Also Seeking Astronomy Nerds

[SilvercatMoonpaw]

Would it be possible to have two non-moon Earth-like planets in the same one-star solar system?

Purpose: I'm trying to see if you could have a non-spaceflight human civilization in the same solar system as an FTL-capable human civilization with the non-spaceflight civilization still having a quick means of asking the FTL civilization for help (i.e. radio, or maybe cobbling together a rocket ship).

(I realize questions like this can be hard to give a straight answer for, so I appreciate anything you can supply. :) )

[Nomadic]

It would be possible. The question is more would they be populated with sentience (or any complex life at all). It is believed by many that our moon acts as a giant celestial shield, taking alot of hits that the earth otherwise might (thus life on the planet can develop in much greater peace than if the planet was unshielded). It's not impossible but it's something to think about.

[SilvercatMoonpaw]

The question is more would they be populated with sentience (or any complex life at all). It is believed by many that our moon acts as a giant celestial shield, taking alot of hits that the earth otherwise might (thus life on the planet can develop in much greater peace than if the planet was unshielded).

"Non-moon" meaning "not the orbiting satellite of another planet".

[Lmns Crn]

I don't see why it wouldn't be possible.

The only real concerns I can think of involve both planets' distance from the star they both orbit. Since they're (probably) going to have to be orbiting at similar distances so that their climates are neither too hot or too cold to support human life, there's always the worry of the planets crashing into one another. (With large orbiting bodies like this, even a near-miss can be catastrophic.)

Of course, there's nothing stopping you from just handwaving that right out of the picture if you want to: "The orbits are arranged such that the planets are not in danger of crashing into one another." Voila! Solved.

[Nomadic]

I don't see why it wouldn't be possible.

The only real concerns I can think of involve both planets' distance from the star they both orbit. Since they're (probably) going to have to be orbiting at similar distances so that their climates are neither too hot or too cold to support human life, there's always the worry of the planets crashing into one another. (With large orbiting bodies like this, even a near-miss can be catastrophic.)

Of course, there's nothing stopping you from just handwaving that right out of the picture if you want to: "The orbits are arranged such that the planets are not in danger of crashing into one another." Voila! Solved.

Space is huge, the chance of 2 planets colliding like that is beyond astronomically small. The zone that supports habitable worlds is large enough to have more than one in it without that chance ever becoming an issue.

[brainface]

Pretty sure planets can be in the lagrange points of other planets too, i.e. two planets orbiting at the same distance from the star, but on opposite sides of the star.

At least one pair of moons (jovian i think?) also share an orbit, where one passes the other every so often in some crazy orbit (they may orbit in opposite directions?). I'd look it up if i wasn't on the clock. :)

[Stargate525]

With slight alterations in atmospheric composition, both Venus and Mars are habitable. So ours had the potential for three.

Take what you will from that.

[Ishmayl-Retired]

I think the whole reason neither of those planets are habitable is exactly because of their locations - so even if those alterations were made, their locations would not provide for permanent life-forming atmospheres.  Earth just happened to be in the right place (if you believe in that sort of happenstance).

[Llum]

I think Mars is near the tail end of the habitable zone. That or the habitable zone (wich moves outwards) will slowly envelop Mars.

[Nomadic]

I think the whole reason neither of those planets are habitable is exactly because of their locations - so even if those alterations were made, their locations would not provide for permanent life-forming atmospheres.  Earth just happened to be in the right place (if you believe in that sort of happenstance).

Venus is too close. Mars however is in a perfect location. It's issue is that it's too small to sustain a thick enough atmosphere. An earth sized planet in mars position with a still active core could support life.

[Stargate525]

I think the whole reason neither of those planets are habitable is exactly because of their locations - so even if those alterations were made, their locations would not provide for permanent life-forming atmospheres.  Earth just happened to be in the right place (if you believe in that sort of happenstance).

Vesus' atmosphere is too thick, Mars doesn't have one of any real note. Replace both bodies with an earth-like, magnetosphere-equipped body, and you get two habitable worlds. One's very hot, the other is very cold, but they are habitable.

[Ishmayl-Retired]

I think the whole reason neither of those planets are habitable is exactly because of their locations - so even if those alterations were made, their locations would not provide for permanent life-forming atmospheres.  Earth just happened to be in the right place (if you believe in that sort of happenstance).

Venus is too close. Mars however is in a perfect location. It's issue is that it's too small to sustain a thick enough atmosphere. An earth sized planet in mars position with a still active core could support life.

Ah, gotcha.  I stand corrected by the Wanderer.

[Matt Larkin (author)]

Someone has already said everything I would have added.

Therefor I will only say two planets in opposite orbit, as brainface mentioned and the first thing I thought of here, is a damn cool feature.

[Polycarp]

And it would eliminate the "my year vs. your year" problem endemic to sci fi, since both years would be exactly the same length.

[Stargate525]

And it would eliminate the "my year vs. your year" problem endemic to sci fi, since both years would be exactly the same length.

Not to mention that actually getting from one to the other will be cake, energy-wise. Simply drop out of orbit, sit still, and wait for the other planet to come to you.