At the bottom of this entry, I give the Weekly Pondering 9 assignment, for those of you in ASTR 1/2. Any text that you need to read is on Blackboard or linked to here.

How Did Our Solar System Form, and How Does Science Answer this Question?

How is knowledge generated in science? Let us take the example of our Solar System. This week we are learning that the Solar System formed from a piece of a nebula that contracted under its own gravity; as it contracted, it began to rotate faster and faster, and it formed a disk-shaped object we call a protoplanetary disk. This object contained a large sphere of gas in its center, referred to as a protostar, which evolved into our Sun. The gaseous, rocky, and icy material surrounding this protostar formed all of the other components of the Solar System: the eight planets, asteroids, comets, dwarf planets, and various other tiny debris. How do we know all of this?

Let us begin with a more terrestrial example. Suppose you are driving at night on a desolate road. You come to a T junction, and stop to look both ways. In the distance, you see two lights. Pausing for a moment, you pull into the intersection and drive away. How did you know you could do this? By seeing two lights, how did you determine that you could safely accelerate through the intersection? We can ask a more radical question: how did you know that the object emanating these lights was a vehicle?

As we interact with the world, we create models of how the world works. We get out of bed in the morning and prepare to fall downwards; we do not ever prepare to be pulled toward the ceiling or wall. When we go to bed at night, we never worry that a venomous alligator will suddenly emerge and consume us. We do these things because, since we are constantly building a model of how our world works, we make certain conclusions about the world. We have observed many cars throughout our lives, and therefore we can put into context these two headlights that we see on a desolate road. In other words, we don’t know with absolute certainty that these two lights are not emus with flashlights, but we use our model of the world—built from our experiences (as we scientists would say, data)—to make a reasonable conclusion that these lights are indeed from a vehicle. We then use our model to estimate how far away the vehicle is and how fast it’s going, and subsequently decide if we can safely pull into the intersection. We never have absolute certainty, but we can still have very high certainty. We justify our models by testing them; if they work, we keep them; if they fail, we modify them.

How do we know precisely how certain we should be? This is complex question that I don’t think can have just one answer, and, in part because this question is so difficult to answer, it is useful to have some rules that will guide us. The idea is to follow rules which will give us the best chance of understanding the world around us. Let start with these:

• We subject all ideas to criticism and experimentation.

• Experimentation is always limited, but this limitation varies considerably. Newton’s Second Law has been tested thoroughly, for example—every bridge, plane, car, and building is built using this principle. There are other concepts and equations in science that have been tested far less. We can be transparent about this by always stating experimental results honestly and accurately. We perform as many tests and observations as possible to avoid being overly sure that our ideas are correct.

• A hypothesis is a possible answer to a scientific question. It can be anything from a wild guess to an educated guess, or perhaps a bit better. A theory is an explanation for a set of phenomena that is well-supported with evidence.

• A good scientific hypothesis makes unique, testable predictions. If we give an explanation that could predict many possible outcomes, we cannot test this properly since it is very difficult to prove it wrong.

• A scientific claim is not correct because of a scientific consensus; rather, a scientific consensus will form around an idea that is supported by a vast array of evidence. This is similar to the adage that “all squares are rectangles, but not all rectangles are squares”.

Let us now turn to the formation of the Solar System. How do we begin to answer this question? The first step could be to list some experimental facts about the Solar System. We want to focus on facts that are particularly hard to explain, because we want to criticize our hypotheses as much as possible. For example, we can begin with the following facts:

• All of the planets orbit in the same direction around the Sun, and all in the same plane.

• All of the planets orbit in nearly circular orbits, with the exception of Mercury.

• The four closest planets are all rocky, and the four farthest are all gaseous. All of the rocky planets are relatively near the Sun, and all of the gaseous planets are relatively far from the Sun. This we describe as saying that the planets are differentiated.

• The oldest objects in the Solar System (as far as we know) are about 4.6 billions years old.

We can then propose different hypotheses and see if they fit the data. For simplicity, let’s propose these two:

1. The Sun captured planets from different parts of the galaxy, due to its gravity.

2. The planets and the Sun formed together.

If hypothesis #1 were correct, why would the planets orbit in the same plane? Why would they orbit in the same direction? Why would all of the rocky planets be relatively close to the Sun? If planets were captured from different parts of the galaxy, it would be highly unlikely that the Solar System would have all (or any) of these properties. Hypothesis #1 does not fit the data, and thus we conclude that it is highly likely to be wrong. Hypothesis #2 can potentially explain all of these facts, but it is not precise enough to test.

Remember that we want to make unique, testable predictions. The claim that the Sun and planets formed “together” is rather ambiguous; what does this mean, exactly? We want to be as precise as possible so that we can make specific predictions; then, we can compare those predictions to the data. In general, the easier it is to prove our hypothesis wrong, the more easily it can be tested and the better it is.

This leads us to a clearer version of this hypothesis, which we will call the Nebular hypothesis: this states that the Solar System formed from a piece of a nebula (a cloud of gas and dust) that contracted under its own gravity; as this piece contracted, it begun to spin faster and faster and eventually formed a disk. How do we determine is this hypothesis is correct?

First, we can subject it to the results of experiments on Earth. We know that all experiments done on Earth conserve angular momentum. This means that the moment of inertia times the angular velocity is constant, so long as there are no external forces speeding up or slowing down the rotation. The moment of inertia can be precisely defined based upon the mass of the object and how far that mass is from the center of rotation. As this quantity increases, the angular velocity (the rate at which the object is spinning) decreases; and as it decreases, the angular velocity increases. This is the conservation of angular momentum, and no experiment has been conducted that contradicts this principle.

As the early Solar System began to shrink, its moment of inertia would have decreased (this is determined based on a mathematical definition), and so experiments done on Earth lead us to state with a high degree of likelihood that the early Solar System would have begun to spin faster and faster. This, in turn, would have flattened the Solar System out to produce a disk. This is similar to how a ball of pizza dough forms a disk when spun, and, therefore, can be tested by experiments on Earth. Thus, we have explained why the planets orbit in the same plane, in the same direction, and why the planets’ orbits are nearly circular. We have a lot more work to do—since, for example, we have not yet explained why the planets are differentiated. We would try to answer this question similarly, however—for example, by taking the properties of atoms and molecules as determined by experiments on Earth and fitting our hypothesis to this data.

I hope the introduction sheds some light on how we would construct a scientific model for how the Solar System formed, and how we would test it. The argument that I have presented thus far, however, is incomplete. Try to think of at least one way that the nebular hypothesis can be tested without relying just on experiments on Earth, and describe this in a paragraph or two. Include any pictures that are relevant.