How Can Two Tidally Locked Planets Coexist Without Catastrophic Repercussions?

[Helland]

Hello! I was wondering if there are anyone here who could help me with a little physics and logic pertaining to the behavior of planets.

I've been working on a science fiction story set in a fictional planetary system containing an earth-sized planet and a mars-sized planet. It's important to me that this system makes somewhat sense. I know there are way too many factors to account for to get everything perfectly right, but I don't want to mess up on the basics

What's important to the story is the following:

1 - The planets are tidally locked, so that the mars-sized planet can always be seen in the sky over one side of the earth-sized planet.
2 - The planets are so close to each other that the illumination of the mars-sized planet lights up the night sky enough to affect the eco-system of the earth-sized planet (for example, the light being reflected creates a different way to camouflage).
3 - They're not close enough to potentially crash into each other obviously
4 - The rotational period is as short as possible (meaning, not so long that the temperature change between night and day makes it unlivable, but not so short that it affects the relations between the two planets either).


The numbers and such that I’ve come up with so far, based on comparisons with what I’ve read about the relations of Earth, the moon, Mars, Pluto and Charon:


The Larger Planet:
Mass: 5.3211877 * 10^24 kg
Equatorial Radius: 6,218.465 km2
Surface Area: 485,036,000 km2
Surface Gravity (eq.): 9.182 g
Rotation Period: 32h 4m 4.9s (how long I’d like it to be, but I have no clue)
Orbital Period: 323.337245 days
Axial Tilt: 12.92934 °
Mean Surface Temperature: 17 °C
Min. Surface Temperature: -95 °C
Max. Surface Temperature: 63.5 °C
One year is equal to: 118.3% of an Earth year

The Smaller Planet:
Mass: 7.02396788 * 10^23 kg
Mass compared to Planet: 13.2 %
Equatorial Radius: 4088,845 km2

Distance between them: 105,546 km (based on the distance between Charon & Pluto)


So I guess the main thing is; how close can the two planets be to each other and how fast can they spin around each other without catastrophic repercussions?

And one last thing - this one might be better suited for a different forum, so just ignore it if it's too of topic: Is it possible for a certain area of the planet to have low enough tectonic plate movement to preserve a crater proportionally equal to Mimas' Herschel crater for billions of years? Even if the planet is earth-sized with a magnetic field, atmosphere and everything to a certain approximation of what Earth has?

 

[Ich]

So I guess the main thing is; how close can the two planets be to each other and how fast can they spin around each other without catastrophic repercussions?

They should be safely outside the http://en.wikipedia.org/wiki/Roche_limit" .
Then, they're almost certainly doubly tidally locked, so the rotation period of the larger planet equals the orbital period of the smaller planet, somewhere around 100 h, I think.

Is it possible for a certain area of the planet to have low enough tectonic plate movement to preserve a crater proportionally equal to Mimas' Herschel crater for billions of years?

After billions of years, erosion alone would have erased the crater.

 

[Helland]

They should be safely outside the http://en.wikipedia.org/wiki/Roche_limit" .
Then, they're almost certainly doubly tidally locked, so the rotation period of the larger planet equals the orbital period of the smaller planet, somewhere around 100 h, I think.

Ahh, thanks! That's exactly what i was looking for. I thought i had heard of a limit like that on a documentary years ago, i tried all kinds of google searches without finding it

100 h sounds reasonable, i guess it was a bit wishful thinking of me to believe the smaller planet could orbit in only 32 h. Sorry for being kinda clueless here, least i could have done was look up 'orbital period'...

After billions of years, erosion alone would have erased the crater.

Yeah, i thought i probably had to resort to some ficticious explanation for that

anyway, thanks a lot! I got the most importaint thing i was looking for.

 

[Ich]

100 h sounds reasonable, i guess it was a bit wishful thinking of me to believe the smaller planet could orbit in only 32 h.

No, that's possible, in a distance of ~55000 km. There are a number of orbital period calculators out there, try them. The ~100 were a guess based on your given distance (105000 km).

Is it possible for a certain area of the planet to have low enough tectonic plate movement to preserve a crater proportionally equal to Mimas' Herschel crater for billions of years?

Another idea: if the crater wall were much higher than the atmospere (say, 30 km), it could last a bit longer. I don't know, however, if Earth could support such a structure. I doubt it.