Comfy Planets 
Conditions on the surface of Mars are fairly similar to those here on Earth. The planet has a thin atmosphere, weather, and some frozen water at its poles and mixed with the orange dirt. Yet Mars is not a particularly comfortable home for life, because it’s outside the “habitable zone” -- the distance from the Sun where the temperature is just right for liquid water at the planet’s surface.
In fact, of the hundreds of planets discovered in other star systems, only a few are inside their habitable zones. One that closely resembles Earth is known as Kepler-22b. It was discovered by the Kepler spacecraft -- an observatory that’s hunting for Earth-like planets in Earth-like orbits around 150,000 Sun-like stars. The planet is about twice as wide as Earth, so it’s probably a dense, rocky world like Earth.
Just because a planet is inside the habitable zone, though, that doesn’t mean it has life -- or even that it’s an especially good home for life. Mars is so small, for example, that its gravity isn’t strong enough to hold onto a dense atmosphere for long. And other worlds might be too big, or made of the wrong material.
Still, the habitable zone is the right place to begin the hunt for inhabited planets -- worlds like our own Earth.
And you can hunt for Mars all night. It’s in the east as darkness falls, in the constellation Leo. It’s putting on its best display of the year, so it shines like a bright orange star all night long.
Script by Damond Benningfield, Copyright 2012