12.5 Billion B.C.T. - The First Galactic Habitable Zones Were Formed

The "habitable zones" of the first galaxies began to be formed.

     The Earth occupies a privileged position within the solar system.  If the Earth had been located much closer to the Sun, then, like Venus, it would have been too hot to support life.  If the Earth had been located slightly farther from the Sun, then, like Mars, the Earth would have been too cold to sustain life.  Our Earth sits in a so-called "habitable zone," a ring around the Sun within which liquid water can exist on a planet.  Without liquid water, any kind of life as we know it is highly unlikely.

     Scientists now posit that just as our solar system has a "habitable zone" so too do entire galaxies.  Although most galaxies contain millions of stars, many of these stars appear to be located in uninhabitable zones.  Scientists theorize that the habitable zone of a spiral galaxy (like our own Milky Way) may encircle its center, just as the habitable zone of the solar system encircles the Sun.  Inside or outside this band, the galaxy is sterile and life cannot exist.

     The question that arises is why?  It is easy enough to understand why a single star should have a habitable zone, but it is not so easy to understand why a galaxy should have a habitable zone.

     In response, scientists note that it is a delicate matter to make a planet like our Earth.  To make an Earth, you need the right materials: heavy, rock-forming elements like silicon and aluminum, and iron for the core.  At our location in the Milky Way, these elements are fairly abundant, but farther out, these elements become scarcer, and corollarily any existing rocky planets would tend to be smaller.  These planets would also be cooler because there would be less of the heavy radioactive elements, that in the case of the Earth, warm up the interior by radioactive decay.

     Without a warm interior, an Earth-like planet cannot have plate tectonics -- the gradual movement of the planet crust.  This means that the planet cannot keep water circulating through its atmosphere, and so it is likely to run down to a frozen state.  That is what happened to Mars.  Mars is actually within the solar system's habitable zone, but Mars froze because, being smaller than Earth, it cooled down too quickly and lost the ability to maintain plate tectonics.

     In contrast, further in towards the center of a spiral galaxy -- such as our Milky Way -- other factors frustrate the development of life.  Stars are more densely crowded together, and so conditions would be stormier.  Exploding stars -- "super novae", for instance -- may bathe nearby planetary systems in hazardous radiation.  Additionally, planets are likely to suffer heavier bombardment by comets, which are sent charging through a star's planetary system by the gravitational pull of other stars.  On Earth, the deadly effects of gigantic comet collisions may have choked life several times, thereby delaying the evolution of complex organisms.  If this bombardment had been more frequent, life might never have taken hold at all, or it may have been constantly prevented from evolving into complex forms.

     Based on these observations and conjectures, scientists suggest that most galaxies might have only limited regions where Earth-like planets could both form at all and be able to support life.  Indeed, it may be that because many galaxies have less heavy elements than the Milky Way, their habitable zones might shrink out of existence, leaving no room for life.

     In the final analysis, it may be that all that glitters is not gold -- that the Universe is likely filled with millions of galaxies with billions of solar systems, ... all of which are as barren and sterile as our Moon.