Habitable Zones (HZ's)

Astronomers have recently taken a second look at the likelihood of life elsewhere in our galaxy and have found that second look discouraging. For decades we have realized that Earth is at a highly favorable distance from the Sun. Were it much closer, our planet would be so warm the oceans would evaporate. Were it much farther from the Sun, Earth's oceans would freeze. Earth is nearly in the middle of what many astronomers refer to as the Circumstellar Habitable Zone, or CHZ, for short.

On a larger scale, our Sun itself seems to be located in a similarly favorable position in the Milky Way galaxy. As we learned earlier, the core of the Milky Way is a dangerous place because of the activity associated with the massive black hole there. Moreover, most of the stars in the core of the Milky Way are so old that they formed from material only slightly enriched in the heavy elements on which life relies. Moreover, planets similar to the Earth are built of such elements, and if they are in short supply, so will be planets. Similarly, if the Sun lay near the outer edge of the Milky Way, we would again be in a region of the galaxy poor in the critically needed heavy elements. Thus, we seem to be in a Galactic Habitable Zone (orGHZ).

References: "Refuges for Life in a Hostile Universe." Gonzales, Guillermo, Donald Brownlee, and Peter D. Ward. Scientific American 285. October 2001. Pg 60.