Guillermo Gonzalez is a Post-doctoral research fellow in astronomy at the University of Washington. He earned his Ph.D at the University of Texas.

The central disk of gas, dust, and stars undulates, as does the sun. The size of the sun's bounce could be another indicator of design.

Science education and science news coverage have helped people understand the importance--for life's survivability--of Earth's relationship to the sun and other solar system bodies. But researchers are just beginning to recognize and report the importance-for life-of our solar system's special orbit.

Most readers probably know that our solar system orbits the center of the Milky Way. A few may have learned that it bounces up and down relative to the plane of the galaxy as it moves along its orbital path. We call that range of up and-down motion the sun's (or solar system's) Z-axis. But how many realize that the Z-axis, or bounce range, must fit within certain limits for life to be possible?

Let me fill in a few of the details. First, we must remember that our galaxy is not perfectly flat. It is shaped like a disk but it has some thickness. It is three dimensional. All the material in the disk, gas, dust, and stars, circling the galaxy's core also undulates up and down to varying degrees as it orbits (see Figure 1).

Objects with a big bounce, say on the order of 400 light years (or more) up and down from the plane, are exposed to intense radiation bombardment-X-rays, cosmic rays from supernova explosions, and a variety of radiation from the galactic center. If these rays were to strike Earth, for example, they would destroy our ozone shield, exposing life to harmful ultraviolet rays, and ultimately cause global cooling, pushing Earth's temperature range below what is tolerable for life.2,3

Astronomers have measured the sun's bounce to be 228 light years up from midplane and down from midplane. At the moment, our solar system is near the midplane, the place where the radiation protection is greatest. In another 8.25 million years we'll reach the top of our bounce (228 light years up), then we'll start downward again, passing through the midplane and reaching the bottom of our bounce (228 light years down). Then we'll come back up again to where we are. This whole up-and-down cycle takes about 33 million years.

How typical is this bounce pattern? Among the nearby solar-type stars between 4 and 6 billion years old (roughly the same age as the sun), only about a third have a Z-axis as small as (or smaller than) the sun's.4 In other words, the majority of stars bounce more and thus are exposed to greater radiation danger.

The solar system's bounce pattern may help explain the periodic extinctions apparent in the fossil record. The shifting gravitational tide, on a galactic scale, tugs comets loose from the cloud in which many reside, the Oort cloud, and hurtles them toward the sun where they can collide with planets.5 We see evidence that comet collisions with Earth caused at least a few of the mass extinctions.

Ironically, the primary force driving most stars' vertical motion is, in our sun's case, the very force that flattens its bounce, keeping it in the safe zone. That force is the gravity of giant molecular clouds. Stars begin their existence inside these giant clouds, and, as they orbit the galaxy, they are pulled this way and that by the gravity of other giant clouds they encounter, clouds moving at different velocities and different distances from the galactic core. These encounters tend to "heat up," or amplify, stars' bouncing motion. In the sun's case, however, the opposite effect is in evidence. The sun's "chance' encounters actually reduced, or restrained, the vertical undulations. This could be one more piece of evidence for "anti-chance," or, as we Christians would describe it, the involvement of our loving Creator.

References

1. C. R. Brackenridge, 'Terrestrial Paleoenvironmental Effects of a Late Quaternary-Age Supernova," Icarus, vol. 46 (1981), pp. 81-93.
2. M. A. Ruderman, "Possible Consequences of Nearby Supernova Explosions for Atmospheric Ozone and Terrestrial Life," Science, vol. 184 (1974), pp.1079-1081.
3. G. C. Reid et at, "Effects of Intense Stratospheric Ionization Events," Nature, vol. 275 (1978), pp. 489-492.
4. B. Edvardsson et al, "The Chemical Evolution of the Galactic Disk. 1. Analysis and Results,' Astronomy & Astrophysics, vol. 275 (1993), pp. 101-152.
5. J. J. Maltese, et at, "Periodic Modulation of the Oort Cloud Comet Flux by the Adiabatically Changed Galactic Tide," Icarus, vol. 116 (1995), pp. 255-268.

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