Rotating Question Curious About Astronomy? Ask an Astronomer

Can the theory of formation of the solar system explain all the observables?

Is there any theory of the origin of Solar system which can explain these three things:

1) The chemical elements distribution between different planets (the Sun has very little of heavy elements, Venus and Earth lots of it, Jupiter and Saturn have little again);

This is explained by the Lewis Model. In the early Solar System, which was a cloud of gasses, the inner parts were warmer than the outer parts. In the inner region, only things like metal or rock could condense, so the inner planets (Mercury, Venus, Earth, and Mars) are composed chiefly of metal and rock. As you move out to the cooler outer regions, it gets cool enough for things like water ice, and then ammonia and methane ice to condense.

The reason why the outer layers of the gas giants (Jupiter, Saturn, Uranus, and Neptune) are composed of lighter elements is that these planets grew larger than the Earth, quickly. There are two reasons why. One is that, in the outer regions, it was cool enough for a larger range of materials to condense -- not only rock and metal, but also things that condense at cooler temperatures such as water ice and ammonia ice, so there was more "raw materials" for the planets to be made of. The other reason is that ice sticks together better than rocks and metals, so when the ice that had condensed in small pieces ran into other pieces of ice, it tended to make bigger pieces, rather than bounce off or fragment as pieces of rock do. The outer planets originated as big planets made of ice and rock. The were massive enough that their gravity allowed them to accummulate hydrogen and helium, which the inner planets did not have enough mass to hold on to, and grow to their current titanic proportions.

2) The upturned direction of Venus rotation around axis (in contrast with other planets);

An old idea about Venus' rotation (which is clockwise as viewed from the north as opposed to counter-clockwise like the other planets) is that gravitational influences of the Sun slowed it down until its rotational period equaled its orbital period. That situation is called a spin-orbit resonance. Mercury is in a slightly different type of spin-orbit resonance. However, that will not explain why Venus is rotating backward.

Another idea is that an impact with a large body gave Venus its strange spin state. This is not supported by any good physical evidence, and it is strange that Venus would end up rotating slowly backwards instead of tipped over at a random angle, like Uranus.

There is no widely-accepted explanation of Venus' rotation right now. Personally, I think that's good. It means that there is still a Great Mystery in the solar system waiting to be revealed, and reminds us astronomers not to get too full of ourselves.

3) The division of momentum (the Sun has less than 2% as I remember).

I don't think this is a problem according to the Cameron Model (which explains how the Solar System formed out of a spinning cloud of gas and dust.) There's no reason why we would expect the Sun to contain most of the angular momentum in the Solar System, especially since the planets are constantly stealing it through tidal interactions.

March 1999, Dave Kornreich (more by Dave Kornreich) (Like this Answer)

Still Curious?

Get More 'Curious?' with Our New PODCAST:

Related questions:

More questions about Planets: Previous | Next

How to ask a question:

If you have a follow-up question concerning the above subject, submit it here. If you have a question about another area of astronomy, find the topic you're interested in from the archive on our site menu, or go here for help.

Table 'curious.Referrers' doesn't existTable 'curious.Referrers' doesn't exist

This page has been accessed 23025 times since September 12, 2002.
Last modified: October 18, 2005 7:31:28 PM

Legal questions? See our copyright, disclaimer and privacy policy.
Ask an Astronomer is hosted by the Astronomy Department at Cornell University and is produced with PHP and MySQL.

Warning: Your browser is misbehaving! This page might look ugly. (Details)