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In Search of the Milky Way's Habitable Zone

Our Milky Way Galaxy is structured much like billions of other spiral galaxies. The galactic disk contains a lot of interstellar matter (like dust and gas), as well as young and intermediate-age stars. While young stars can be found scattered throughout the Galaxy, the stellar population tends to be older in the bulge around the galactic center.
Image Credit: NASA/STScI
by Leslie Mullen
for NASA Astrobiology News
Moffett Field - May 24, 2001
Our Milky Way Galaxy is unusual in that it is one of the most massive galaxies in the nearby universe. Our Solar System also seems to have qualities that make it rather unique. According to Guillermo Gonzalez, Assistant Professor of Astronomy at the University of Washington, these qualities make the Sun one of the few stars in the Galaxy capable of supporting complex life.

For one thing, the Sun is composed of the right amount of "metals." (Astronomers refer to all elements heavier than hydrogen and helium as "metals.")

Moreover, the Sun's circular orbit about the galactic center is just right; through a combination of factors it manages to keep out of the way of the Galaxy's dangerous spiral arms. Our Solar System is also far enough away from the galactic center to not have to worry about disruptive gravitational forces or too much radiation.

When all of these factors occur together, they create a region of space that Gonzalez calls a "Galactic Habitable Zone." Gonzalez believes every form of life on our planet - from the simplest bacteria to the most complex animal - owes its existence to the balance of these unique conditions.

Because of this, states Gonzalez, "I believe both simple life and complex life are very rare, but complex life, like us, is probably unique in the observable Universe."

"I think this is a very, very interesting idea," says Dr. William Borucki, a research scientist in the Planetary Studies Branch of the NASA-Ames Research Center. "I'm delighted to see this theory. I like how Gonzalez has imagined the consequences of planets existing at different parts of the Galaxy. Now scientists need to check the math to make sure it all adds up."

The Theory in Detail

Our Milky Way Galaxy is structured much like billions of other spiral galaxies. The galactic disk contains a lot of interstellar matter (like dust and gas), as well as young and intermediate-age stars. While young stars can be found scattered throughout the Galaxy, the stellar population tends to be older in the bulge around the galactic center.

Many of these older stars are gathered together into globular clusters, which orbit the nucleus of the Galaxy in a region known as the galactic "halo." Strong emissions of infrared radiation and X-rays from the galactic center indicate clouds of ionized gas rapidly moving around some sort of supermassive object, quite possibly a black hole.

There are billions of stars in the Milky Way Galaxy, and some are more metal-rich than others. Part of this is a condition of age: The older a star, the more metal-poor it tends to be. That's because the most ancient stars formed from just hydrogen, helium, and lithium.

When the most massive of these stars exploded, nuclear reactions fused these light elements into heavier ones. These heavier "metals" became part of the raw material from which a second generation of stars formed. Each stellar explosion led to a greater abundance of available metals. A metal-rich star, therefore, has material that came from many previous generations of stars.

Our Sun is unusually metal-rich for a star of its age and type. Scientists aren't sure why. It could be that the Sun formed in a part of the Galaxy that had an abundance of metals, and then migrated to its present position.

Based on studies of extrasolar planets, metal-rich stars are more likely to have planets orbiting around them. One reason for this may be that a certain minimum amount of metals is needed to form rocky bodies (including the cores of the gas giant planets). A metal-rich interstellar cloud that collapses to form a star would therefore be more likely to form planets than would a metal-poor cloud.

Besides requiring a metal-rich star, a Galactic Habitable Zone excludes stars too close to the galactic center. Our Sun is a nice distance away from the galactic center, about 28,000 light years.

Being in the outer region of the Galaxy protects our Solar System from the huge gravitational tug of stars clustered near the galactic center. If we were closer in, the combined gravity of all those stars would perturb the orbit of comets in the Oort cloud.

The Oort cloud, which circles the outer perimeter of our Solar System, contains trillions of comets. The gravitational disturbances caused by other stars would send many of those comets in our direction - increasing the rate of comet impacts and endangering - if not eventually wiping out - life on Earth.

Staying away from the galactic center has an additional advantage. The center of the Galaxy is awash in harmful radiation. Solar systems near the center would experience increased exposure to gamma rays, X-rays, and cosmic rays, which would destroy any life trying to evolve on a planet.

"Large, complex organisms are much more sensitive to environmental perturbations than simple life," says Gonzalez. "Our hypothesis deals exclusively with complex life, more specifically, aerobic macroscopic metazoan life. The effects of radiation would damage the ozone layer, as well as increase radiation levels at the surface of a planet from secondary particle cascades in the atmosphere."

Keeping out of the way of the Galaxy's spiral arms is another requirement of the Galactic Habitable Zone.

The density of gases and interstellar matter in the spiral arms leads to the formation of new stars. Although these spiral arms are the birthplaces of stars, it would be dangerous for our solar system to cross through one of them.

The intense radiation and gravitation of a spiral arm would cause disruptions in our Solar System just as surely as if we were closer to the center of the Galaxy.

Luckily, our Sun revolves at the same rate as the Galaxy's spiral-arm rotation. This synchronization prevents our Solar System from crossing a spiral arm too often.

"At our location, our orbital period is very similar to that of the pattern speed of the spiral arms," says Gonzalez. "This means that the time interval between spiral arm crossings will be a maximum, which is a good thing, since spiral arms are dangerous places. Massive star supernovae are concentrated there, and giant molecular clouds can perturb the Oort cloud comets leading to more comets showers in the inner solar system."

The unusually circular orbit of our Sun around the galactic center also tends to keep it clear of the spiral arms. Most stars the same age as our Sun have more elliptical orbits.

"If the Sun's orbit about the galactic center were less circular," says Gonzalez, "the Sun would be more likely to cross spiral arms."

Thus, thanks to a lot of unusual characteristics of our Sun, our Solar System is lucky enough to lie in a Galactic Habitable Zone. Gonzalez argues that these characteristics made it possible for complex life to emerge on Earth.

More than 95 percent of stars in the Galaxy, says Gonzalez, wouldn't be able to support habitable planets simply because their rotation is not synchronized with the rotation of the galaxy's spiral arms.

Add all the other factors involved in keeping a solar system habitable, and it seems that the odds of finding another solar system in a Galactic Habitable Zone are close to impossible.

"This is a good theory," says Borucki. "I think this idea is a spark that will initiate similar research. Like a spark plug, it can't drive the car, but it provides the necessary impetus to get the car moving."

What's Next?

Gonzalez says he plans to continue his studies on the limitations of life in the Universe. He and his colleagues are working on a paper that discusses such dangers from space as transient radiation sources and large comet or asteroid impacts.

This article was originally pubished as part of Astrobiology News
for the NASA Astrobiology Institute.

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Reflections From a Warm Little Pond
University Park - May 8, 2001
Back in 1953, Jim Kasting said, scientists thought they had the origin of life figured out. Chemists Stanley Miller and Harold Urey at the University of Chicago had simulated that crucial instant around 3.9 billion years ago when a batch of simple inorganic molecules, zapped by a bolt of lightning (or maybe just the sun's warmth during a break in the clouds), fell together to form the prototypes for the complex organic compounds that life is made from.

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