How many habitable planets can you have orbiting a single star?
In our solar system, only one planet is actually habitable in a narrow sense of the word: Earth. Mars is too cold with air too thin, and Venus just the opposite.
But… that’s happenstance. If you swapped the positions of Mars and Venus, and maybe swapped a significant fraction of Venus’ atmosphere, their temperatures would be a lot more suited for us*. That’s because both are in our Sun’s habitable zone, the range of distance from our star where liquid water could exist on a planet’s surface.
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?
Artwork showing the TRAPPIST-1 planetary system, seven Earth-sized planets orbiting a cool red dwarf. Credit: NASA/JPL-Caltech
At some number you’d hit a limit. The finite region of space means planets would get too close together. ...