Why does the solar system appear to be unique?
Astronomers have discovered over 250 planets outside of our solar system residing in over 200 different planetary systems. However, all of these extrasolar planets exhibit characteristics that would eliminate the possibility of another planet residing in the same planetary system that could possibly support advanced life for a brief time or even primitivelife for a long time.
Why does the solar system appear to be unique? Many astronomers have suggested that the solar system is different because it formed in a manner very different from the extrasolar planetary systems so far discovered. For such a claim to be held as fact, though, astronomers need to develop a model for the formation of extrasolar planetary systems. Researchers could then confirm the model through observations, demonstrate what is different about the manner in which the solar system formed, and show which features in the solar system’s formation make the solar system appear unique. Such a quest may also uncover several more design features in the solar system that make it suitable for the support of life.
Theoreticians have recognized that all of the large extrasolar planets either orbit close to their stars or manifest significantly eccentric (noncircular) orbits. That observation led them to conclude several years ago that these planets must have formed far enough away from their stars that they could accrete substantial amounts of gas and subsequently suffered substantial inward migration as they interacted with the dust and rocks that remained in the star’s protoplanetary disk. However, astronomers could not match theoretical models with the statistics of extrasolar planet characteristics. These difficulties informed the theoreticians that their models were too simplistic.
A breakthrough has been achieved through the work of Philip Armitage, an astronomer at the University of Colorado. He inserted into a protoplanetary disk model the mass loss from photoevaporation caused by the light emitted both by the star and nearby bright stars. He also accounted for “viscous evolution,” that is, allowance for changes in the viscosity of the protoplanetary disk.1 His model produced results that matched not only the distribution of extrasolar planet distances from their stars but also the distribution of extrasolar planet masses.
Such a good fit between theory and observations implies that the mechanisms for the formation and evolution of extrasolar planets are well-enough understood to justify certain deductions about the solar system’s extraordinary formation history. Armitage’s model, though it explains quite well the characteristics of the 200+ extrasolar planetary systems, is not able to explain the solar system’s features (an ensemble of distantly orbiting gas giant planets with near-circular orbits combined with large rocky planets with stable near-circular orbits, with one of those rocky planets manifesting all the features that life requires) without the introduction of exceptional fine-tuning.
The formation of the solar system either implies this exceptional fine-tuning or it implies the solar system formed through some exotic and unique process. Either way, it adds to the already overwhelming evidence that the solar system must have been supernaturally designed for the support of life and of human beings in particular.
- Philip J. Armitage, “Massive Planet Migration: Theoretical Predictions and Comparisons with Observations,” Astrophysical Journal 665 (August 20, 2007): 1381-90.