Galactic Habitable Zones
The Galactic Habitable Zone (hereafter GHZ) extends the idea of a CHZ to describe the regions of the Milky Way Galaxy where conditions are not inhospitable to the development of complex life. The basic idea is that a number of physical processes that may either favor or hinder the developement of intelligent life depend strongly upon location (and era) in the Milky Way.
Metals (elements other than Hydrogen and Helium) are the building blocks of planets. Terrestrial planets are formed exclusively from metals. Metals also form the cores of Jovian Planets and there is evidence suggesting that it is not possible to have Jovian planets without metals. Recent studies have found that stars with extrasolar planets have high metallicities whereas well-studied stars that do not have extrasolar planets have low metallicities. Some researchers have argued that there is a threshold value of metallicity necessary for terrestrial planet formation.
The amount of metals in the interstellar medium varies with location in the Milky Way. These elements are produced in nuclear reactions in the cores of massive stars and distributed through supernovae explosions into the interstellar medium. Two metallicity trends are apparent in the Milky Way disk: metallicity has been generally increasing over time and metallicity is larger near the center where star formation is more substantial than toward the disk's edge.
Supernovae pose a great danger to the development of complex life. If one occurred within 10 parsecs of Earth the high-energy photons and protons would obliterate the ozone, leaving land animals unprotected from the Sun's ultraviolet radiation (marine life would be largely unaffected). This danger predominantly occurs when the Sun passes through a spiral arm. Researchers have estimated that lethal supernovae at the Sun's location occur with a frequency on the order of 1 Gyr-1. This frequency is much higher in the inner regions of the Milky Way than the outer regions. There is also a time dependence to the supernova danger since they were much more common soon after the disk formed than they are today. Our solar system orbits in the Milky Way at a rate pretty close to the rate at which spiral arms rotate, thus we pass through a spiral arm very infrequently.
Another threat to life comes from the massive black hole at the center of the Milky Way. Although presently inactive, periodically large amounts of matter will spiral into the black hole and be devoured. As matter falls into the black hole it is heated to extremely high temperatures and will release very energetic photons and charged particles. This radiation would threaten life in the inner regions of the Milky Way.
Another potential problem is that our solar system is surround by a vast swarm of comet nuclei known as the Oort Cloud. Periodically one of these objects passes through the inner solar system and we see it as a long period comet. However, if our solar system passes close enough to another star so that its gravity greatly disturbs the Oort Cloud, a host of comet nuclei projectiles may assault the inner solar system. This type of perturbation would occur far more often in the packed inner regions of the Milky Way than the sparse outer regions.
The GHZ depends on the balance of two opposing trends. Metallicity decreases as one moves outward in the Milky Way, decreasing the number of potential planets. On the other hand, there are a number of environmental dangers to life associated with the packed inner regions of the Milky Way. Keep in mind that although all of the GHZ factors mentioned above are rooted in solid science, several are very difficult to quantify. The GHZ is an area of research that is still in its infancy and its validity has been challenged by some researchers.