I would like to set a story in a solar system that has multiple habitable planets.

Is it possible for more than one plant to occupy the same orbit around a star? ( imagine a few beads on the same wire hoop). Would they have inevitably smashed into one when they were being formed? Would they have to be too similar in size and makeup for it to happen by coincedence?

Could you have binary planets? ie instead of a moon orbiting around a plant, two similar sized planets orbitting around eachother, travelling together around a star. If such a thing were possible would their oceans have wicked-crazy tides?

In the Firefly universe, mankind has populated a solar system "with dozens of planets and hundreds of moons".
You could easily have a gas giant in the habitable zone, but one with multiple moons. Gas giants can have very large moons, large enough to have an atmosphere and could possibly sustain life. Their surface gravity would be smaller than Earth's or you can make them Earth-like. If the gas giant is big enough, the moons can be big enough as well. Of course having a gas giant very close means you would have trouble developing astronomy since much of the sky is occupied by the giant planet. It would also mean possible strong magnetic effects.

Sometimes the planet's tidal forces can cause the moon to start falling apart. This is why Jupiter, Saturn, Uranus and Neptune all have rings. They are debris of former moons. If the moons are populated, you have yourself a nice apocalyptic story.

If you want a really exotic system, try two habitable planets orbitting a twin star system. In a double star system, there exists an orbit in the shape of number eight with both stars in the middle of the loops. Have your planets occupy the same orbit but on the opposite sides of the star system. They could only come close every several years.

This can also be done with moons around the gas giant. If the moons are at different heights, it means they circle the host planet with different velocities. This would mean that moon face each other only on rare occasions.

See the wikipedia article on this.

Pluto's orbit is highly un-planetary. Most likely it was formed in a different way than all the other planets. That's why Pluto was reduced from a planet into a planet-like object. All conventional planets have orbits in the same plane (ecliptic) because of conservation of the angular momentum, making it a two-dimensional problem instead of a three-dimensional.

I hope I'm not being overly academic. I'm giving out real science since that was being asked for.

I'm trying to figure out which situation would fit my story best and I have a few more questions about seasons and day lengths.

In the the scenario where two planets have a figure 8 orbit around binary stars:
- when the a planet is on the "outside" of either star I assume it would have a night and day (like Earth) with every roation about its axis.
- when it is moving "between" the stars it would have 2 periods of light and 2 periods of night for every rotation about its axis. does that sound right martin?
- as for seasons, I assume a planet with a tilted axis would experience one cycle of seasons while going around each star, but that it would also be way hotter when the planet was between the stars. If the planet was habitable when between the stars, would it be so cold as to be uninhabitable when on the "outside"?

For habitable moons orbitting a gas giant:
- the rotation of Earth's moon is such that one face is always toward the Earth and one side away. Is this true of all moons?
- If the moons around the gas giant were like Earth's moon, it would experience night when "behind" the gas giant and day (on part of its surface) when "infront" of the gas giant (ie between the sun and the gas giant). Is this right?
- if the moon rotated more quickly it would experience a day-night cycle with each rotation about it's axis when not "behind" the gas giant, but continual night when "behind" it. Is this right?
- would there be seasons?

For binary planets:
- they spin about a point between them right? Would that orbit be in the same plane as their orbit about their star?
- as they rotate around eachother would each planet move closer and farther from their sun? (in other words is it the point between them that traces a smooth orbit around the star). If so would this create seasons independant of those created by the angle of the axis of each plant.

I hope that is at least somewhat intelligible. It would be easier to articulate these questions diagramtically I think

Figure 8 orbit:
- Probably best to draw a picture, put in ray lines from each star. I visualise a conventional day/night cycle when at the "top" or "bottom" of the eight, as one of the stars will be eclipsed. As you get to the side of the eight there would still be a day/night from the closer star, but half the night would be a twilight from the further star. Half the day may be brighter too. It would also depend how far apart the stars were.
- At the cross of the orbit you would have no night at all. Depending on the relative brightness of the stars you may have a bright day and dull day.
- Habitable: well I guess it depends on lots of things. You could postulate something similar to Aldiss' Helliconia where you have a very hot period and a very cold period. Once again, the stars wouldn't have to be the same temperature.
- I think that if you humans had interstellar travel then they would potentially also have enough technology to survive on such a planet.
- As for native life forms, who knows? At the moment we know of exactly one planet that sustains life. Who's to say what life could adapt to elsewhere?

Moons around a gas giant:
- Not sure on tidal locking. I think not necessarily but there would be a tendency toward it.
- Tidally locked moons would have one side in daylight when not behind the planet. This area would change as the moon orbited the planet. When behind the planet there would be an eclipse for part of the orbit, so the area of the planet who's "turn" it was to get sunlight would miss out. However, that area would, I think, change throughout the gas giants year. Once again, I'd draw pictures.
- Non tidally locked moons would have a conventional day/night except during times of eclipse.
- Could be conventional seasons on a non tidally locked moon if the moon has a tilt with respect to the star.

Binary Planets:
- Yes, the definition of a binary planet is that their center of mass is outside either body. That's why the Moon is, well, a moon, not part of a binary system.
- Yes, I'd imagine the centre of mass would trace a smooth orbit with each planets being some curve back and forth over that. Whether that affected the seasons would depend on how far apart they were but I imagine the relative distance would be small compared to the orbit around the star, so axial tilt would still play a big role in the seasons.

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But isn't the L3 Lagrangian point in earth's orbit around the sun exactly opposite from the earth? That's certainly within the habitable zone.

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...two planets orbiting each other closely enough that they share an atmosphere...

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8-shape orbit binary stars:
DavidS has covered it nicely. But one thing you should think about: not all stars burn bright as the Sun. Having a binary system could mean you have a regular star and a white dwarf. You could even have a neutron star or a black hole but that would not be a very healthy neighborhood. If a planet regularly makes a pass close to a black hole it would no doubt be torn to bits if not swallowed. A white dwarf is your best bet. This things usually have 1.6 Sun's mass or less but emit far less light.
Such binary systems are quite common. You could have such exotic phenomenons as the white dwarf sucking matter of the regular star but that again could cause trouble to a planet that got in between. Perhaps with a very very strong magnetic field, the gas jet from one star to another could be pushed aside. That could cause the planet to be periodically in a time of complete aurora borealis for the entire planet, not just the poles. Also, the mass the white dwarf gathers would periodically explode, creating what is called a nova. It would also make life close by very uncomfortable. Maybe the perioid of these explosions is locked with your planet's revolution, putting the planet safely behind the brighter star when this happens.

Anyway, make one star a white dwarf and you have normal days/nights with the dwarf star a moon-like object on the night sky. When the planet makes a swing aroung the white dwarf, the distance to the hot star could be great enough for the planet to pass to the edge of the habitable zone. You could have something like George R R Martin's Song of Ice and Fire where harsh winters last for years.

Habitable moons orbitting a gas giant:

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the rotation of Earth's moon is such that one face is always toward the Earth and one side away. Is this true of all moons?

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As a side note: the Moon is virtually stealing the Earth's angular momentum, causing the Moon to slowly drift further away from the Earth and the Earth to spin slower and slower. This is a slow process, though, so I wouldn't make a story based on it. I think Earth's day was 23 hours long about 300 million years ago.

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If the moons around the gas giant were like Earth's moon, it would experience night when "behind" the gas giant and day (on part of its surface) when "infront" of the gas giant (ie between the sun and the gas giant). Is this right?

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You could also use a planet as a light source. Not making nuclear reaction, that would make it a star. Jupiter has constant storms. Perhaps you could have very strong lightning or that lightning could ionize gases so much that they create a sort of fluorescent lamp effect. Such light would be eerie at best, sort of like our moonlight.

Moon's seasons could be tricky. Depends on the ratio of periods of the moon's revolution and rotation and the revolution of the gas giant around the sun. The combinations are literally endless. You can have moons that have very favourable conditions and moons with very poor conditions but maybe the latter has great mining resources and the miners must get off the moon quickly when the conditions diminish.

For instance, Mercury has a very strange rotation/revolution ratio. The Sun would rise on Mercury, travel to about one third of the sky, then retreat and set on the same spot it rose before. Then the Sun would once again rise on the same spot, travel the entire length of the sky, set, then reappear where it just set, travel back to one third of the sky, then turn and set again.

Twin planet system would revolve around a joined center of mass (as do all multi-body systems). That means that the center of mass acts like a body in itself, travelling around the star.
Ursula Le Guin had a twin planet system in Dispossesed, Urras and Anarres. I don't remember any moons being mentioned.

DavidS, not all centers of mass are outside both bodies. If the ratio is large enough, the center of mass would be only slightly different from the center of mass for the bigger body. The center of mass for our Solar system is in fact well within the Sun, due to the Sun being by far the heaviest object.

[This message has been edited by MartinV (edited December 14, 2010).]

Same thing with 2 planets on a single orbit. The very formation process of planets "scoops up" all debris in the orbit, to form in the first place.

The one thing that I don't think anyone has mentioned is that, depending on the star, the habitable zone can be pretty big. If you pick the right star, even with an identical solar system as our own, Mars and Earth might both be in the habitable zone, or Earth and Venus. Granted, one would be a bit on the colder side than the other, but a thicker atmosphere to lock in more heat could counteract this effect.

Since this is fiction after all, feel free to use any exotic setup you like, but in my mind, the most scientifically plausible is simple two planets on their own orbits that both happen to lie in the near and far region of the star's habitable zone.

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My thoughts too were regarding the Trojan Points (the L4 and L5 Lagrangian points). Unlike the other Lagrangian points, they have stable orbits even for non-negligible masses. If perturbed from these two points, the bodies form kidney shaped orbits around the points, which is cool...

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Actually, Brendan, this is wrong. The 4th and 5th Lagrangian points are unstable while the first three are stable. The kidney shape to which you are refering to are the equipotential curves. L4 and L5 are in fact as peaks of two hills. They are stable points but if you perturbate the position perpendicular to the Sun, your mass will 'fall down hill' and never come back again.

Here are two references that back up what I said (the wikipedia article above also states the same).

http://www.wordiq.com/definition/Trojan_point
http://www.astronomy.com/~/link.aspx?_id=e3acba05-9eae-4596-9af5-adf56d039992

Quote from the second article above

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The Lagrangian points L1, L2, and L3 are unstable, so that slight displacement of an object may result in its rapid exit. The other points, L4 and L5, 60° ahead of and behind the planet, are stable

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Good luck

Back in the days of my youth, I wrote a computer program that simulated such orbits, and iteratively mapped out the amount of time a planet would stay in-system. With very few exceptions, all possible starting positions and velocity vectors led pretty rapidly to the planet being ejected from the system.

My take is that in our current existance, with certain planet side changes, three planets are currently within the theoretical 'habitable zone' of our solar system. Those planets are: Venus - needs a change of atmosphere to lower temp to the theoretical 86 degree average based on orbital position, a small amount of mass, a stronger magnetic field, and a moon for proper volcanic activity to initiate part of the carbon cycle; Earth - already habitable; and Mars - needs significant added mass, needs significant atmosphere added, also needs a moon and a stronger magnetic field.

There is the theory that solar systems pass through nebula and the planets can, depending on the density of the nebula, gain a lot of mass quickly. a pair of steroids in orbit around each other could become planets in orbit around each other over time.

There is another theory that says "no planet can develop inside the orbit of Jupiter and no planet can for outside the orbit of Saturn. If this theory is true, all other planets were captured at some time during the history of the sun. Again the math is ugly but is possible. In this case, binary planets are quite plausible.
An observation based on this is that if stars can leave nebula, then so can planets and planetoids. If this is true, then the space between might be as dense with planets, moonlets and asteroids as our solar system is. Gravitational effects on planets already in the solar system would when something passed could really dramatic.

As for planets orbiting each other, they simply need a common center. Our moon is actually orbiting the sun on its own, and so is the earth. They simply have a wobbling orbit as they pass around the sun. A pair of planets would also have similar motions.
Another thing to consider is how far apart are the planets. If they both have life, and intelligent life develops, the fires on the other planet might push them toward space technology a whole lot faster, just to see what is there. They might even work out a way to communicate with each other long before the telescope was created by building fires in patterns across the landscape.
They could be farther away, appearing to be nondescript disks until telescopes are created. Fires might not be noticed until the telescope was created.

Usually, having two planets on completely different orbits indicates one was perturbed drastically. It would be possible for the two to be essentially on the same orbit but not collide or even really effect each other's orbit. They would pass each other's orbital plane when the other was more than a quarter of the way around. The effect on astronomers when they see the other planet approaching or passing might be interesting to follow.

On the binary suns, the real question is about how far away are the two suns from each other. There are examples where two stars are just outside the Roche limit of each other. Beyond a certain distance (I have no idea what that is), it would be like one sun. The farther apart the stars are from each other, the farther the planets have to be to even out the gravitational effect.

Also consider the types of suns. The proper heating environment for life is within the Roche limit for red dwarfs and would be still within the deadly radiation limit for blue stars.