Goldilocks Zone

I have 4 inhabitable planets in my solar system. Granted the civilization in my novel has terraforming, but does anyone have any advice as to how I should space out my planets. I don't want them so close they collide or steal one another's moons.

How far apart are Venus and Mars in concentric orbit?

Distance from Sun to Venus: 67,230,000 miles.
Sun to Mars: 141,640,000 miles

141,640,000 - 67,230,000 = 74,410,000 miles between them.

The diameter of Earth is about 7,926.3352 miles.

That gives space for 9,387.7 Earth-sized planets, if you squeeze 'em end to end.

I know I passed first year physics in mechanics, but I'm not going to calculate how the gravity would work out in such a situation. But this assumes your system orbits a star like ours. I don't know if systems with other kinds of stars would act differently.

In our system, planets are increasingly widely-spaced as you move out from the Sun. We don't know if this is typical of other systems, but if it is, then fitting planets into the habitable zone may become more challenging.

There's a rather terse abstract at http://adsabs.harvard.edu/abs/1993Icar..101..108K that deals with this issue. According to the authors, a brighter star will have a wider habitable zone - but because of the increasing distance between planetary orbits that I mentioned above, the number of planets that are likely to be found in each zone may remain the same.

Wikipedia suggests that the continuously habitable zone for our system runs from 0.725 to 3.0 AU's, which would just about span Venus (0.7AU), Earth (1.0AU) and Mars (1.5AU) but not Jupiter (5.2AU). Venus is too hot to be habitable, something that has been tentatively attributed to a runaway greenhouse effect. You might speculate that if the atmosphere of Venus had been less dense (perhaps because the planet itself was smaller, and didn't retain such a dense atmosphere), the runaway effect wouldn't have occurred, and Venus could be habitable. Conversely, Mars is (probably) too cold for life, but if it had been more massive and had retained a denser atmosphere, it would likely be warmer.

So if you swapped Mars and Venus in a solar system very much like ours in other respects, you might get three habitable planets. The inner one might be uncomfortably warm, the outer one uncomfortably cold, but there could be liquid water on each, and that's what counts in what we call habitability.

That only gets you three habitable planets. A possible alternative would be, instead of filling your CHZ with rocky planets like Earth, Mars and Venus, to move a gas giant (like Jupiter) into the third slot. Then give it an extensive moon system, including a couple of large moons similar to Titan, where Earth-like conditions could occur. Of course the moons need to orbit far enough from the giant to be outside its radiation belts, and you have to hope that the presence of a giant in the inner system doesn't perturb the orbits of the other planets.

I suspect that the laws of orbital mechanics and planetary physics are fairly unforgiving, and that the odds of having multiple habitable planets in a single system are on the high side of immense. Still, it's a big universe, and if your civilization can nudge things in the right direction by terraforming promising "near-misses", why not?

National Aeronautics and Space Administration Monday announced the discovery of a possible Earth-like planet in the so-called "Goldilocks zone." That means it might be charitable. The world could be able to support life as it is known to us. The world, 600 light-years away, is dubbed Kepler-22b. It was detected by the orbiting Kepler room telescope. Studying this world will be the next big National Aeronautics and Space Administration undertaking if it is viable for humans. http://www.newsytype.com/13826-planet-could-support-life/

The discovery of this planet opens to many possibilities that there may be other planets out there that are also habitable. There might be also living creatures with the same intelligence to human or even higher. Who really knows? The distance of this new found planet is 600 light years away from us. It is very impossible for us to reach the place. I guess we leave this problem to the next generation. After all, we are all digging thins out for them.

There is always the engineering 'cheat.' Choose a planet on the edge of the Goldilocks Zone and have a group fix it up by tweaking things. The group could be dissidents from one of the planets who are faced with certain death if they don't come up with a decent place to live.

Another thought. If the continuously habitable zone ranges from .724 AU to 3 AU, and in our system Venus is at .7 AU, Earth is at 1 AU and Mars is at 1.5 AU, you could fit additional planets at 2 AU, 2.5 AU and 3 AU IF Jupiter wasn't present. That would give you six planets in the range.

You could also 'cheat' by having a double planet, with both planets being inhabitable.

If you really wanted to push things, have some unknown group create a group of double planets in the Goldilocks Zone. Imagine having six pairs of planets available.

You can also push the definition of inhabitable. What is inhabitable for our type of carbon based life form may be too hot or too cold for other life forms.

You could give the outermost planet an atmosphere with an unusually strong greenhouse effect.

Could you not put more than one planet on the same orbit? Even if it's so that the two planets are on opposite sides of the sun at different times?

Moons around a Hot Jupiter could do it. Place a Hot Jupiter in a Goldilocks zone and have earth-sized moons orbiting it.

Sky would be kind of oppressive, though.