# Are two Earth-Like planets able to be in the same system while sharing five habitable moons between them?

What I want is to know if I could have seven Earth like (What I mean by this is having an Earth-like atmosphere and climate, in this scenario size doesn't matter and nor does gravity) planets in the same system (This is not in our solar system but a different one containing what I will specify and other non habitable planets I have not mentioned) two Earth sized or larger Planets and five moons split between them however. I just want to know if this works and is probable.

• – Mołot Dec 17 '16 at 22:25
• It might help if you clarified what you mean by Earth-like.  One Earth- sized planet cannot orbit another;  they could both orbit their common center of mass.  You mention different sizes; can you characterize how different a body can be from Earth in size and still be considered Earth-like? – Peregrine Rook Dec 17 '16 at 22:28
• @PeregrineRook I believe it has been fixed, does this help – Mendeleev Dec 17 '16 at 22:40
• Well, it’s a start. Does “Earth-like atmosphere” mean just an 80% nitrogen 20% oxygen mix at ≈14.7psi, or does it include climate? Having different sizes may help establish stable orbits, but planets’ gravity is affected by their size and mass; how much gravity is too much? While we’re at it, please clarify: are you talking about a star system like our solar system, but with Earth being bigger, Mars being much bigger, and those two planets having five large moons between them? Or are you talking about a system where seven Earth-like bodies are clumped together and orbiting the star as a group? – Peregrine Rook Dec 17 '16 at 22:53
• (Cont’d) …  and (4) How many planets with Earth-like climates can you have in stable orbits in the same stellar system (see Can you have two or three planets in a stable orbit together?, How many planets can fit in the goldilocks zone with stable orbits?, How to Make Nine Different Earths in One System Possible, and maybe Orbit of a double planet).  Those threads suggest that you could have two planets in the same orbit, 60° apart. Another possibility is to put them at the same orbital distance but different orbital planes. – Peregrine Rook Dec 18 '16 at 19:58

...but I'm sure you're wanting more details than that, so here's some research!

# Roche Limits & Hill Spheres

Or: How I Learned to Respect Gravity

.

## Hill Sphere

Every object has a set gravitational field; you, me, the earth, cheese, and the galaxy all are gravitationally attracted to one another. Gravity though is a function of mass and distance, which means that the larger and closer an object is, the more gravitational pull it will exert. This also means that if an object is moving fast enough, but close enough, or a large center of mass, then it will orbit that mass. This zone is the Hill Sphere.

The Hill Sphere is a zone in which an object can safely orbit another body. Outside of that, the body will fling off into space. So the solar system as we know it exists within the sun's Hill Sphere.

Now, as mentioned before, gravity is also a function of mass. So if two similarly-sized objects get close to one another, then they both enter one another's Hill spheres, and thus will become gravitationally attracted to one another. And if they're in stable orbits before hand, then they can possibly be knocked out of orbit, sending them flying away or crashing into the parent body, which brings me to...

## Roche Limit

There's an inner limit to the Hill Sphere called the Roche Limit. Past this limit, the gravitational forces experienced by any orbiting body become so great that it literally rips the body apart. Within this limit, either a body will become shredded into a ring system or just crash into the planet.

Those of you reading this may remember the comet Shoemaker-Levy 9, which was ripped apart by Jupiter after it got too close. This is why it was ripped apart. As the comet drew closer and closer, the gravity of Jupiter overpowered the gravity c=holding the comet together, ripping it into pieces.

## Conclusion

So now that you know about these two forces, let's look back at your scenario. One star, two Earth-size planets within the habitable zone, and 5 moons split between them. The scenario is doable, but there's a catch. The planet should have to be far enough apart that their Hill spheres do not cross one another, yet close enough to both fit into the Goldilocks zone. Otherwise, their moon systems will become entangled, and the moons will careen into either of the planets or the star. Then ya won't have any habitable Worlds.

Very rare setup, definitely. But is this scenario doable? Absolutely.

• Well, if the moons and/or planets crash into each other, once all the dust settles the resulting planet might be habitable afterwards. I mean, it happened here. But you won't have the pretty multi-world system the OP wants. – Random832 Dec 18 '16 at 4:47

Short and concise: Yes, but no

Longer. But not that long:

Possible: Yes. Everything is possible.

Works: Uhm... Might. Yeah it might, depending on their distribution, etc. But you'll run into a multi-body-problem (3+) - so the system is unlikely to remain in equlibrium for any longer period of time, due to the various forces each of these massive bodies applies to the others.

Probable: No. Quite improbable. The three-body-problem makes it quite improbable and from there it is a downward spiral.

7 Earth-like planets is a chain of 7 planets with a good chance of spawning life1.
I cannot give you the math right now but we're getting into the ranges where current computers cannot calculate the chance of this happening without making use of compound datatypes spanning multiple registers and passes.

1Assuming that is what you mean by Earth-like. If you do not require them to be able to spawn life of their own similar to our earth you could limit their rate of mutation by having less radiactivity on them, but that doesn't really change anything, as you still want them to be ablr to support life.

• I’m not sure you and I are reading the question the same way.  Suppose you have two big planets (A and B) with radius approximately $2×$ the diameter of Earth and perhaps more dense than Earth.  Add five smaller bodies (C, D, E, F and G) with radius approximately $\frac12×$ the diameter of Earth and perhaps less dense than Earth.  A and B will be $≈64×$ as massive as the others, so how hard would it be for the small ones to orbit the large ones?  Put A and B in orbits with radii of $0.8~\rm AU$ and $1.2~\rm AU$, so their climates will be livable and their orbits will not interfere.  … (Cont’d) – Peregrine Rook Dec 17 '16 at 22:28
• (Cont’d) …  And the question isn’t asking about the possibility/probability of all seven bodies developing life independently.  A scenario where life develops on one and colonizes the other six would answer the question. – Peregrine Rook Dec 17 '16 at 22:29
• @PeregrineRook if you take a look at my answer you will see that there's not much more than a side not on the chance of spawning life. It really doesn't make a difference for the stability of the system if the planets develop life or not. But if they can support life the way earth does, they are equally likely to create life - because everything is there already. The real issue is the sheer coincidence of 7 such planets in the same system, orbiting each other stable.. – dot_Sp0T Dec 18 '16 at 9:03
• I’ve taken another look at your answer, and I see that there are 7 complete(ish) sentences, 3 of which mention spawning life (and one of which doesn’t really say anything).   My comments, coincidentally, also total 7 sentences — 2 of which mention spawning life.   So I didn’t take a ‘side note’ and blow it out of proportion.   If the real issue is the stability of the orbits, why did you mention it in less than half of your sentences? – Peregrine Rook Dec 18 '16 at 16:15
• @PeregrineRook nvm then. It is clearly my fault and I apologize for it and any uneasyness I have caused you. – dot_Sp0T Dec 18 '16 at 16:54

Can a system of two binary planets sharing five moons exist? Yes certainly in fact there are multiple arrangements that could be at play. The moons could orbit one planet the other planet or both in any combination.

The question of plausibility is much harder to answer though short answer is no. They key point is that this can only be solved numerically with a computer via an n body simulation (or in this case a 7 body simulation) There are in principal an infinite number of possible solutions to this problem all dependent on initial conditions and outside factors as this is a chaotic system which implies an exponential deviation per change in initial conditions.

This would make it nigh impossible for the system to remain stable(as it will tend towards a more stable configuration over time(with a single world being the most stable configuration). The odds are this planetary arrangement if it were to exist it probably would not last long as the presence of other bodies/worlds, cosmic radiation, cosmic dust, the host star(including gravity the varied light emission over its lifespan stellar winds, and outflows caused by magnetic storms on the surface of the host star) and on a more fundamental level Gravitational waves insure that any orbit will eventually decay.

As it is I doubt you could keep this stable long enough for sentient life to develop as the various planets and moos masses as well as all these external factors will highly perturb this system into pure chaos.

If you want multiple worlds suitable for life you might be better off having separate planets achieve habitable conditions. In our solar system at some point of time Venus Earth and Mars seem to have had liquid oceans(and all have lost them as some point so given the right atmospheric and geologic conditions to adapt to the stars evolution over time. Then there is the potential for habitability around the moon or moons of a gas giant planet as well.