Chapter 40 — The Right Star

The Sun is a G-type main sequence star. Approximately 5 percent of the stars in the Milky Way are G-type. G-type stars are distinguished by moderate luminosity, stable hydrogen-burning over billions of years, moderate surface temperature producing substantial visible light output, and relatively low flare activity.

Most stars in the galaxy are cooler, dimmer M-dwarf stars comprising approximately 70 to 80 percent of all stars. M-dwarf habitable zones are so close to the star that planets there are tidally locked—one face permanently toward the star—and subject to intense flares and ultraviolet radiation. O-, B-, and A-type stars are hotter and more massive but far too short-lived: a massive O-type star may burn for only a few million years before exploding as a supernova, far too short for complex life to develop.

The Sun's stability over more than 4 billion years has provided the continuous, modulated energy input required for life's development and maintenance. Its luminosity has increased gradually enough for geological and biological feedback mechanisms to maintain habitable temperatures. Of all the star types available in the galaxy, the one that best supports long-duration habitable planets is the minority type the Sun happens to be.

Gonzalez, G. and Richards, J.W. The Privileged Planet. Regnery Publishing, 2004. Chapter 4. Ward, P.D. and Brownlee, D. Rare Earth. Copernicus Books, 2000. Chapter 10.

Chapter 41 — The Galactic Address

Not all positions within a galaxy support life equally. The galactic center is hostile: high radiation levels, dense stellar clustering, frequent supernova events, and disruptive tidal forces. The outer regions of the galaxy lack sufficient concentrations of heavy elements—carbon, oxygen, iron—produced by stellar nucleosynthesis and recycled through generations of stars. Complex chemistry and rocky planets require these elements. The outermost galactic regions cannot build Earth-type planets.

Astronomer Guillermo Gonzalez developed the concept of the Galactic Habitable Zone: the region within the Milky Way where conditions necessary for complex life are most likely to be met. Earth sits between the hostile center and the element-poor outskirts. Additionally, the Sun's orbit is nearly circular—it does not repeatedly cross the spiral arms of the galaxy, which would expose it to higher stellar density, radiation, and supernova rates.

Gonzalez and Richards estimated that the Galactic Habitable Zone represents approximately 5 to 10 percent of the galactic disk. Earth occupies a position within this zone and additionally lies in a region particularly suitable for scientific observation: a point Gonzalez and Richards developed into the argument that the universe appears designed for discovery as much as for habitability.

Gonzalez, G., Brownlee, D., Ward, P. "The Galactic Habitable Zone: Galactic Chemical Evolution." Icarus 152 (2001): 185–200. Gonzalez, G. and Richards, J.W. The Privileged Planet. Regnery Publishing, 2004. Chapter 15.