September 29, 2011

Living In The Galactic Habitable Zone

It wasn’t long ago that Carl Sagan preached his “theory of mediocrity,” that “we live on an insignificant planet of a humdrum star, lost in a galaxy, tucked away in some forgotten corner of a universe in which there are far more galaxies than people” (Cosmos). A recent study indicates that Earth remains a rare bird in the celestial aviary.

Bulletin! Life found in Milky Way! Astrobio.Net reported the finding: “We know for certain that life exists in the Milky Way galaxy: that life is us.” OK, maybe that is not news, but the article did confirm the idea of a Galactic Habitable Zone (GHZ) outside of which life is unlikely. Author Gemma Lavender reminded readers of other requirements “such as atmospheric composition, a carbon cycle and the existence of water” that must also be satisfied. Then she briefly revisited the debate between the Copernican Principle (championed by Carl Sagan and Frank Drake) and the “Rare Earth” hypothesis, advanced by Peter Ward and Donald Brownlee, who first proposed the Galactic Habitable Zone in their book Rare Earth. But how wide is the GHZ?

Lavender entertained a new model that proposes more habitability in the inner zones than Ward and Brownlee described, despite the increased danger of supernovae, because of higher concentrations of heavy elements there. A supernova can quickly sterilize a planet, the team led by Michael Gowanlock (NASA Astrobiology Institute) admitted, but life in the fast lane near the galactic nucleus also has benefits—more raw material for rocky planets. Other astrobiologists are not so sure. Regardless of who’s right, one item stood out in the study: “The team discovered that at some time in their lives, the majority of stars in our Galaxy will be bathed in the radiation from a nearby supernova, whereas around 30% of stars remain untouched or unsterilized.” Artwork of an unlucky planet getting sterilized by its star going boom served as a reminder that not every star in a galaxy can be counted on to provide a stable habitable zone.

From this admittedly optimistic study, it appears that 8% of 30% of stars in the GHZ remain candidates for having Earth-like planets where life can thrive. That’s 2.4% the stars, before considering all the other factors listed in The Privileged Planet, such as the right crustal composition, plate tectonics, an abundance of water, the right kind of star, the right kind of atmosphere – at least 20 requirements. In the film, a simple calculation using conservative estimates of 1 in 10 for each factor put the odds at a thousandth of a trillionth that a planet would have all the conditions necessary for life. Earth still appears the winner of a gigantic cosmic lottery.