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Re: Cidri -- Density -- Big Planet.

When I was in High School, I was wondering if there was 
a density of a planet where it could get really large, and 
keep about 1 gee of surface gravity.  (I was wondering, 
because… Cidri.)

No constant density.  As worlds got larger their radius went 
up (so lower surface gravity), but the large mass 
increased the surface gravity faster.

So larger mass wins out.  As worlds get larger, you have 
to keep lowering the density.  However, low density means 
that worlds can get quite large, and still keep a 1 gee 
gravity.  If I recall correctly, you could get worlds with about 
triple Earth’s radius, on a world with no iron core, just rock.  
Beyond that surface gravity just started going up.

(And that does not count rock compressing into denser 
forms, deep in the core of such a world.)

Note that Jack Vance wrote some stories set on “Big Planet”
("Showboat World”, and others).  Big Planet formed with 
almost no metals, and was HUGE.  Well shiny, heavy 
metals like iron.  It had plenty of metals like calcium, 
aluminium, lithium, and ones with low atomic masses.

If Cidri is hollow, then you can make it as large as you want.
(But the inside of the world is in zero gravity, not counting 
centrifugal force due to rotation.

Warm regards, Rick.