Quote:
Originally Posted by eburacum45
Looking at the question of binary stars first; there appears to be two ways that you can have a planet in a stable orbit in a binary system.
(from this page )
http://www.solstation.com/habitable.htm
First the two stars might be close together in the centre of the system- a good rule of thumb is that the stars should be separated by no more than one third of the radius of the planet's orbit; for Earth that would be 1/3 AU, or a bit less than fifty million kilometers. In this case the planet would orbit both stars.
Or the second star could be separated from the first by more than 3.5 times the planet's orbital radius; for Earth that would be 3.5 AU, nearly as far as Jupiter. In this case the planet would only orbit one of the stars, not both - figure-of-eight orbits are almost impossible.
I'd add a safety margin to both of these figures - make the stars in the first case ten million kilometers closer together or so, and in the second case make the second star at least 4.5 AU distant- but that is up to you. |
That's pretty sound advice. A different sort of "binary" -- a Jovian class planet in the inner solar system -- is observationally quite common. The orbit of such an intruder has to meet conditions similar to but less stringent than those on a binary star for there to be stable planetary orbit in the habitable zone. If the gas giant itself is in the habitable zone then there's the possibility of its possessing a habitable moon.
Another thing to keep in mind is the life cycle of stars. Don't cast a late cycle degenerate star such as a white dwarf in any role but a very distant companion. Black holes and neutron stars are pretty much a no-no anywhere in a life-bearing system.
Quote:
Originally Posted by eburacum45
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As for the moons, there is an interesting thread here about multiple moons
Planet with two moons: double eclipse possible?
It looks like you could have at least two moons orbiting an Earth-like planet, although it would be a little difficult to explain how they were formed. One moon at 50 or 140 thousand kilometers, and one at 380 thousand kilometers seems to be stable. You could then add a single close-in asteroid-type moon, a bit like Phobos; note that this moon will eventually hit the planet. And for a fourth moon you could have a smallish retrograde moon at about 500,000 km or so, going the other way round the world. |
In the solar system moons around terrestrial planets are uncommon and seem to be the result of "unusual" mechanisms such as asteroid capture (Phobos and Deimos) or mega-impact (Luna). I'm not sure that a captured moon would necessarily have a weird orbit: Phobos and Deimos have very vanilla (circular, prograde, low inclination) orbits.
If you like your moons' orbits weird the author could consider a pair of co-orbital moons. Retrograde orbits, high inclination orbits, and orbits at or below geosynchronous are all plausible ways to add variety. A debris disk could substitute for a really close in moon.
I've noticed that when people design worlds they usually make it just like Earth only better: more moons, more suns, a bigger planet, a brighter night sky etc. Don't over do it.