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This artist’s impression shows the planet HD 85512b orbiting the Sun-like
star HD 85512 about 35 light-years from Earth. This planet is about 3.6 times as
massive as the Earth is at the edge of the habitable zone around the star, where
liquid water, and perhaps even life, could potentially exist.
CREDIT: ESO/M. Kornmesser
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We know for certain that life exists in the Milky Way galaxy: that life is
us. Scientists are continually looking to understand more about how life on our
planet came to be and the conditions that must be met for its survival, and
whether those conditions can be replicated elsewhere in the universe. It turns
out that looking at our entire galaxy, rather than focusing just on life-giving
properties of our planet or indeed the habitability of regions of our own solar
system, is a good place to start.
How far our planet orbits from the sun, along with other factors such as
atmospheric composition, a carbon cycle and the existence of water, has told
astronomers much about the conditions that are required for life to not only
originate, but to survive on rocky worlds. This distance from a star is referred
to, quite simply, as the 'Habitable Zone' or sometimes the 'Goldilocks Zone''
because conditions here are neither too hot or too cold for water to be liquid
on the planet’s surface —
conditions
just right for life as we know it to thrive. [
Gallery: The
Strangest Alien Planets]
Copernican theory tells us that our world is a typical rocky planet in a
typical planetary system. This concept has spurred some astronomers to start
thinking bigger, way beyond the simplicity of any one planetary system and
instead towards much grander scales.
Astronomers are exploring whether there is a Galactic Habitable Zone (GHZ) in
our Galaxy – a region of the Milky Way that is conducive to forming
planetary
systems with habitable worlds. The Galactic Habitable Zone implies that if
there are conditions just right for a planet around a star, then the same must
go for a galaxy.
This concept was first introduced by geologist and paleontologist Peter Ward
and Donald Brownlee, an astronomer and astrobiologist, in their book, '
Rare Earth.' The idea of
a GHZ served as an antagonistic view point to the Copernican principle.
Despite scientists such as Carl Sagan and Frank Drake favoring the theory of
mediocrity based on the Copernican model, which supports the probability of the
universe hosting other forms of complex life, Ward and Brownlee were certain our
Earth and the conditions within our galaxy that allowed such life to evolve are
both extremely rare. Their answer to the famous Fermi paradox – if
extraterrestrial aliens are common, why is their existence not obvious? – is
that alien life more complex than microbes is not very common at all, requiring
a number of factors, each of low possibility, to come into play.
In short, Ward and Brownlee were suggesting that much of the galaxy was
inhospitable
to complex life. In their view, only a narrow belt around the galaxy was
fertile: the Galactic Habitable Zone.
Since then, many astronomers have looked at the idea of the GHZ. Not all
believe that it necessarily supports Ward and Brownlee's Rare Earth
hypothesis.
One recent assessment of the GHZ, by Michael Gowanlock of NASA's Astrobiology
Institute, and his Trent University colleagues David Patton and Sabine
McConnell, has suggested that while the inner sector of the MIlky Way galaxy may
be the most dangerous, it is also most likely to support habitable worlds. [
Infographic: How
Alien Solar Systems Stack Up]
A multi-wavelength image of the Milky Way's center. It is towards the
galactic center where the highest number of stars and rocky planets reside, but
also where the most supernovae occur.
CREDIT:
NASA/JPL-Caltech/ESA/CXC/STScI.
Their paper, accepted for publication in the journal Astrobiology, modeled
habitability in the Milky Way based on three factors: supernova rates,
metallicity (the abundance of heavy elements, used as a proxy for planet
formation) and the time taken for complex life to evolve. They found that
although the greater density of stars in the inner galaxy (out to a distance of
8,100 light-years from the galactic center) meant that more supernovas exploded,
with more planets becoming sterilized by the radiation from these exploding
stars, the
chances
of finding a habitable planet there was 10 times more likely than in the
outer galaxy.
This contradicts previous studies that, for example, suggested the GHZ to be
a belt around the galaxy between distances of 22,800 light-years and 29,300
light-years from the galactic center. What's noticeable is that our sun orbits
the galaxy at a distance of about 26,000 light-years – far outside GHZ proposed
by Gowanlock's team.
Why is their proposed galactic habitable zone so different? [
How
Do Astronomers Find Alien Planets?]
"We assume that metallicity scales with planet formation," Gowanlock
said.
Heavy elements are produced by dying stars, and the more generations of stars
there have been, the greater the production of these elements (or ‘metals’ as
they are termed by astronomers). Historically, the greatest amount of star
formation has occurred in the inner region of the Milky Way.
"The inner galaxy is the most metal-rich, and the outer galaxy is the most
metal-poor. Therefore the number of planets is highest in the inner galaxy, as
the metallicity and stellar density is the highest in this region," Gowanlock
said.
However, amongst so much star formation lurks a danger: supernovae.
Gowanlock’s team modeled the effects of the two most common forms of supernovae
– the accreting white dwarfs that produce type Ia supernovas, and the collapsing
massive stars of type II supernovae.
Measurements of the galactic abundance of the isotope aluminum-26, which is a
common by-product of type II supernovas, have allowed astronomers to ascertain
that a supernova explodes on average once every 50 years. Meanwhile, previous
studies have indicated that a supernova can have a deleterious effect on any
habitable planet within 30 light- years.
A supernova sterilizes an alien world in this artist's impression.
CREDIT: David A Aguilar (CfA)
"In our model, we assume that the build-up of oxygen and the ozone layer is
required for the emergence of complex life," Gowanlock said. "Supernovae can
deplete the ozone in an atmosphere. Therefore, the survival of land-based
complex life is at risk when a nearby supernova sufficiently depletes a great
fraction of the ozone in a planet's atmosphere." [
Supernova
Photos: Great Images of Star Explosions]
The team discovered that at some time in their lives, the majority of stars
in our galaxy will be bathed in the radiation from a nearby supernova, whereas
around 30 percent of stars remain untouched or unsterilized.
"Sterilization occurs on a planet that is roughly [at a distance] between 6.5
to 98 light-years, depending on the supernovae," Gowanlock said. "In our model,
the sterilization distances are not equal, as some supernovae are more lethal
than others."
Although the outer regions of the galaxy, with their lower density of stars
and fewer supernovas, are generally safer, the higher metallicity in the inner
galaxy means that the chances of finding an unsterilized, habitable world are
ten times greater, according to Gowanlock's model. However, their model does not
stipulate any region of the galaxy to be uninhabitable, only that it’s less
likely to find habitable planets elsewhere.
This explains why our Solar System can reside far outside of the inner
region, and it also gives hope to SETI – Gowanlock's model proposes that there
are regions of the galaxy even more likely to have life, and many SETI searches
are already targeted towards the galactic center. [
Field
Guide to Alien Planets]
However, not all are in favor of the new model. Ward and Brownlee noted that
the sun's position in the galaxy is far more favorable because planets that
dance around stars that are too close to the galactic center are more likely to
suffer from a perturbed orbit by the gravity of another star that has wandered
too close. Others question some of the assumptions made in the research, such as
the accuracy of the percentage of
planets
that are habitable in the galaxy (1.2 percent), or that tidally-locked
worlds can be habitable.
“The authors may be making some assumptions that aren’t too well justified,”
said Jim Kasting of Penn State University and author of "How to Find a Habitable
Planet." "They seem well ahead of the rest of us who are still pondering these
questions."
However, others believe that the research is promising. "This is one of the
most complete studies of the Galactic Habitable Zone to date," said Lewis
Dartnell, an astrobiologist at University College London. "The results are
intriguing, finding that white dwarf supernovae are over five times more lethal
to complex life on habitable worlds than core collapse supernovae."
The GHZ isn't static; the research paper written by Gowanlock's team points
out that over time the metallicity of the galaxy will begin to increase the
farther out one travels from the galactic center.
"This is why stars that form at a later date have a greater chance of having
terrestrial planets," Gowanlock said. As a result, perhaps the heyday for life
in our galaxy is yet to come.
This story was provided by Astrobiology Magazine, a web-based
publication sponsored by the NASA astrobiology program.
