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Let's assume that for whatever strange -- probably magical -- reason, the Moon was suddenly and instantaneously replaced by a planet identical to Earth in terms of mass and size. Its orbital path is identical to that of the former Moon. Aside from deciding how the planet got there in the first place, and where the former Moon went, what would the effects of its appearance and presence be on us Earthlings? For some extra fun, let's also assume this other Earth-like world has its own biosphere, and surface conditions similar to Earth (atmospheric composition and pressure, surface water, soil composition, etc).

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    $\begingroup$ I think this is more of a google search, seeing as I found several articles and websites on this. The biosphere and conditions similar to earth though aren't outlined in the google searches so I suppose it can be answered here. $\endgroup$ – Jefferey Dawson Jul 14 at 20:36
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    $\begingroup$ Please specify: do you réally want the twin Earth to just pop into existence where Moon is now? Think carefully: sudden gravitational effects, rotational & orbital mechanics all point to sudden and educational disaster! Or are you looking to replace Moon with a twin Earth, resulting in a binary planet? $\endgroup$ – elemtilas Jul 14 at 21:17
  • $\begingroup$ Does this moon replacement have a magnetosphere? Because otherwise solar winds would strip the atmosphere back to nothing quite quickly $\endgroup$ – Cygnus X-1 Jul 14 at 21:31
  • $\begingroup$ I think your meaning is clear. Moon, gone. Eartg2, where moon was, in orbit moon was in. Some compensation in the orbit parameters to account for the shift in the center of mass. Earth2 is in any way you ask the same as Earth. $\endgroup$ – cmm Jul 15 at 2:38
  • $\begingroup$ What are the effects of X on us? is the standard too-broad question. Please narrow it down. $\endgroup$ – L.Dutch Jul 15 at 3:09
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The initial effect will be catastrophic, as the two planets adjust to a new orbital equilibrium; elliptic orbits around their common center of gravity midway between the two planets (see figure). The semi-major axis will be equal to half the current distance between the Earth and the Moon, while the semi-minor axis will me far smaller (I can't off-hand figure out exactly how small). The orbital period will also be shorter than the current lunar month.

The tidal forces between the two planets will be very strong, particularly at closest approach. This is likely to also cause strong seismic activity, as seen on Jupiter's closest major moon, Io.

The planets will eventually be tidally locked, with the same sides facing each other. As rotational speed is bled off, the orbits will increase because the angular momentum of the planets is transferred to the angular momentum of the orbit. This will reduce the eccentricity of the orbit, since the effect is strongest at closest approach.

Once the planets are tidally locked, there will be no 'lunar' tides as we know them any more, as high tides always face directly away from and towards the orbiting body. The height of these tides will however vary with the distance between the planets, with the highest tides at closest approach (and correspondingly lower tides at the 'sides' of the planets). There will also still be solar tides, which account for almost one-third of tidal forces affecting the Earth.

An additional complication is that there will be some 'wobble' in the positions of the sister planets in the skies, since the planets rotate at constant speeds, while they pass each other at much higher angular velocities at closest approach than when they are farthest apart. This will create a pull or push on the planets' surfaces that will move tides from side to side during close approach.

enter image description here

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If you magically replace the Moon with a copy of the Earth, two things happen:

1-) Both planets will immediately have their surfaces ravaged by giant tsunamis. The Earth has about 100x the mass of the Moon, so tides will be extremely strong.

2-) They will spiral towards each other and eventually collide. The collision would happen in less than four days. The reason is that there are forces acting on the Moon keeping its orbit almost in equilibrium (its orbit is expanding slowly). The force pulling the new planet towards Earth would be almost 100x stronger, so the new vector would mean catastrophe for us.

The last time the Earth collided with another planet, the debris formed the Moon. But it was a Mars-sized planet last time. This time, a new Super-Earth will be formed. It will start out in a Hadean phase, so it won't be inhabitable. With the heat of the impact all water will evaporate into space, and without the asteroids for a new bombardment, this new resulting planet may never develop a hydrosphere.

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    $\begingroup$ Lunar gravity is about 1/6th Earths. Where did your 100x number come from. I do not find a 4-day collision credible. First order, the Earth2 would be in the moon's orbit, at the moon's distance. $\endgroup$ – cmm Jul 15 at 2:35
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    $\begingroup$ You are right about the mass ratio. $\endgroup$ – cmm Jul 15 at 2:39
  • $\begingroup$ @cmm acceleration adda a little speed every second. In space, without a medium to give you a terminal speed, you can easily get very fast in very little time. If you accelerate at just 1m/s^2 for example, you will be going at almost 0.003c after just one day. $\endgroup$ – Renan Jul 15 at 2:59
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    $\begingroup$ #1 It's 82x -- #2 They won't "spiral into each other." The new moon won't have enough velocity for the larger gravity well, so it will fall into a crazy elliptical orbit. However, space is BIG so this will continue for a long time with the planets separating a little each year due to tidal forces. They will not collide. $\endgroup$ – Benjamin Jul 15 at 4:23
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    $\begingroup$ What @Benjamin said. :-) $\endgroup$ – Klaus Æ. Mogensen Jul 15 at 8:36

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