WASHINGTON, March 31 (UPI) -- Several billion years from now, the sun will stop being the relatively calm, steadily shining, middling star we know so well.
On that day -- the late astronomer Carl Sagan called it "the last perfect day on Earth" -- the sun's familiar white-yellow light will fade, turning an orangish-red. The hydrogen fuel for its nuclear-fusion reactor depleted, the million-mile-wide ball of hot gas will begin to grow -- and grow.
Eventually, its diameter will become big enough to swallow Earth, summarily ending our home planet's existence.
Bad news for whatever creatures happen to populate Earth at that time, but good -- albeit temporary -- news for the outer planets and their moons. There, the sun will melt any water ice and possibly provide a stable environment long enough for life to emerge and flourish.
That is the conclusion of an international team of astronomers that has been studying the effect of red-giant stars on outer planetary systems, like Jupiter, Saturn and possibly Uranus and Neptune.
"Our result indicates that searches for life-giving worlds outside our solar system should include planets around old stars," said Bruno Lopez of the Observatoire de la Cote d'Azur in Nice, France.
The key is whether outer planets contain water ice, which under the right conditions can be melted to provide liquid water and, if the planet is large enough, an atmosphere.
Planets containing water and breathable air are presumed to exist only within an area called a habitable zone around a star. Exobiologists estimate that the sun's habitable zone ranges from about 0.95 to 1.67 astronomical units -- one AU being the average distance between the sun and Earth, or about 93 million miles (about 150 million kilometers).
A habitable zone around another star depends primarily on that star's size, brightness and output of ultraviolet light, which can be lethal to life forms if too intense.
The other factor is the star's age. As sunlike stars grow brighter and expand, they push their habitable zones outward, creating periods of warmth and, possibly, life to planets and moons that were too far away originally.
The research team focused on the potential habitable zones of stars reaching the sub-giant and red giant phases. For such stars, with masses of up to slightly less than 10 times the sun's mass, there may be a temporal window within which life could gain a foothold.
The earliest known fossils on Earth are about 3.5 billion years old. With the planet's age estimated at about 4.5 billion years, that means the bacteria creating those first fossils emerged about 1 billion years after Earth's formation. Scientists think life probably evolved considerably earlier, but the fossil evidence is missing, buried or destroyed by Earth's active geology.
Based on inferences gathered from analyses of carbon isotopes, which provide reliable estimates of age, a case can be made that the first living organisms appeared on Earth -- assuming they were not planted here by asteroid or meteorite impact -- a few hundred million years earlier than the oldest fossils.
That means it is theoretically possible for life to begin on a planet about 500 million years or so after its formation -- or after the emergence of a habitable zone.
Astronomers estimate the red-giant stage of stars lasts from a few hundred-million years to maybe 2 billion years -- or just about enough time to allow life to develop.
"The temporal transit of the habitable zone does not appear incompatible with the possible duration for the development of life," said team member Jean Schneider of the Observatoire de Paris.
How many stars in the sun's part of the Milky Way galaxy constitute candidates for second-chance life on outer planets? The research team said about 150 such stars lay within about 100 light-years, or close enough for next-generation observatories to search for signatures of life in the atmospheres of planets that may be orbiting them.
When doomsday finally comes for Earth, the planet that might benefit most immediately is its next door neighbor, Mars.
"Mars will be in the habitable zone for a couple billion years, so Martian life may get a second chance," said another team member, William Danchi of NASA's Goddard Space Flight Center in Greenbelt, Md.
Even Earth's life could receive a second chance in the process. As the sun grows and eventually envelopes the planet, Earth's crust will be pulverized by the solar mass. Millions and millions of rocks will be blown out farther into the solar system.
Many of those rocks will carry terrestrial microbes, which are capable of surviving in space indefinitely, and some no doubt will land on other planets and moons in the solar system as meteorites. If those worlds lie within the new habitable zone, there is a chance some of the microbes will find niches to revive and prosper.
"Transport of existing life between worlds could jump-start its emergence on outer planets," Danchi said, "allowing it to exist even if a star's habitable zone transit is too fast for life to be newly created."
In the Stars is a weekly series examining new discoveries about the cosmos. E-mail: [email protected]