What would happen to the Earth if this happened?

Question

Physically speaking --
What would happen if INSTEAD of the moon, Earth had a second planet in orbit around it?

First, assume that the planet was the size of the moon.

Second, suppose that the planet was the same size as Earth.

Would the two planets rip themselves to shreds?

Answer 1

Firstly, there is no difference between a moon and a planet in this sense. If it was the same size as the moon, and a satellite of the Earth, it would still be called our moon.

If the "moon" was Earth size, the tidal forces would be chaotic for us, but not enough to rip the planets apart. Certainly the stresses on Earth's crustal plates would mean dramatic crustal movement with huge quakes and volcanic eruptions, and of course the ocean tides would be horredous, but the Earth as a whole would remain intact.

Answer 2

The first part of the question is like asking "What if, instead of the Moon, the Earth had . . . well, the Moon orbiting it?" The so the first question is easy to answer . . . nothing would change, since we have a body the same size as the Moon orbiting at the same distance that the Moon does today.

The second question is a bit more interesting, as the system is gravitationally unstable. The two worlds would have a center of mass somewhere out in empty space, in between the two, and would orbit that center of mass, which would, in turn, orbit the Sun, except this center of mass, called a barycenter, would describe a pronounced circle around the path of the system's average orbit around the Sun. As a result, interactions between the two worlds and the Sun will likely result in the two planets either colliding, or the system being broken up, with one planet spiralling inwards toward the Sun.

Answer 3

The answer to the first question is obvious: no difference means no change.

The answer to the second question is an increase in the tidal force on Earth by a factor of 4.3 (cube root of 81) and a decrease in the sidereal month, to about 19.4 days (instead of 27.3 days). These two effects would be synergestic with regard to heating the crust, and there would probably be an increase in volcanism and earthquakes.

However, there would be no danger that the larger moon would come crashing down on Earth. The system is stable. If you want to check, just set up an open-integration loop to solve the three-body problem for two Earth masses in circular orbit around a mutual barycenter while that barycenter orbits a 331950 Earth mass object (the sun) at a distance 400 times larger than the separation between the Earth masses. It's stable.

Answer 4

A lot would depend on the density of those satellites.

The moon is about 2000 miles across, with a density of 3.4 g/cc.

If you were to replace it with a satellite that was also 2000 miles across, and a density of 3.4 g/cc, then there would be no change in the system.

If you had something the size of the moon, but made of solid iron (7.86 g/cc), then the moon would have 0.35 G - more than twice the gravity it currently exerts. This would have a significant effect on Earth's tides, and would possibly make the Earth much warmer than it is now (through stirring the molten core and increasing vulcanism - much as Jupiter affects its moon Io).

A satellite the same size and density as Earth, but at the same distance as the moon is currently probably wouldn't be a stable system. It's likely that the two bodies would eventually crash into each other - possibly forming a larger planet in this orbit, depending on how long ago the system formed.

Answer 5

Depends on the size of the hypothetical planet. If it is significantly bigger than the Moon, tides would be more severe. But I doubt the planets would rip themselves to shreds.

Answer 6

First case...no change whatsoever. All you've done is change the name of moon to planet...do you honestly think that makes any difference wahtsoever? (Think about it.)

Second...tides would be much larger. No other effects beyond wild speculation of how it might have affected the early development of the Earth.

Last question: No.

Answer 7

In theory, there should be no change, as long as the mass, rotational velocity, and angular momentum were still the same.