Extreme forms of life could exist on eccentric planets outside the solar system with conditions unlike those on Earth, U.S. scientists say.

Extreme exoplanets, from scorching hot worlds with molten surfaces to frigid ice balls, could maintain some form of life even though they don't inhabit a star's so-called habitable or "Goldilocks" zone, they said.

"When we're talking about a habitable planet, we're talking about a world where liquid water can exist," said Stephen Kane, a scientist with the NASA Exoplanet Science Institute at the California Institute of Technology in Pasadena. "A planet needs to be the right distance from its star -- not too hot and not too cold."

Not all exoplanets have Earth-like orbits that remain at a fairly constant distance from their stars.

Astronomers have identified many planets that travel in very oblong, 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," Kane said in a release from NASA's Jet Propulsion Laboratory 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."

While those conditions are very different from Earth's, this might not preclude them from being able to support alien life, Kane said.

"Scientists have found microscopic life forms on Earth that can survive all kinds of extreme conditions," he said. "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. There have even been studies performed on Earth-based spores, bacteria and lichens, which show they can survive in both harsh environments on Earth and the extreme conditions of space."

The habitable zone around stars might be larger than once thought, the researchers said, and planets that might be hostile to human life might be the perfect place for other, more extreme forms of life.

"We may find some surprises out there as we start to determine exactly what we consider habitable," Kane said.