[meteorite-list] Dying Stars May Bring Life to Frozen Worlds

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Mon Mar 28 15:53:22 2005
Message-ID: <200503282052.j2SKqxJ04102_at_zagami.jpl.nasa.gov>

NASA-GODDARD SPACE FLIGHT CENTER
NEWS RELEASE


Susan Hendrix March 28, 2005
NASA Goddard Space Flight Center
301 286-7745

RELEASE: 05-14

DYING STARS MAY BRING LIFE TO FROZEN WORLDS

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 of the evolution of habitable zones around stars 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 Dr. Bruno Lopez of the
Observatoire de la Cote d'Azur, Nice, France. Dr. Lopez is lead author of an
article about this research 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. This research was funded by grants from NASA and
Institut National des Sciences de l'Univers/Centre National de la Recherche
Scientifique. For images and more information, refer to:


http://www.nasa.gov/centers/goddard/news/topstory/2004/0801frozenworlds.html

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Received on Mon 28 Mar 2005 03:52:58 PM PST