#twinearth

[scriptastronomer]

Hi! In my current story, I have two Earth-like planets (probably a bit smaller than Earth) that orbit each other close enough to see each other (a bit bigger than the moon). Is this impossible, or how far apart would they need to be? Could anyone survive on these planets? Thanks in advance!

As long as the two planets were farther than their Roche limit from each other and closer than their Hill sphere, they would orbit each just other fine. 

However, if they were as close as their Roche limits would allow (that’s about 6400 km for the Earth), they’d be quite un-nice places to be. Tidal forces would be extremely strong and they would be planets full of earthquakes, volcanoes, and devastating tides.

Actually, that sounds pretty cool. :-)For a more friendly pair of planets, I’d have them a bit farther apart - about Earth-Moon distance. If the Earth had a twin-Earth that was at the distance of the Moon, both planets would orbit each other quite nicely, and life could continue on each.

There would be a couple of differences, however. The full twin-Earth would look thirteen times larger than the full Moon (in area - about 3.6 times in diameter) and be a lot brighter. The twin- Earth would shine about 33 times brighter than the full Moon. Nights would be very well lit.

There would be much larger and much stronger ocean tides. Tides would cover much more land than they do now, and cities would be built farther from the coasts. There are places on Earth that have very strong and dangerous tides, and these would be even more dangerous when powered by the twin-Earth. Of course, twin-Earth would also have larger tides as well.Each planet could have more earthquakes and volcanoes (as they’re being gravitationally flexed by their twin), but the might not be that much of a problem. The geologies of planets can be pretty flexible, so you’d get to decide that. Make one nice and stable and one a volcano hell, and it’d be believable.

If twin-Earth and Earth had always been together, they might be tidally locked to each other. Each would hang in the sky at the same place, with the same sides facing each other. Each would have a day as long as a month.

Or not. Maybe they started out with a lot of rotation and there hasn’t been enough time for them to lock into each other. Up to you. They’re your happy little planets,