[meteorite-list] OT: INTERSTELLAR DIPLOMACY and Hello

From: lebofsky at lpl.arizona.edu <lebofsky_at_meteoritecentral.com>
Date: Fri, 10 Oct 2008 04:37:31 -0700 (MST)
Message-ID: <51144.>

Hi all:

The mass of Gliese 581c was determined by its gravitational pull on its
star. This was how it was detected. So, the mass is known. The next thing
to do is to guess its composition. If it is made of Earth-like rock, then
its radius is the cube root of the mass (mass goes as radius cubed), so
1.5 times the radius of the Earth. If you do the calculation, the gravity
would also be 1.5 times Earth (not too bad). If it is an icy world (not
sure if this makes sense given its distance from the star), then the
planet has to be bigger for the same mass and the surface gravity drops to
maybe only 1.2 or 1.3 times that of Earth.

This is the kind of thing I do with my students for gravity on other
worlds. Assuming that a planet is made up of about the same stuff: double
the radius, the mass is eight times as much (volume and mass are
prooportional to the radius cubed). However, since you are on the surface,
you are twice as far from the center of mass. Gravitational pull goes as
1/radius squared, so decreases the pull by 4 times. So 1/4 times 8 = 2
times gravity for two times radius (gravity is proportional to the radius
for objects made of the same material).


On Thu, October 9, 2008 9:41 pm, Darren Garrison wrote:
> On Thu, 9 Oct 2008 19:50:12 -0700, you wrote:
>> Gliese i581c is a planet that may be 5 times the mass of Earth.
>> If there are intelligent inhabitants, wouldn't they be like incredibly
>> strong on Earth? A race of Hulks come to mind. Sending them 500 voices of
>> little children sounds like inviting a lot of trouble. I don't think it
>> would be a bad thing if they don't pay a visit.:-)
> I was thinking of some of the mechanical problems related to life on
> higher-gravity worlds-- thought up a few points, but then decided, why
> reinvent the wheel? I'll google up some good stuff:
> http://www.sciforums.com/Has-anyone-thought-about-creatures-would-be-on-h
> igh-gravity-planets-t-24869.html
> a short mention here:
> http://www.molvray.com/sf/exobio/astron.htm
> and 4 points here (not all of which I agree with)
> http://io9.com/325109/four-theories-about-creatures-from-high+gravity-wor
> lds ______________________________________________
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Received on Fri 10 Oct 2008 07:37:31 AM PDT

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