Science fiction stories, books and TV shows traditionally place only one habitable planet in each star system. Frankly, I think this is a waste of a perfectly good star. If you’re going to write about human beings travelling out into the galaxy then you may as well have them find something truly spectacular – a system brimming with exiting, strange and habitable worlds. It seems that in thereal world habitable planets are exceptionally rare anyway, so if you’re going to write something fantastical it may as well be completely incredible.
The Goldilocks Zone
You probably already know about the Goldilocks zone (or habitable zone). It’s the orbital region of a star system that is not too hot, not too cold, just right for life – mainly because the temperatures allow water to exist in its liquid state. The size of the zone and its distance from the centre of the system depends on the type of star it encircles. Earth sits neatly in the middle of our own habitable zone and Mars sits just beyond its outer edge.
It is entirely possible for more than one planet to exist inside the habitable zone and therefore support human life. I’m no physicist but I’d say its possible for two, three, perhaps even four planets to form within this range. For example, move both Mars and Venus a little closer to the Earth and they would be much nicer places to live. In this set-up you might have an ice-world (with a climate like that of Antarctica), a tropical or desert world (hot and humid but capable of supporting life) and a nice Earth-like planet in the middle. Of course, this relies on them having the right atmosphere too.
Planets Outside the Goldilocks Zone
It is theoretically possible that planets outside the Goldilocks zone could also support human life. A combination of internal pressure, extreme volcanic activity and an unusually thick atmospheres could theoretically heat planets sufficiently to make life possible even on the outer edges of a star system. This would probably be due to intense lunar activity exerting tidal stresses on the planets mantel and crust. Needless to say, it would be a pretty horrible to place to live.
Lets not forget that moons could also be life-sustaining. This gives us a big opportunity to really cram some planets into our hypothetical star system. Imagine a large gas giant sitting neatly in the centre of the habitable zone with several Earth-like planets orbiting. These planets would experience relatively extreme seasonal changes but would otherwise be potentially valuable pieces of real-estate.
All the moons in our own star system are smaller than the Earth but there’s no reason why moons as large, or larger than the Earth couldn’t exist out there.
Now here’s where things really get exciting, if a little confusing. It is possible for planets to actually share the same orbit. Remember that out-dated science fiction idea of a second Earth, orbiting directly opposite our own on the other side of the sun? This is entirely possible. The second Earth would sit in the Earth-Sun L3 Lagrange point (and the Earth in second Earth’s L3 point).
Lagrange points are positions relative to large objects (in this case the Earth and Sun) in which the gravity of both objects balance each other out. This allows an object in the Lagrange point to remain stationary in relation to one of the objects (in this case the Earth). There are five Lagrange points, as the illustration shows.
A large gas giant in the Goldilocks zone could theoretically hold a small habitable planet in each of its five Lagrange points.
If you’re interested, it really is worth reading more about Lagrange points here. It might look like a complicated subject but it’s actually a very simple concept, and one that science fiction really hasn’t explored thoroughly enough.
Another possibility for a shared orbit it for two planets of equal mass to orbit both each other and their star. They would revolve around a common barycentre, while also revolving around the star (imagine the moon was the same size as the Earth and exerted an equal gravitational force). Caprica and Gemenon are an example of this phenomenon in science fiction.
Now, if you’ve done your homework on Lagrange points then consider this: if Caprica and Gemenon were slightly further apart, they might sit in each others L4 and L5 points (Caprica in Gemenons L4 and Gemenon in Capricas L5). They would therefore remain stationary relative to each other, orbiting their star at the same speed. They would essentially have two common barycentres; their star, and a point directly between them, the time it takes them to orbit both barycentres being exactly the same. Confused? Good.
One final idea to consider is that of binary and multiple-starred systems. You can use the ideas above to cram several habitable planets into the orbit of one star, then double the whole thing up.
Star systems can have two, three, four (two sets of binary stars) or more stars. Of course, once you start getting into much larger numbers the term ‘star system’ starts to give way to the more appropriate ‘star cluster’, but the point I’m trying to make is that you can have as many habitable planets in your star system as you like, providing the system is big enough. Do try and limit yourself though, otherwise it could all get rather silly.