13 Apr. 2011

 

Our solar system is unusual

 

Some decidedly unexpected conclusions have been drawn in an article by Sean N. Raymond and Philip J. Armitage (University of Colorado, Boulder) and others, soon to appear in the journal Astronomy & Astrophysics. By considering the physical and dynamical parameters of the numerous planetary systems now known, and simulating 152 possible evolutionary scenarios based around reasonable assumptions, the UC Boulder team have shown that some characteristics of our solar system are a real rarity. In particular, the presence of 4 rocky planets and 4 gas giants and the absence of a dusty debris disk in the inner solar system, is unusual according to these models.
From these simulations it is evident, as if more evidence were needed, that the presence of gas giants in a planetary system can determine the evolution of the rocky planets. The rocky planets form relatively close to the star, from the heavy materials (silicates, iron, nickel etc.) which remain after the stellar wind has blown away most of the lighter elements.
The gas giants, in contrast, form beyond the so-called "frost line", where volatile molecules like methane, ammonia, CO
2 and water can exist in the solid state (in our solar system this line lies at a radius of about 2.7 AU from the Sun, in the asteroid belt).
As seen in many models, there is an evolutionary phase in which the gas giants, after gravitational interactions amongst themselves, can either migrate to often more eccentric, stable orbits, or be expelled from the system. If these migrations occur before the rocky planets have finished their formation (something which takes 10-100 million years) the perturbations generated in the inner regions of the system are sufficient to inhibit the growth of the rocky planets, creating chaos amongst the accreting planetesimals.
According to Raymond and colleagues, only 40% of systems manage to create more than one rocky planet, 20% make just one, and the remaining 40% have none. The simulations also show that where one or more rocky planets are present, there is often abundant dust, even after billions of years, created by the remnants of collisions between planetesimals. In our solar system this seems to be all but absent, except for the modest quantity which scatters sunlight and causes the Zodiacal light. How important might have been these differences for the appearance of life?

 

by Michele Ferrara & Marcel Clemens

credit: University of Colorado, A&A