A "Just Right" Galaxy

Slide 21 of 22

The table above shows the probabilities that the galaxy will match what is required for life to exist. Galaxy size is important, since galaxies that are too large tend to have a number of violent events that disturb solar orbits. Small galaxies have insufficient material to maintain star formation for long enough time periods to form rocky planets.¹

The type of galaxy is also important, since elliptical and irregular galaxies (comprising most galaxies at this point) support new star formation poorly, if at all. Spiral galaxies are optimal for the formation of rocky planets.²

The galaxy location is important, since most galaxies are part of much larger galaxy clusters. We are in a very small galaxy cluster (known as the "local group"), in which we are the "big guys" among the members of the cluster. The closest galaxy to ours is Andromeda, which is 2 million light years distant. This may seem like a large distance (and it is relative to other galaxy clusters), but even so, we are scheduled for collision with Andromeda in 3 billion years. In fact, the Andromeda galaxy is closing on our galaxy at 500,000 kilometers per hour. This pace will accelerate until the two galaxies collide in 3 billion years. According to astrophysicist Chris Mihos of Case Western Reserve University in Cleveland, Ohio, "It will be a major car wreck, and we're the Yugo in this one."³ Other galaxy clusters are much more dense then ours, resulting in frequent galactic collisions. When galaxies collide, bad things happen to stellar and planetary orbits. The long term stability of our Solar System and galaxy would not be possible in most other galaxy clusters.

All stars are formed in nebulae and our Sun is no exception. In order to gain enough heavy elements to form planets, our system must have formed close to a recent supernova. Heavy elements were not formed during the Big Bang. They are only formed inside the furnaces of stars and distributed through supernova events. Carl Sagan used to have a famous saying in his Cosmos series on PBS,  "We are star stuff." He had a unique way of saying it that I can still visualize today. His point was (also explained in the series) that everything that we are made of was distributed during the explosion of a large nearby star. If the solar nebula was too far from the supernova event, insufficient heavy elements would have been present for life chemistry. If the supernova event occurred too early, then the heavy elements would have been dispersed before the Solar System would have formed. If the supernova event occurred too late, then the nebula would not contain enough heavy elements for life chemistry. In addition, it is possible that the supernova event itself could destroy all life.

White dwarf binaries are necessary to form the element fluorine, which is required by living organisms. Since this element is only formed in these kinds of stars, they must have been present near where the Solar System formed.⁴

References

1. In a recent survey of globular cluster 47 Tucanae, scientists found zero extrasolar planets out of 37,000 stars searched (Astronomers Ponder Lack of Planets in Globular Cluster from the Hubble Space Telescope).
2. Cowen, R. 1992. Were spiral galaxies once more common? Science News 142: 390.
   Dressler, et al. 1994. New images of the distant rich cluster, CL 0939+4713 with WFPC2. Astrophysical Journal Letters 435: L23-L26.
3. Irion, R. 2000. A Crushing End for Our Galaxy. Science 287: 62-64.
4. Davies, R.E. and R. H. Koch. 1991. All the observed universe has contributed to life. Philosophical Transactions of the Royal Society of London, series B 334: 391-403.