It wasn't all that long ago that the idea of other planets where humans could live was considered a fantasy. Now we have lots of data coming in from exoplanets, those outside our solar system, and they come in all sizes, shapes, and flavors. Could they support life? The common theory is that only planets in which water would be in liquid form are suitable for us to live in, and these fall into the "habitable zone." Our solar system has three such planets: Venus, Earth, and Mars. Since we can't live on Mars and Venus as they are, you can see that habitability actually depends on many factors.
The idea of a habitable zone is a bit squishy, because having liquid water depends on a laundry list of other things, including the existence of an atmosphere, what’s in it, and more. But it’s a useful concept as long as you don’t look at it too closely†.
So technically, three planets orbit the Sun in its habitable zone. But how many could you fit in there?
At some number you’d hit a limit. The finite region of space means planets would get too close together. They’d interact gravitationally, and celestial hijinks would ensue: They’d create chaos, and some planet or planets would have their orbit messed up, dropping them into the Sun or ejecting them from the system entirely.
Scientists have crunched the numbers to find out how many planets could be in a system's habitable zone. It depends on the size and heat of the star, and a few other factors. But the answer will have you imagining a system where people could send mail to their relatives on the next planet over. Read how it might work at Bad Astronomy.
(Image credit: NASA/JPL-Caltech/R. Hurt/IPAC)
