How does the nebular theory account for the difference in planets?

Question

How does the nebular theory account for the difference between the inner and outer planets?

Answer 1

Actually, Uchiha Madara, that's not quite right. Our Sun has never exploded; instead a previous star exploded and our solar system formed from the debris of that supernova.

According to the nebular theory this debris then formed into a clump (the Sun) surrounded by a flattened disc. Inner parts of the disc were spinning much faster than the outer parts of the disc, because they felt a stronger gravitational pull from the Sun (Kepler's laws, right?) As a result, friction built up immensely in the inner parts of the disc as rings of material spun at different speeds and rubbed against each other, but further out where everything was moving more slowly friction was much less. As a result, temperatures in the inner parts of the disc reached up to thousands of degrees while the outer parts remained very cold.

Now, hydrogen is the most abundant element in the Universe - it was created in the Big Bang - and oxygen is a very common element created in supernova explosions like the one that made our Solar System. As a result by FAR the most common compound in the disc was water - hydrogen bound to oxygen.

In the inner parts of the disc - from the Sun out to about 4AU - it was too warm for water to freeze and so the only solid matter was grains rock and metal (of which there wasn't much.) This stuff built up into the inner planets, Mercury, Venus, Earth and Mars, which are small and made of rock and metal.

Beyond about 4AU, water froze into ice - and because water is so common there was a much, much greater amount of solid material around to make planets from. So solid balls of rock, metal and ice built up in the outer Solar System, reaching tens of times the mass of the Earth. These icy worlds grew so large that they could hold on to the light gases - the hydrogen and helium - in the nebula, and as a result they grew even bigger - into balls of gas up to hundreds of times the mass of the Earth. This is why the outer planets, or gas giants, Jupiter, Saturn, Uranus and Neptune, have solid cores of rock and metal, surrounded by ices, surrounded by thick layers of hydrogen and helium liquid and gas.

As the gas giants formed they swirled up the gas and dust around them and created "mini-nebulas" of their own, which got hot close to the planet and cooler further from the planet due to friction between layers and Kepler's laws as described earlier. That's why Jupiter, Saturn etc.'s inner moons are made mainly of rock, and their outer satellites contain more ice - they formed like solar systems in minature.

When the Sun finally ignited, it released a huge amount of energy and sent a shock wave blasting through the Solar System (this is probably what Uchiha Madara means when he/she talks about the Sun going supernova, which it never has.) This blasted the remaining gases out of the solar system. The inner planets, being made of a little rock and metal, couldn't hold on to this gas and so stayed with thin atmospheres. The outer planets, being huge balls of rock, metal and ice, managed to hold on to their gas because of their huge gravity, even though the Sun blasted most of it away.

Now the gas was gone, for the next few hundred million years rocky and icy planetesimals continued to bombard the inner and outer planets. Obviously large balls of gas can't crater, but the inner planets, and the moons of the outer planets, bear the scars of that cratering. The Solar System's formation still hasn't finished, because there's still a load of planetesimals out there, as asteroids, meteoroids and comets.

So in summary the nebular theory accounts for the differences between inner and outer planets by introducing a difference in temperature where they formed. The inner planets formed where it was hot enough for only rock and metal to condense, whereas the outer planets formed where it was cool enough for ice to form and build huge, icy planets which could hold onto even huger envelopes of gas and grow into gas giants.

Of course the nebular theory is not perfect, because it predicts we should always find small, rocky planets close to a star and large, gas giants further away - which is true in our Solar System, but most of the planets found around other stars are gas giants orbiting close to their star!  (This doesn't mean they're very common, because they're the easiest type of planet to detect - for all we know most solar systems are like ours and have so far escaped detection.) We think the nebular theory is true because of all the other evidence supporting it - planets orbiting in same plane and direction, observing dust discs around other stars, comets, asteroids and meteorites appearing to be the same age, impact craters from heavy bombardment etc. so we assume these "hot Jupiters" must have formed further out as the nebular theory predicts and somehow migrated inwards - we don't quite know how that happens yet but that's the beauty of science! We're always making new discoveries and seeing how they fit in with what we already know.

Hope that helps, :)

Procyon

Answer 2

Inner planets are terrestrial because the sun going supernova exploded any remaining gas close to it, leaving only rocks and other solids. Outer planets are gassy because the sun's explosion did not reach as far and did not blow away the gas.