Habitable Zones in the Galaxy

The concept of habitable regions can be applied to our Milky Way galaxy as well. We (and a few other astrobiologists) suspect there are geographic regions that can be plotted from the center of our galaxy that are habitable regions in a way analogous to the habitable zones around stars. Our galaxy is a spiral galaxy (the other types are elliptical and irregular galaxies). In most galaxies the concentration of stars is highest in the center and diminishes away from the center. Spiral galaxies are dish-shaped (round, but flat if viewed from the side), with branching arms when viewed from the top. But viewed from the side they are quite flat. Our galaxy has an estimated diameter of about 85,000 light-years. Our sun is about 25,000 light-years from the center, in a region between spiral arms where star density is quite low compared to the more crowded interior. In this position we slowly orbit the central axis of the galaxy.

Like a planet revolving around a star, we maintain roughly the same distance from the galactic center, and this is fortunate. Our star—by chance—is located in the "habitable zone" of the galaxy. We suspect that the inner margins of this galactic habitable zone (GHZ) are defined by the high density of stars, the dangerous supernovae, and the energy sources found in the central region of our galaxy, whereas the outer regions of habitability are dictated by something quite different: not the flux of energy, but the type of matter to found.

At the present time, we cannot do more than crudely designate the limits of this habitable region. Its inner boundary is surely defined by celestial catastrophes occurring closer to the center, but we cannot yet estimate how close to the center of the galaxy that boundary is. Perhaps it extends 10,000 light-years from the center, perhaps more. However, we do have at least a vague idea of the forces that impose this inner limit. Life is a very complex and delicate phenomenon that is easily destroyed by too much heat or cold and by too many gamma rays, X-rays, or other types of ionizing radiation. The center of any galaxy produces all of these.

Among the lethal stellar members of any galaxy are the neutron stars called magnetars. These collapsed stars are small but astonishingly dense, and they emit X-rays, gamma rays, and other charged particles into space. Because energy dissipates as the square of distance, these objects are no threat to our planet. Closer to the center of the galaxy, however, their frequency increases. Any galactic center is a mass of stars, some the lethal neutron stars, and it seems most unlikely that any form of life as we know it could exist nearby.

An even greater threat comes from exploding stars known as supernovae. As stars grow old, they burn up their hydrogen and eventually collapse on themselves. Some of them then explode outward with terrific force. Any star going supernova would probably sterilize life within a radius of 1 light-year of the explosion and affect life on planets as far as 30 light-years away. The very number of stars in galactic centers increases the chances of a nearby supernova. Our sun and planet are protected simply by the scarcity of stars around us.

The outer region of the galactic habitable zone is defined by the elemental composition of the galaxy. In the outermost reaches of the galaxy, the concentration of heavy elements is lower because the rate of star formation— and thus of element formation—is lower. Outward from the centers of galaxies, the relative abundance of elements heavier than helium declines. The abundance of heavy elements is probably too low to form terrestrial planets as large as Earth. As we shall see in the next chapter our planet has a solid/liquid metal core that includes some radioactive material giving off heat. Both attributes seem to be necessary to the development of animal life: The metal core produces a magnetic field that protects the surface of the planet from radiation from space, and the radioactive heat from the core, mantle and crust fuels plate tectonics, which in our view is also necessary for maintaining animal life on the planet. No planet such as Earth can exist in the outer regions of the galaxy.

Not only is Earth in a rare position in its galaxy; it may also be fortunate (at least as far as having life is concerned) in being in a spiral rather than an elliptical galaxy. Elliptical galaxies are regions with little dust which apparently exhibit little new star formation. The majority of stars in elliptical galaxies are nearly as old as the universe. The abundance of heavy elements is low, and although asteroids and comets may occur, it is doubtful that there are full-size planets.

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