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by Robert Roy Britt (www.space.com)
May 28, 2002
It took 4.5 billion years for Earth
to generate and evolve a life form that could think, reason, and finally
fly off the planet. That's a long time, even by cosmic measures. Perhaps
too long.
At a time when the only known
sentient species has earnestly and optimistically begun to search for life
on other planets, several scientists within that species have found a host
of reasons to guard the optimism. Throughout the galaxy, hazards to planet
formation and sustained evolution are so serious and varied that life may
be exceedingly rare. Intelligent life, presumably, would be the rarest of
all.
We may, it turns out, be very lucky
to be here. However, we may also turn out to be very alone.
Location, location, location
Guillermo Gonzalez, an Iowa State
University expert in stellar evolution, says there are relatively small
bands and patches of the Milky Way Galaxy that he considers to be
habitable regions. There are places where conditions are just right for
the formation of planets and where things stay calm enough, long enough,
to allow the evolution of anything but the lowest forms of life.
Our Sun happens to be in one of
these Goldilocks zones. For now, at least.
Gonzalez has examined the structure
of the galaxy and the amount of heavy elements distributed through it. The
central region of the galaxy, he says, is far to cramped and chaotic to
expect Earth-like planets to have much chance of developing and remaining
stable.
Planetary systems, if they are like
ours, are expected to include outer belts of comets, like our own Oort
cloud which extends beyond Pluto. Near the center of the galaxy, which all
astronomers agree is more densely packed with stars, close encounters
between stars would gravitationally boot more of these comets into the
inner reaches of a solar system, where the planets would be.
Further, because there is a greater
concentration of heavy elements -- carbon, iron and other stuff that
weighs more than hydrogen and helium -- near the galactic center, Gonzalez
said more comets and asteroids would probably develop.
"Comet showers should be more
common," Gonzalez said at a meeting titled "Astrophysics of Life" earlier
this month at the Space Telescope Science Institute (STScI).
Conversely, the outer reaches of
the galaxy are relatively lean in the heavier elements, making planet
formation difficult according to present theories. Other researchers have
doubted this assertion, suggesting that regardless of the abundance of
heavy elements in a star's environment, the quantities can vary greatly
within a solar system, as has been observed in our own.
Galactic Habitable Zone
There are other hazards, however.
The pronounced spiral arms of the Milky Way are regions where star
formation is more frequent and intense. As with the center of the galaxy,
gravitational chaos and heavy radiation in the arms is not conducive to
long-running biological evolution.
Between the spiral arms is the only
safe place, Gonzalez has been saying in recent years.
However, while stars orbit the
galactic center, they are all on different courses in relation to the
spiral arms and the main, fairly flat disk of the galaxy. Some stars,
during their lifetimes, cross the spiral arms, and others do not. Other
stars travel above, below and perilously through the main plane.
The Galactic Habitable Zone, which
it has come to be called is, then, actually several shifty places whose
bounds are as-yet unclear.
"We don't know where it is
exactly," Gonzalez said. "We think it's in the thin disk of the galaxy. It
excludes the center of the galaxy, and it excludes the outer edge of the
galaxy."
And where are we?
"We're between spiral arms. We're
going to stay between spiral arms for a long time."
Though Gonzalez figures the Sun
"undoubtedly" experienced more dangerous regions of space in the past, he
says it isn't possible to figure out where we were prior to a few hundred
million years ago. What he does know is that our solar system moves around
the galaxy at a pace and direction that is similar to the nearest spiral
arms, so we will not soon crash through one or be overtaken.
Bully stars
Like Gonzalez, John Bally would
love to know where the Sun was born. Bally, of the University of Colorado,
examines the birthplaces of stars and has learned that the vast majority
are generated from giant clouds of hydrogen that spawn not one, but many
stars in a huge and dense nursery of nearly simultaneous birth.
It's anything but a pleasant womb.
The clusters are violent, chaotic
places whose largest stars -- superhot, massive, short-lived objects --
bathe the smaller ones in heavy doses of ultraviolet radiation that can
destroy the seeds of planets before they ever form.
Here's what happens:
When stars form, some or perhaps
most leave a circle of debris -- gas and dust -- that develops into
rotating mass called a protoplanetary disk. From this material, planets,
asteroids and comets are thought to form. At least that's how it probably
happened in our solar system. Scientists are only beginning to spot and
study these dusty disks around other, relatively nearby stars, and they've
seen clumps that hint at planets in the making.
But nearby stars share the relative
calm pocket of space -- one of Gonzalez's habitable zones -- that our Sun
benefits from. Most star formation occurs in clusters, and it the bulk of
it takes place in the spiral arms.
In these clusters, a few massive
stars shine thousands or even millions of times more brightly than our
Sun. Their UV radiation eats away at the dust disks of other stars, and
can strip the planetary seeds from all nearby stars over the course of a
million years or less.
Limited time
Meanwhile, other wild interactions
are taking place in the dense star clusters. When a cloud of hydrogen
forms stars, it does so because it contracts and begins to spin. Clumps of
greater density form here and there, Bally explained, and these clumps
gain spin and collapse to form stars.
"Nearby interactions with multiple
stars in a rich cluster can truncate and even completely eliminate
protoplanetary disks," Bally said.
All the while, the rotation and
gravitational forces can fling stars hundreds of light-years from their
birthplaces. After a few million years, many of the stars escape the worst
radiation environments. And the large, bully stars pay a price for all
their energetic activity -- they typically die within 40 million years.
In an interview, Bally said his
research and that of others shows that around most stars, there is a tight
time constraint on when planets must form before the star's dust disk is
blown away.
"I'm not saying planets can't
form," he said. "But you have only 100,000 years to a million years."
Given the roughly 300 billion stars
in our galaxy, Bally's limits would still allow for plenty of planets out
there, but it could also mean there are far fewer than some researchers
have expected. "Either planetary systems form very fast," Bally said, "or
we will find planet development to be rare. Something like 5 percent of
stars will have planets."
However, if a planet can form
quickly -- and theorists are not sure just how long this process takes --
then the radiation is irrelevant, Bally said, and his constraints would be
largely lifted.
Our own Sun may have been born in a
cluster and later tossed out, but Bally said it's not yet possible to
figure out if that was the case.
Other perils
If a planet finds itself around a
star that fortuitously plans to hang out for a long time in a habitable
zone of the galaxy, its odds of supporting life -- especially any kind of
intelligent life -- are still slim. One only needs consider our own solar
system, where intelligent life exists on just one out of nine planets.
Most planets probably do not end up
in habitable zones around their stars -- slim orbital paths where
radiation from the star is just enough to support life but not so much
that it evaporates the oceans away. [The terms "habitable zone" and
"Goldilocks zone" were originally devised to describe these favorable
swaths around stars.]
Problem is, planetary habitable
zones shift, too.
Kevin Zahnle, an astrobiologist at
NASA's Ames Research Center, said our Sun has gotten significantly
brighter during its roughly 4.6 billion-year life. It emits 30 to 40
percent more radiation than when Earth was born. Luckily, and possibly
because life is present and moderates the change by evolving and modifying
the atmosphere, Earth's surface temperature has remained about the same,
he said.
Until other possible Earth-like
planets are found and studied, no one can say whether it is commonly
possible to preserve such a delicate balance.
Eventually, Earth will be
overwhelmed by the change. Within the next 5 billion years or so, the
aging Sun will have swollen so much that it envelops and vaporizes Earth.
In just a billion years, the Sun could be 11 percent brighter than now,
turning the planet into an inhospitable greenhouse. Had it taken another
billion years for humans to evolve, only some real lowlifes would have
been around to deal with the problem.
Only the smart survive
Long before we fry, another
asteroid or comet will strike Earth. Every 100,000 years or so, leading
experts agree, an impact large enough to threaten civilization occurs. If
other planets are anything like our own, they too would face this peril of
bombardment.
Christopher Chyba of the SETI
Institute has theorized that only the smart can survive. A civilization
must evolve to the point that it can detect and then either detour or
destroy threats from space, lest it be rendered extinct or, at best,
plunged back into a Dark Ages existence.
"There is a kind of selection
effect for long-lived civilizations," Chyba said in comments to a group of
reporters during the STScI conference. "If you want to be long lived, you
need to become technical because you need to be able to observe the impact
environment around you and respond to that environment in some way to
mitigate its effects on your planet."
Chyba pointed out that it took 700
million years or so for life to begin on Earth. The planet had to cool
down after its initial formation, and then it weathered a barrage of
asteroid impacts. The largest objects might well have evaporated the
oceans, he said, preventing the origin of life or resetting it if it had
already occurred.
"Our solar system tells you that
life isn't going to be much younger than a billion years," Chyba said.
"It's going to take that long for the planet to be capable of supporting
life at all. It couldn't be 10,000 years. It couldn't be a million years."
Even in an ideal world, there are
hazards and limits to life at both ends. Along the way, it is no picnic.
Life is tough, these theorists all recognize. But it is not impossible. At
least one planet has proved that.
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