New research indicates that life might be able to survive on some of the odder exoplanets discovered so far - from scorching hot worlds with molten surfaces to freezing balls of ice.
Habitable planets are usually defined as those where liquid water can exist. But not all exoplanets orbit, like Earth, at a fairly constant distance from their stars. Many travel instead in very eccentric orbits that vary greatly in distance from their stars.
"Planets like these may spend some, but not all of their time in the habitable zone," says Stephen Kane, a scientist with the NASA Exoplanet Science Institute at the California Institute of Technology in Pasadena.
"You might have a world that heats up for brief periods in between long, cold winters, or you might have brief spikes of very hot conditions."
Though planets like these would be very different from Earth, this might not preclude them from being able to support alien life.
"Scientists have found microscopic life forms on Earth that can survive all kinds of extreme conditions," says Kane.
"Some organisms can basically drop their metabolism to zero to survive very long-lasting, cold conditions. We know that others can withstand very extreme heat conditions if they have a protective layer of rock or water."
He points out that there have even been studies performed on Earth-based spores, bacteria and lichens, which show they can survive in space.
The team's research suggests that habitable zone around stars might be larger than once thought, and that planets that might be hostile to human life might be the perfect place for extremophiles, such as lichens and bacteria, to survive.
And as Kane points out, "Life evolved on Earth at a very early stage in the planet's development, under conditions much harsher than they are today."
In fact, many life-harboring worlds might not be planets at all, but rather moons of larger, gas-giant planets like Jupiter.
"There are lots of eccentric and gas giant planet discoveries," Kane says. "We may find some surprises out there as we start to determine exactly what we consider habitable."