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Comment by kqr2 | Hacker News Reader

According to http://news.yahoo.com/s/afp/20100929/sc_afp/usastronomyplane... , surface gravity might only be slightly higher than earth's.

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How is that possible?

Density of a planet's interior is not uniform, and gravity tapers exponentially with distance. So, a planet with a dense core and a less-dense middle area could easily have less gravitational force than you might expect since the surface is so far away from the dense core.

Surprisingly not so.

If you have a body with a spherically symmetric mass distribution, from the outside it always has exactly the same gravitational field as a point mass at the centre.

Proof: http://hyperphysics.phy-astr.gsu.edu/hbase/mechanics/sphshel...

Wait, but didn't I specifically say I was referring to a body without a spherically symmetric mass distribution? Maybe I'm incorrect in my understanding of symmetric, but I was definitely referring to a non-symmetric case in my text.

Spherically symmetric means that it looks the same no matter which way you rotate it. You talked about a dense core, less dense middle-region planet, which is spherically symmetric.

If the planet had an ultra-dense region off to one side then it wouldn't be spherically symmetric and it would have gravity slightly stronger on one side than on the other... it's hard to imagine such a planet existing though.

Ah, gotcha. Thanks!

> and gravity tapers exponentially with distance.

Inverse square law.

It's more massive, but the radius is bigger as well. It could be as low as 1.5 Gs. Modifying the gravitational formula (http://en.wikipedia.org/wiki/Earths_gravity), it's m/r^2, so 3/(1.4^2) = 1.53 Gs. That's neglecting any local variations.

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