Maximum number of earth-like planets in a system

I am trying to build a planetary system with a maximum number of earth/near earth like planets/moons. What would the maximum number of possible earth like planets be?

I am assuming that some would be free standing and that some would be moons of Gas Giants that were still in the goldilocks zone. All don't need to be exactly like earth but need to be habitable to some degree by large numbers of people.

• – BartekChom Jul 29 '15 at 13:19

3 Answers

I stumbled upon a site a few months back that went into detail on this issue. The site is made by a man who knows what he is talking about. Read the whole thing for more detail on how he came to his conclusions. The question is, is the system artificial, or constructed? If its a single star system you could fit 36 planets realistically. Double that for a double star system. Of course this really couldn't exist natually, the odds are really against it. But as a construct, absolutely. http://planetplanet.net/2014/05/23/building-the-ultimate-solar-system-part-5-putting-the-pieces-together/

To answer, I split this question into multiple parts (each of which may have subparts):

1. How many Goldilocks zones can there be in a multi-star system?
2. How many orbits can be in a single star's Goldilocks zone?
3. How many planets can be in a single orbit?

For 1, take a look at this question. While it does not address the important question directly, what I read in the answers is that there is only one Goldilocks zone - either one of the stars is faint and distant and the planets orbit the other, or else the stars orbit each other closely and the planets orbit their shared center.

For 2, the conservative zone is about 0.5 AU, centered on Earth's orbit, for Sun-like stars. Stars that are somewhat smaller than the Sun have smaller Goldilocks zones (but live longer so life has more time to evolve); stars slightly large and brighter than the Sun (but burn out faster giving less time for life to evolve). Stars with types other than F, G, and K are unlikely to support life.

Because the habitable zone moves during the star's lifecycle (Venus used to be when the Sun was cooler, and Mars will be eventually as the sun gets hotter), I find it somewhat unlikely that more than two orbits could fall into the zone for much of a star's life. However, since both Venus and Mars are almost in the zone and we don't really know how much they are hurt by not having a massive moon (which is necessary for tectonics and a magnetic field), I'll say 3 orbits.

For 3, you can consider rocky planets or the moons of gas giants. There are stable solutions that involve two planets sharing the same orbit. You might be able to replace each of those planets with a pair of planets orbiting their common center of mass while that center orbits the Sun. This way gives you a total of 1*3*2*2=12. Though I'm not sure if all the doubling is really safe so 6 would be a much safer number.

Although gas giants have many moons, they are unlikely to have more than 4 with significant mass, and the presence of gas giants in the inner system invalidates the doubling cheats we used in the rocky case. So just 1*3*4=12. Note that with gas giants you do have a larger goldilocks zone, but this is cancelled out by the fact that gas giant orbits must be farther apart.

TL;DR: 4 is reasonable, 6 is plausible, 12 is possible.

• "Baby bear zone", surely? – colmde Jul 29 '15 at 7:06
• Red dwarfs(M type) can support life for trillions of years. So you could have a multiple red dwarf system where the mass is the same as say our sun but it will last much longer(powers of 10 longer) than a single star of the same mass. Now yes this does mean the planet has to be closer to get the same temperature range and much closer to get the same brightness but still, this proves that red dwarfs can actually support life as we know it. – Caters Jul 20 '16 at 16:08

If you don't mind long-term instability, a Klemperer Rosette https://en.wikipedia.org/wiki/Klemperer_rosette

can be extended to any number of bodies

Unfortunately, these are not stable, although it is suggested in the Wiki article that a 12-body version may be stable since each body is at the one of the Lagrange points of two others.

• Literally just humans inhabiting it is enough to destabilize it. – PyRulez Jul 30 '15 at 1:15