# Can smaller habitable planets (or moons) orbit a larger habitable planet?

I am doing some research for my fantasy novel, but I'd like to really understand how my planet system would cause life to grow if at all possible.

My idea would be, there is a large core planet that is orbiting a sun. Two smaller planets (or moons) are in an orbit around it on polar opposite sides of the core planet. Is this even possible? Would the necessary means to spark life occur?

• Perhaps you have not heard, but Jupiter has putatively habitable moons. Jun 20 '17 at 12:30
• Not sure why this is being flagged to close for being too broad. There is clearly a direct question about whether a particular planetary configuration is possible. Jun 20 '17 at 12:59
• The Planet is orbiting the sun and the moons are orbiting the planet on polar sides. Could all 3 bodies sustain life? Jun 20 '17 at 13:09
• i may be mixing up terms. My apologies. What I mean is that the moons would be orbiting on opposite sides of the planet. Jun 20 '17 at 13:14
• I think I used binary orbit because it was the closest thing I could find anywhere. Jun 20 '17 at 13:15

First off, let me point out that if you have a central planet which is orbited by other bodies, those other bodies must necessarily be considered moons and cannot be planets. This is due to the official definition of a planet stating that it must orbit the central star. The objects orbiting your central planet are not primarily orbiting the central star and thus would be considered moons or satellites.

### Orbital Dynamics

What you're proposing is that your central planet is orbited by two moons that are on opposite sides of the planet. This condition provides some strong constraints on your system as dictated by the laws of physics.

1. If we are to consider that the moons are orbiting around the central planet, then the planet must be fairly massive. If all three of your bodies are of a similar or comparable mass, you'd have a much different (and not necessarily stable) type of orbit than what you're describing (see this link for example). The fact that you have two moons orbiting your planet, without your planet having a noticeable orbit about the moons, implies the planet has most of the mass of the system. This results in one of two scenarios, either

• your central planet is very large (e.g, the size of Jupiter or Saturn) with moons the size of Mercury or Ganymede, or
• your central planet is relatively small (e.g., the size of Earth or Venus) with moons the size of asteroids (e.g., Phobos and Deimos, the moons of Mars)

Of course you could have any gradation between the two possibilities I just listed. The main point is, your planet must be much more massive than your moons in order to be the dominant gravitational player in its system.

2. You also state your moons are orbiting such that they're on opposite sides of the planet. Such an orbital configuration is known as a 1:1 resonance. It necessarily implies that your moons are orbiting at the exact same distance from the planet and take the same amount of time to complete a single orbit. While such a system is technically stable, I can't say with certainty that it is stable for a long period of time. More than likely, small perturbations will occur that will build up over time and cause your moons to leave their 1:1 orbital resonance. This will result in unpredictable outcomes, but my best guess is that the moons may start gravitationally interacting if they get close enough after no longer being on the same side of the planet. Eventually this may result in them exchanging energy and one getting kicked to a closer orbit while another getting kicked to a further orbit. But that's just a guess.

So short answer, yes your system is possible but it comes with some constraints and is likely not going to be stable for a long time period.

### Habitability

You also ask about the habitability of such a system. This is a bit tricky and there's no single answer. There are so many conditions that affect habitability. To make matters worse, we only have a single example of a habitable planet. However, there are general conditions we believe must be true for a planet to be habitable (for humans at least).

The big condition is that your system must be at a good temperature such that water can exist as a liquid. If your central planet is a Jupiter/Saturn analog, this is not going to happen since such planets have large, mostly hydrogen gaseous envelopes. There's not really any pools of water for humans. However, your moons could still host water. It really depends on how far out your planet is from the star. These types of planets form very far from the star, where water doesn't easily exist as a liquid so either your planet must migrate in to the inner stellar system early on (and stop at a respectable distance), or else some sort of tidal effect has to keep your moons much warmer than they would be other wise (see Enceladus, for example). The former means your chosen moon orbits are likely to be less stable and the latter means you're going to need other moons to help provide the heating, again making your orbits less stable. If your central planet is Earth-like, it can certainly host water and be as Earth-like as you want, but the moons are unlikely to be big enough to be habitable.

There are various other conditions for habitability such as nice atmospheres or magnetospheric protection from the star but one could write a whole book on this subject. I'll leave off by pointing you to this question on habitability.

• Thanks! That helps a lot. I know it's science fiction so I can basically make anything up. But I always like to have a little of the basics so it can be at least somewhat believable. Your answer helps a lot. Thank you again! Jun 20 '17 at 22:25
• @Drask Glad I could help. At the risk of offering unsolicited advice, I'd say be careful about "making anything up". I'll agree than in sci-fi you can probably make up any type of technology and explain its existence away as quantum-mumbo-jumbo theory, but you can't circumvent known physics. To a knowledgeable reader, it would be very jarring to have a scenario which we know in today's world to be physically impossible or to contradict known physics. A quantum teleporter is a valid made up concept but a planetary configuration which cannot possibly form is not. Jun 21 '17 at 12:47
• Let's say the core planet is very, very large and orbits 2 suns. Could that fix the gravitational pull on the moons and heat problems? Jun 22 '17 at 11:34
• @Drask If anything, making your system have binary stars will just make things more complicated. In order to have a nice stable planet/moon orbit, they're going to need to be pretty far from the double stars so as to not be gravitationally perturbed by it. One possibility that might work is to have a system where your core "planet" is actually so large, it's basically a failed star. Then, make your actual star into a small, dim star like an M class dwarf. Jun 23 '17 at 13:20
• @Drask Your moons can orbit the brown dwarf and the brown dwarf/M dwarf star can have a very wide binary such that the moons aren't appreciably perturbed by it. If the moons are close enough, the brown dwarf could supply the necessary heat. This brown dwarf would also be skirting the line between planet and star so it'd be hard to say which it actually was. Jun 23 '17 at 13:20

A planet and moon both habitable seems feasible to me, given the planet's orbit is in the stars 'goldilocks' orbital region. As an example, if an Earth/moon system were recreated except larger. So the Earth moon system was created early in our solar system by a large Mars sized collision with early Earth. We ended up with a rare larger than normal moon, compared to Earth, 1/6th the size. Looking at the other planets, no other moon comes close to being this big compared to the planet. Anyway, if instead of a collision creating two bodies the size of the Earth and moon you increased both masses by 2.5, you could end up with an Earth 2.5 times the size of ours, likely the edge of feasible, and a moon the size of Mars, also the edge of feasible. So yes both could sustain life, imo, assuming other conditions are met. (Water, organics, solar output, magnetic fields, etc.)

On the 1:1 resonant moons, I'd have to agree with the previous comment, very unlikely to be stable. Now if you want an advanced civilization to create an artificial one, that could work. Or use a very large Jupiter to have a couple of habitable planets in orbit and maybe even habitable planets at L3 and L4 (Lagrange points), though I am unsure of their stability too I don't think it's be too big a stretch. (Jupiter's Lagrange asteroids are called Trojans and appear stable but none are particularly big.)

• I'm sad to hear about the 1:1 resonance. That was the key point in my theory; plot wise. I don't think my people are world building advanced just yet. Unless this was super ancient technology that has been long forgotten. Jun 27 '17 at 20:26
• What would you say is the closest I could get to 1:1 resonance? Jul 1 '17 at 21:08