Tuesday, October 26, 2010

Our Strange Solar System

We used to think the solar system formed in a nice orderly way.  The inner or rocky planets Mercury, Venus, Earth, and Mars, then the asteroid belt, and the outer or gas giant planets Jupiter, Saturn, Uranus, and Neptune all accreted from an initial disk of gas and dust some 4.6 billion years ago.

Turns our we may have been completely wrong.  There were problems with our old view of the solar system - Uranus and Neptune are too big and Mars is too small given their locations.  No one really understands the formation of the asteroid belt.  New computer models, however, bolstered by the discover of hundreds of exoplanets orbiting other stars over the past few years, cause some astronomers to believe they have the answer  They claim that the early solar system was a much more dynamic place planets bouncing around a bit until they finally settled into their present orbits.

These models show Jupiter, Saturn, Uranus, and Neptune forming much more closely together between 5-12 A.U. from the Sun (1 A.U. or Astronomical Unit is the average distance from the Earth to the Sun, about 93 million miles).  Jupiter and Saturn were unstable, however, and got into a gravitational shoving match which eventually ejected Uranus and Neptune out to 20 A.U. and 30 A.U. respectively.  That's why Uranus and Neptune are too big for where they are - they didn't form there!

Jupiter and Saturn meanwhile became coupled in a 3:2 resonance (Jupiter completed 3 orbits for every 2 by Saturn) and together they moved inward toward the Sun eventually reaching a distance of 1.5 A.U. (where Mars is now).  Jupiter would likely have went further into the Sun if not braked by Saturn's mass to which it was coupled.  This inward movement compressed the disk of rocky planetesimals that was later to accrete the four inner planets.  That's why Mars is too small for its position it formed further in from the compressed planetary disk and then migrated outward with the rocky, iron-rich planetesimals that formed the S-type asteroids.

Then Jupiter and Saturn migrated back out they kicked water and carbon-rich planetesimals into the outer asteroid belt to become the C-type asteroids.  Many of these bodied, kicked further inward, possibly supplied Earth with much of its initial water through collisions.  In addition, the collision of the early Earth with a Mars-size impactor formed the Moon.

While this all sounds crazy, it's being proposed by serious planetary scientists, is consistent with observations and the laws of physics, and the models work.  It also fits into what we see with many of the exoplanets of weird systems where massive gas giant planet are orbiting very close to their stars.  Given slightly different starting condition, our solar system may well have ended up like that as well.

Read more about it at The "New, Improved" Solar System over at Sky & Telescope.

One disturbing implication of this research is that "nice" solar systems like we have may be very rare in the universe which, of course, means life as we know it may be rare too.  This will be interesting to follow and see if it hold up to further scrutiny.

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