world-recipes"
and likely billions of possible semi-Earth combinations that
could be made from them. It's NOT going to be the same old
solar system repeated over and over again.

What if Gliese 581g was made of low-density rock like my
sample world (above)? It would be 3 times the mass, 5 times
the volume, 1.70 times the diameter, 2.92 times the surface
area, and have a surface gravity of just 1.04 gees, ocean
depths the same as ours. But Gliese 581 doesn't seem to
be the right kind of star for that blend of materials. Some
K2 light-metal-rich dwarf somewhere...

Planet-building is fun.


Sterling K. Webb
--------------------------------------------------------------------------------------
----- Original Message -----
From: "Carl 's" <carloselguapo1 at hotmail.com>
To: <meteorite-list at meteoritecentral.com>
Sent: Wednesday, September 29, 2010 7:46 PM
Subject: Re: [meteorite-list] (OT) Habital Planet Discovery Announcement


>
> Good evening All,
>
> Sorry for being really dense about stuff like this but I don't quite
> understand how a planet with a mass three to four times and a diameter
> 1.2 to 1.4 times Earth will only have it's gravity only slightly
> higher? How much is "slightly"? Thanks for the link, Count.
>
> Carl2
>
>
> Countdeiro wrote:
>>Hello List,Maybe...just
>>maybe...http://news.yahoo.com/s/afp/20100929/sc_afp/usastronomyplanet_20100929210707Best
>>to all,
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Received on Thu 30 Sep 2010 01:27:11 AM PDT