Galactic Habitable Zones

Galactic Habitable Zones

Life on other planets?

In April 2008, the fifth Astrobiology Science Conference convened in Santa Clara, California. According to its mission statement, the SETI Institute, which hosts these conferences, seeks to “explore, understand and explain the origin, nature and prevalence of life in the universe.”

One of the sessions at the conference focused on how the habitability of the galaxy varied in space and time. Two of the talks in that session each highlighted an issue important for human life here on Earth. The first talk, by Australian astronomer Charles Lineweaver describes the regions of the galaxy meeting three requirements for life (from strictly natural processes):

  1. enough elements to form terrestrial planets,
  2. sufficient time for biological evolution, and
  3. an environment free from life-extinguishing supernovae.

This galactic habitable zone is an annulus (ring) around the galactic center that brackets the corotation radius. At this radius, stars orbit the galaxy at the same speed as the spiral arms, thus minimizing passages through the spiral arms. According to Lineweaver&’s calculations, his galactic habitable zone contains less than 10 percent of all the stars ever formed in the Milky Way Galaxy.

Another talk, by Kansas University astronomer Adrian Melott, described a 62-million-year periodicity (cycle of increase and decrease) in the level of biodiversity seen in the fossil record. His research argues that the passage of the sun through the galactic plane—which increases the comet and asteroid bombardment in the solar system—as the cause of the periodicity. If correct, the habitability of a star&’s planets depends not only on its location but also on its path through the galaxy. Since the sun oscillates through the galactic plane every 30 – 35 million years, the solar system would now be starting a passage through the galactic plane, thus beginning a period of increased bombardment (and possible extinction).

Both of these papers, as well as many others presented at the conference, articulate the many factors that must exist simultaneously for a planet to be suitable for advanced life like humans to exist. Such research continues to demonstrate the scientific reasonableness of believing that a supernatural Creator fashioned and designed Earth as a unique habitat for human life.