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, CO2
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?
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