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Doomed planets, new life
Dying stars
can temporarily heat up frozen planets long enough to bring them new life,
scientists say—but not much longer. If intelligent beings happened to evolve during this time, they would have
nerve-wracking lives once they figured out the situation.
Posted March 30, 2005
Courtesy NASA Goddard Space Flight Center
and World Science Staff
Sometimes, the very fact that a planet is
approaching its doomsday can buy it a brief new life, researchers have found.
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In this artist's conception, a
sun-like star grows into its red giant phase, increasing in size and luminosity. Energy in the form of heat can now reach a once-frozen and dead moon. The icy surface quickly melts into liquid water, filling in old craters with warmer seas. The stage is now set for the possible formation of new life.
(Credit: NASA)
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Dying stars may warm previously frozen worlds around them to the point where liquid water temperature exists long enough for life to form, according to a new analysis by an international team of astronomers.
“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, Nice, France. Lopez is lead author of an article about this research to appear in the Astrophysical Journal.
Known forms of life require liquid water. Therefore, astronomers believe each star has a “habitable zone,” a doughnut-shaped region in space around it that is neither too hot nor too cold for liquid water to exist. Planets within that zone can possibly harbor life.
Dying stars undergo a sort of last gasp called red giant stage, in which they expand tremendously before completely running out of fuel. This process pushes their habitable zones outward, where it can warm and liquefy water on new planets.
Unfortunately, according to the group’s calculations, this revival would gives life barely enough time to evolve to the bacterial stage, at least judging by how long it took for organisms of various levels of complexity to arise on Earth.
If a lucky break somehow let creatures of human intelligence to arise on one of these last-chance worlds, these beings might have
a nerve-wracking existence after they figured out what was going on.
They might have a precious short time to enjoy their stay in the universe, unless they figured out some way to get out in time. Stars eventually burn out their fuel and die, abandoning their entourage of planets to a frigid, black existence.
Our sun is estimated to be about halfway through its life, meaning we have about 5 billion years left to hatch an escape plan, if we’re still around then.
The Sun’s habitable zone is presently estimated to range from about 0.95 to 1.67 times the average Earth-sun distance. Other stars have habitable zones of various sizes and distances, depending on their brightness and other properties.
The researchers focused 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. On earth, it’s estimated that it took half a billion years to one billion for life to emerge. It took another four billion for the first multicellular creatures to arise and to emerge from the sea.
The transit of the habitable zone lasts from a few hundred million years to a couple billion years, for planets whose distance from their sun is between 2 and 9 times the Earth-sun distance, the team found. This is about the same amount of time as the estimate for the development of life, but not much more.
“The temporal transit of the habitable zone does not appear incompatible with the possible duration for the development of life,” said co-author 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 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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