(1) (a) Most would agree that there is only one accurate description of the real solar system and that the real solar system followed one path of evolution. The question is whether we've figured it out or not. In the absence of complete knowledge about the past, can there be more than one theory that could be correct? How are multiple theories reduced to the one theory that explains reality? Until we have compelling evidence of one particular path of evolution for the solar system, we have to accept that several difference paths could be possible. As further pieces of evidence are found, then different theories are eliminated. Leaving only the ones which are consistent with the available evidence.

(b) In the future we will able to explore planetary systems around other stars (it is not a question of "if" but "when"). If we find that the planetary systems are very different from our solar system, does this necessarily mean that our theories about the formation of solar systems are wrong? Explain. Not necessarily - it may be that the theory still holds but the conditions under which other solar systems were made were a little different.

(2) (a) Check that the list above is basically the same as the one in Chapter 8. What is added? That's compare with Table 8.3. (1) is new, (2) & (3) are the same as the table #1, (4) is table #2, (5), (6) and (7) are table #3, (8) is new, (9), (10), (11) are table #4, (12) is new. So, we have a more complete check list - there are surely more details to add - life?

(b) Each person is entitled to pick their own set of facts that they would like a theory to explain - why is Pluto such a misfit and stuck out at the edge of the solar system?. Here is perhaps a more useful catagorization of facts:

(c) Look at the list of 12 facts above and organize them into these 4 main catagories. Do you see anything that is left out from either list? The evolution of life, perhaps? Err - I guess I just answered this one under (a).

(3) What determines in which direction the collapsing nebula spins? Our solar system has a prefered sense of rotation that is anti-clockwise looking down from the north (as if you were looking at it from the star Polaris). Is it just as likely that our solar system could have the opposite rotation? Logic says that no particular direction is preferred - the direction we ended up with is just due to most of the small motions cancelling out. Until we start to observe other solar systems in more detail we cannot say if in fact all solar systems are oriented in random directions.

(4) How does this idea of condensation of different materials according to temperature in the solar nebula and to cosmic abundances lead to just two types of planets - terrestrial and giant - rather than a continuous spectrum or 4-5 different kinds of planets? This is a critical issue - the particularly high abundance of oxygen, nitrogen and carbon, their preference to combine with hydrogen (most abundant element) means that water vapor, ammonia and methane will be abundant gases in the solar nebula. The key issue is to get one of these abundant compounds to become solid - freeze, condense. Once water froze (outside the snow line) then the story snowballs - literally. Once you get a bigg enough snow ball to gravitationally hold hydrogen then the giant planet is inevitable. So, the other objects (inner solar system) are really just the accumulation of the left over material from making giant planets, you could say.

(5) Go back to our table of 12 facts we need to explain. How are we doing at this point? Which aspects of the solar system have we explained? I reckon we have hit 1-4.

(6) (a) If you pick up a fresh piece of lava (having waited for it to cool down, obviously!) would you expect the ratio of potassium-40 to argon-40 to be close to 0 or to a large amount? (Hint: look at Figure 8.17) Argon-40 is a gas - it does not hang around in a molten rock. So if you pick up a fresh piece of lava, any Argon-40 has floated off. There will be all Potassium-40. That tells you that the rock is very young. Over time the Potassium-40 will decay and the Argon-40 will be trapped in the solidified rock.

(b) Next, think about the old meteorites, what is the potassium-40/Argon-40 ratio in the old meteorites? A great deal less than in fresh lava.

(7)The above are listed aspects of the solar system that may have been caused by large impacts that occured quite late in the formation of the solar system. Given the large size of the solar system and all of the objects in it - planets, moons, asteroids, comets - are these a large number of coincidences/ catastophes? Or are these 'mis-fit' aspects just a few "wrinkles" that make our solar system unique? That is, they give our solar system its own special character - just as each litter of labrador puppies look and behave in a predictable way but, on closer inspection, chance has resulted in differences that make each litter different (a floppy ear here, a white patch there, etc.;). My honest answer is that we will not know until we have inspected other solar systems in more detail. There are plenty of examples where a little chaotic behavior makes nature more interesting. That does not mean we cannot try to characterize the system - just that we cannot accurately predict what will happen next. Interesting, eh?

(8) (a) Writing out a "scenario" - printing it in nice type - can make it seem "real". Yet, much of this is just guesswork. We have an idea that something must have caused a particular feature (such as the initial coalescance of condensed grains) but we really have no real idea how this happened. Because the planets have evolved considerably since they formed, they are not likely to be the places where we are going to find clues about the early solar system. If not the planets themselves, where else are we going to find clues about the early solar system and how it formed? I'd put my bets on cometary material - probably an indication of the original material from the solar nebula.

(b) We have completely ignored the issue of life. At what point in the above scenario could life have begun to successfully evolve? The issues of how and where life evolved are perhaps the most challanging and exciting questions to answer. Life could not begin until the aerial bombardment slowed down and the environment settled down. So, about 3.5 - 3.7 billion years ago.