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peter303_ t1_j50vepy wrote

Reply to comment by Effective-Avocado470 in Are Two Tidally Locked Earth in One Solar System Possible? by Thirdy-DOg

Yes, I was going to cite that example too. Charon is fractionally largest "moon" (12%) tied to a "planet", so that could encourage tidal locking. (Earths Moon is second at 2%.)

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SweatyFLMan1130 t1_j51b1o1 wrote

This kind of boggles my mind cause I knew our moon was unusually large as far as such bodies go relative to their planets, but the fact it's only 2% while exerting 17ish% the gravity on its surface as we have on Earth seems counterintuitive. I know that proximity to the center of the mass influences how strong the gravitational attraction is, but damn, that's way more skewed than I had imagined.

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Underhill42 t1_j5373lw wrote

If density is constant then surface gravity increases linearly with radius: an r^(3) mass divided by gravity's r^(2) falloff.

So 10x the gravity means 10x the diameter and 1000x the mass. Crazy!

Density skews things, but even ignoring it is good enough for a sanity check. (I'm pretty sure 2x the density = 2x the gravity at the same size)

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