According to the Wikipedia article List of Firefly planets and moons , there are actually five stars and seven brown dwarfs in the 34 Tauri (the name of the system).
The central planets all orbit the White Sun in the center of the system.
The border planets orbit two other stars that both orbit the White Sun.
All of this information comes from
The Complete and Official Map of The Verse by Geoffrey Mandel as specified in the Wikipedia article.
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They were terraformed, as mentioned in the Train Job.
The planets most distant from the sun must have more greenhouse gases in their atmospheres.
Or they have warmer cores that are leak heat to the surface that keep it nice and warm on the surface.
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According to the RPG, admittedly a source of dubious canonicity:
There are 5 stars in system.
The worlds orbit around their own star at distances where greenhouse effect can be blamed for the temperature in a semi-reasonable manner.
Artificial gravity generators are used to maintain breathable atmospheres on some of the smaller worlds.
More importantly,according to the interview on the Serenity Blue Ray, Joss stuck to the rule of Drama when creating the verse, not to the rule of Science.
Joss didn't worry about it.
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I do agree to some extent that a larger sun with a higher metal index could have both a much wider habitable zone and a greater number of planet.
If one could some how control gravity without too much effort the depthof atmosphere could be controlled.
Some feel Sol could have a life supportingplanet beyond Mars if the planet was larger and the right atmosphere depthand gas mixture.
See the Wikipedia article of habitable zones of stars.
Granted
I don't think JW sweated the details......
Trig
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Our galaxy is currently 'eating' the Sagittarius dwarf galaxy.
It is feasible that during one (or more) of the passes, planets were captured from other systems that were within 5 degrees of the ecliptic to the (Firefly) system.
This would, in fact, produce exactly the type of orbital path distribution seen in the movie Serenity.
If the star is a blue-white, the star itself would be a pin-prick in the sky, but the light perceived by the human eye (with its self-regulating photo-limiter) would seem to be the same whether on an inner or outer system within the habitable zone.
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Is there an in-universe explanation for how it is that the outer planets in Firefly all seem to have temperate climate zones, despite their distance from the sun?  I am assuming here that the outer planets (or moons) must be farther away from their star than Saturn is from our sun.  They should all be frozen, with seas of liquid methane, like Titan.
How is it that the outer planets in Firefly are not frozen?
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<firefly><terraforming>
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The planets most distant from the sun must have more greenhouse gases in their atmospheres, or they have warmer cores that leak heat to the surface, keeping it nice and warm. The planets furthest from the sun have greenhouse gases to maintain the temperature. Alternatively, they have warmer cores that leak heat to the surface.
The planets furthest from the sun have greenhouse gases to maintain the temperature. Alternatively, they have warmer cores that leak heat to the surface.