Galactic Habitable Zone
The notion that the location of a planetary system within a galaxy must also be favorable to the development of life has led to the concept of a Galactic Habitable Zone (GHZ), developed in 1995 by Guillermo Gonzalez although the concept has been challenged.
Planetary habitability theory suggests star systems favourable to life should be located close enough to the galactic center for sufficient levels of heavy elements to form rocky (terrestrial) planets. (This may not preclude life existing on gas giants or gaseous planets which may be more common elsewhere, however life on gas giants (like Jupiter and Saturn) is currently considered less likely.) On the other hand, the planetary system must be far enough from the galactic center, so it would not be affected by dangerous high-frequency radiation, which would damage any carbon-based life. A way for life to evolve despite these opposing requirements is that the Sun may have originated nearer the center but have migrated outwards.
Also, most of the stars in the galactic center are old, unstable, dying stars, meaning few or no stars form in the galactic center. Some types of spiral galaxies in later time periods have been depleted of gas and dust in regions near to the galactic center, resulting in minimal new star formation in those parts of the galaxy. Because terrestrial planets form from the same types of nebulae as stars, it can be reasoned if stars cannot form in the galactic center, terrestrial planets cannot, either.
In our galaxy (the Milky Way), the GHZ is currently believed to be a slowly expanding region approximately 25,000 light years (8 kiloparsecs) from the galactic core and some 6,000 light years (2 kiloparsecs) in width, containing stars roughly 4 billion to 8 billion years old. Other galaxies differ in their compositions, and may have a larger or smaller GHZ – or none at all.
In 2008, a team of scientists described extensive computer simulations in the Astrophysical Journal that show that, at least in galaxies similar to the Milky Way, stars such as the Sun can migrate great distances, thus challenging the notion that parts of these galaxies are more conducive to supporting life than other areas.
Read more about this topic: Habitable Zone
Famous quotes containing the words galactic, habitable and/or zone:
“In sci-fi convention, life-forms that hadn’t developed space travel were mere prehistory—horse-shoe crabs of the cosmic scene—and something of the humiliation of being stuck on a provincial planet in a galactic backwater has stayed with me ever since.”
—Barbara Ehrenreich (b. 1941)
“If its individual citizens, to a man, are to be believed, it always is depressed, and always is stagnated, and always is at an alarming crisis, and never was otherwise; though as a body, they are ready to make oath upon the Evangelists, at any hour of the day or night, that it is the most thriving and prosperous of all countries on the habitable globe.”
—Charles Dickens (1812–1870)
“Just like those other black holes from outer space, Hollywood is postmodern to this extent: it has no center, only a spreading dead zone of exhaustion, inertia, and brilliant decay.”
—Arthur Kroker (b. 1945)