How Unlikely Is Our Planetary System?

The year 1995 marked the first time astronomers discovered an extrasolar planet.¹ It was found orbiting the star 51 Pegasi. Today, scientists know of more than 350 planets residing outside the solar system. Although this sample is not huge, it is large enough that astronomers can compare detailed simulations of the formation and evolution of planetary systems with the measured characteristics of extrasolar planets. Such studies help them determine the rarity of the life-critical features displayed in our solar system.

Three astronomers from the University of Colorado and Google Incorporated published the latest and most detailed comparison in the July 10 issue of the Astrophysical Journal Letters.² They focused on planetary eccentricity, which is a measure of the ellipticity (deviation from a circle) of a planet’s orbit about its star. The eccentricity for a circular orbit is 0, for a completely flat ellipse it is 1, and for parabolic or hyperbolic orbits it is greater than 1.

For advanced life to be possible within a planetary system, all the large and medium-sized planets must exhibit orbits that are nearly circular. Significant departures from near circularity result in too much chaos in a life-friendly planet’s orbit. Of the 353 extrasolar planets discovered so far, none exhibit the combination of just-right mass, just-right distance from its star, and just-right eccentricity to allow a life-sustaining planet to exist in the same system.

The simulations performed by the three astronomers demonstrate why the hospitable features of the solar system’s eight planets are not seen anywhere else in the Milky Way Galaxy. The team first modeled the evolution of multiple-planet systems both with and without the early existence of a large belt of planetesimals (Moon-sized bodies), where the initial planetary configurations were marginally unstable. When planetesimals were not present, the simulations matched the eccentricity distribution observed for extrasolar planets. With planetesimals, the team got two different outcomes. In systems where the total mass of the planets exceeded that of Jupiter the eccentricity distribution of planets was broad. Only in the case where the total mass of planets was less than that of Jupiter did the researchers notice “a preponderance of nearly circular final orbits.”³

However, the latter situation is not conducive to the existence of advanced life. In the solar system, the existence of such large mass planets as Jupiter and Saturn act as effective gravitational shields to prevent Earth from absorbing too many life-exterminating collision events from asteroids and comets. If Jupiter, Saturn, Uranus, and Neptune had masses that added up to less than that of Jupiter, the shields would no longer provide adequate protection.

Additionally, the team studied the fate of multiple-planet systems that start off marginally stable. For planetary systems where the masses of the gas giant planets are large enough to act as effective shields, the results showed that in the vast majority of cases the gas giants evolve into resonance. Such resonance, where the gas giant planets line up frequently, would induce enough chaos within the life-support planet’s orbit as to render advanced life impossible.

This research adds to existing evidence for the solar system’s uniquely fine-tuned life-friendly characteristics. The team also predicts that “a transition from eccentric to near-circular orbits will be observed once extrasolar planet surveys detect sub-Jovian mass planets at orbital radii of a = 5-10 AU.”⁴ (1 AU = distance between Earth and the Sun.) If proven correct, this prediction will further solidify the conclusion that the solar system’s planets have been purposely designed to make advanced life possible.