"Our result indicates that searches for life-giving worlds outside our solar system should include planets around old stars," said Dr. Bruno Lopez of the Observatoire de la Cote d'Azur, Nice, France.

Dr. Lopez is lead author of an article about this research set to appear in the Astrophysical Journal. The search for life on other worlds is a key element of NASA's vision for space exploration.

Known forms of life require liquid water, which sets the definition for the habitable zone as the region around a star where liquid water can exist on the surface of a planet.

If a planet is too close to its parent star, it will be too hot, causing its oceans to evaporate and be lost to space. If the planet is too far, it will be too cold, and its oceans will freeze.

The Sun's habitable zone is presently estimated to range from about 0.95 to 1.67 Astronomical Units (AU) (One AU is the average distance from the Sun to the Earth, about 93 million miles, or about 150 million km.).

Other stars have habitable zones of various sizes and distances, depending on how bright they are (and their spectral type as well).

Stars become brighter as they age, pushing the habitable zone further from the star and possibly bringing a period of warmth and life to planets originally too far away.

The team focused in particular on the movement of the habitable zone when stars reach old age, called the sub-giant and red giant phases.

The team calculated the evolution of the habitable zone for stars with the same mass as the Sun, and for stars with 1.5 and 2 times the Sun's mass.

The team compared the duration of the outward movement or transit of the habitable zone to the estimated time required for the emergence of life. Currently there is only one example for comparison: the development of life on Earth.

The earliest known fossils are about 3.5 billion years old, from bacteria existing about a billion years after our planet's formation.

Life could have evolved even earlier, but older fossils are hard to find since Earth's active geology has long since recycled the oldest rocks through the restless churning of the Earth's crustal plates.

There is indirect evidence that life existed a few hundred million years before the oldest fossils, from the analysis of carbon isotopes. The team used this data to give a range of between half a billion to a billion years for the emergence of life.

The transit of the habitable zone, for planets between 2 and 9 AU from a solar-mass star, lasts from a few hundred million years to a couple billion years, according to the team.

This is about the same amount of time as the estimate for the development of life. "The temporal transit of the habitable zone does not appear incompatible with the possible duration for the development of life," said co-author Dr. Jean Schneider of the Observatoire de Paris, France.

Mars is a small planet with a thin atmosphere that does not hold heat well, so even though Mars is just inside the estimated outer limit of the Sun's habitable zone, it remains a frozen world today.

However, a few billion years from now, the inner limit of the Sun's habitable zone will move out from Earth to Mars.

"Mars will be in the habitable zone for a couple billion years, so Martian life may get a second chance," said Dr. William Danchi of NASA's Goddard Space Flight Center, Greenbelt, Md., also a co-author on the paper.

Terrestrial life may get a second chance as well. Microbes are capable of surviving in space indefinitely, and many live in rocks on Earth. Meteorite impacts are capable of blasting rocks into space, some of which eventually land on another planet in our solar system.

Astronomers have calculated that there is also a reasonable probability that bacteria in rocks could be transported between two planets by meteorite impacts during typical habitable zone transit times.

"As the Sun grows ever brighter and Earth overheats, terrestrial life might hitch a ride on a meteor and find a new home on Mars," said Danchi.

"Transport of existing life between worlds could jump-start its emergence on outer planets, allowing it to exist even if a star's habitable zone transit is too fast for life to be newly created."

The team estimates that about 150 sub-giant or red giant stars are close enough (within 100 light years) for proposed planet-finding missions to observe signatures of life in the atmospheres of planets that may be orbiting these stars.

Using Earth's atmosphere as a model, researchers will look for light emitted by concentrations of molecules indicative of biological processes.

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