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Old November 11 2012, 08:58 PM   #6
Location: Kentucky
Re: New Discovery: Super Earth in a Nearby System

If it had the same density as Earth (iron core) so the density is 5,500 kg/m^3, then the radius and surface gravity would be 1.58 times that of Earth (cube root of four). 1.58 G's wouldn't even be that uncomfortable, and it would be like walking around with a backpack (a 75 pound pack for a 140 pound person).

For reference, here is the density of some known planets:
Mercury 5.427
Venus 5.204
Earth 5.513
Mars 3.94
Jupiter 1.33
Saturn 0.687
Uranus 1.27
Neptune 1.638

Neptune's surface gravity is only 1.14 G's even though it's 17 times as massive as Earth, with 15 times as much surface area. Uranus surface gravity is 0.886 G's, it's mass is 14.5 times Earth's, and it's surface area is 16 times larger than Earth's.

Ignoring Jupiter and Saturn, our own solar system has planets whose density varies from ours (the highest in the system) down to only 23% as much, and since surface gravity is a linear function of density and radius, and mass a function of the cube of the radius, the surface gravity goes up with the cube root of the mass.

If you take 1.5 G's as the limit where a human would feel resonably comfortable, and the density of Uranus as the lower limit of likely density while still having a habitable surface (not a gas giant planet), then the upper end would be a planet with 6.52 times the radius of Earth, 42 times the surface area, and 64 times the mass.

So the "giant" Earthlike planets in the habital zone aren't ruled out as good places to live until the mass is quite extremely large. Unfortunately that also means likely candidates' habitability will come down to surface pressre and temperature, and those can't necessarily be determined except by close inspection. We couldn't venture a guess as to Venus' surface pressure and temperature until we had radar data from bouncing signals off the surface.
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