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CremePuffBandit t1_j4yio4m wrote

Yea, that would be a double planet. Depending on the exact sizes, distance from the star, and distance from each other, it might not be stable in the long term. But in the right circumstances, it could exist for a very long time.

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Effective-Avocado470 t1_j4z1geb wrote

Like the Pluto-Charon system

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peter303_ t1_j50vepy wrote

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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GrizzKarizz t1_j4ytefp wrote

Similar question, if there was another Earth sized planet in our (therefore each other's) langrange point, would that be stable?

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slinkymcman t1_j4yyfsf wrote

No Lagrange points only work because the mass is negligible at them

Edit Lagrange are stable because they don’t impact forces on the other two bodies when you account for rounding errors

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MrMe_1621 t1_j4yzbto wrote

No, the planets would probably end up orbiting each other or colliding. Lagrange points are stable for things like satellites because their masses are insignificant compared to the Sun and Earth. If an Earth-sized planet was at one of the Lagrange points(mostly L1 and L2, I’m not sure about L3-5), the planets would be attracted to each other much more than to the sun.

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sault18 t1_j4zoyr1 wrote

Isn't Theia hypothesized to either have formed or migrated into Earth-Sun L4 or L5 point and then had to be gravitationally nudged out of there by Jupiter and / or Venus?

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pmMeAllofIt t1_j527nft wrote

L4/5 isn't stable for larger object. If Theia collected enough mass it would nudge itself out without the help of other bodies of mass. Being about 10% Earth's mass in the Sun-Earth L4/5 points I believe.

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GrizzKarizz t1_j53hikz wrote

I meant the L3 point, but I assume you are right and that it's not possible.

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MrMe_1621 t1_j53prs2 wrote

Well, of all the points I think L3 would be stable the longest since the attraction between planets is minimized. Over time the extra force might cause them to spiral inwards, but the effects of other planets would destabilize things more quickly.

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rumbletummy t1_j5208qm wrote

If a planet of the exact same size as earth existed, in the same orbit/speed as earth, but 12 hrs (6 months) off, would we ever know?

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ruferant t1_j52dpeg wrote

They made a movie, everything was reversed like a mirror. Edit, 6 months off, somehow I think that's what you mean

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rumbletummy t1_j52n04x wrote

Youn right, 6 months. And thanks for the suggestion!

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Underhill42 t1_j5398l2 wrote

12 hours? Definitely. That's only about half a degree away in our orbit around the sun. The impact wouldn't be long in coming.

6 months though? Then it would be exactly on the opposite side of the sun, in our L3 point.

It was actually once speculated that there might be a "Mirror Earth" there, but it wouldn't be a particularly stable location, and our probes have long since confirmed there's nothing significant there.

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rumbletummy t1_j53m62d wrote

Yeah I messed up with the 12 hrs, probes make sense.

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