There are some obvious questions associated with the Solar System. The first might be "How old is it?" Stellar evolution theory says that the Sun is about 5 billion years old, but that depends on how well we understand how stars work. Is there an independent way of estimating the Solar System's age?

Yes, there is, and it has to do with nuclear fission. The metals in the solar system were formed from previous supernovae, and, included in those metals are certain radioactive elements, such potassium-40, rubidium-87, or uranium-238. (The number refers to the total number of protons and neutrons in the atomic nucleus. Different isotopes of an element have the same number of protons, but different numbers of neutrons.) Potassium-40 and uranium-238 are unstable -- left alone, they will spontaneously decay into other elements. (In the three examples above, potassium-40 will decay into calcuim-40 or argon-40, rubidium-87 will decay into strontium-87, and uranium-238 will decay into lead-206.)

Now radioactive decay proceeds in a very specific fashion. It is characterized by half-life. For example, the half-life of uranium-238 is 4.5 billion years. So, if you start with a block of uranium-238, then after 4.5 billion years, half of it will have turned into lead. If you wait another 4.5 billion years, then half of the remaining uranium-238 would also have turned into lead, so the ratio of uranium-238 to lead-206 will be 1-to-4. In another 4.5 billion years, the ratio would be 1-to-8 and so on.

Other radioactive decays proceed in a similar fashion; the half-life of potassium-40 is 1.3 billion years; that of rubidium-87 is 47 billion years. In their examination of meteorites and moon-rocks, geologists looks for inclusions of these radioactive elements. They then measure the ratio of the parent isotope to the daughter isotope. This defines the age of the rocks. Moon rocks and meteorites give 4.6 billion years for the age of the solar system. (The oldest rocks on Earth are a bit younger, 3.9 billion years, but the Earth has been surfaced due to ancient lava flows.)

(While we're here, there is one other property of nuclear fission that is important. In general, when an atom heavier than iron undergoes fission, the mass of the two particles that are produced is less than the original particle. Thus, energy is produced. We'll come back to this later.)

The next question to ask is "How did the Solar System form?" Any theory of solar system formation must account for the obvious features we see, such as (1) The fact that the Solar System is a fairly flat place, with all the planets within a few degrees of the ecliptic and revolving in roughly circular oribts that are all going in the same direction; (2) The division between the small, rocky terrestrial planets in the inner part of the solar system, and the hydrogen-rich Jovian planets in the outer solar system; (3) The decrease in average planet density from the inner part of the solar system to the outer part; and (4) The existence of Bodes law, with each planet roughly twice as far from the Sun as the previous planet.

The first property of the Solar System, that of its flatness, is simple to understand in terms of the mechanisms we have already studied. The solar system probably began as a large gas cloud, which began collapsing due to gravity. Such a cloud would have had some rotation to start with: since all the gas in the Galaxy is rotating about the Galactic center and (thanks to Kepler's and Newton's laws) the rotational period for the gas further out is longer than that for the gas close in, any cloud of any size would have started with some rotation. As the collapse continued, centripetal force and the conservation of angular momentum would have caused the cloud to collapse to a disk, much like it caused the proto-galaxy to form a disk. The densest region of this disk (the center) became the Sun.

The composition of the solar nebula, like that of everything else in the galaxy, was mostly hydrogen and helium, with a few contaminants. In the disk of the early Solar System, the density would have been high enough for some molecules to form, for example water (or, more properly, ice), and, in time, small bodies, called planetesimals would have condensed out of the gas. At first, these planetesimals (which may only be a centimeter or meter in size) would have grown by further condensation, where one atom or molecule attaches itself to the main body, via molecular forces. After a while, accretion will take over, where two bodies collide and stick together. Note that this works because of Kepler's and Newton's laws. If two planetesimals form at approximately the same radius, then even if one is on the opposite side of the Solar System from the other, their slight difference in rotation speeds guarantees that eventually the bodies will catch up to each other. Thus planetesimals will grow in size, and as they do, they will clear out spaces in the nebula. Note that as the planetesimals grow, their gravitational attraction towards other planetesmals will increase as well, further helping them clear a path.

Eventually, the planetesimals reach a size that they can be called protoplanets. But note that not all protoplanets would be made of the same material. The inner solar system was probably very hot, due to the energy of the Sun. In this area, only elements with high melting temperatures (metals, etc.) could have condensed out, and the protoplanets would have a high density. Conversely, in the cold regions of the outer solar system, light ices could condense in the nebula. This explains the gradual change in planet density.

While planetary formation was going on, the Sun was moving towards the main sequence. In the early stages of the Sun's formation (as gravity was still shrinking it down in size), its appearance would have been more like a red giant. The Sun would have been rotating faster then, and most of its outer portion would have been convecting. Convection in the outer portion of a star causes strong magnetic fields, which, in turn, cause sunspots and solar flares. As a consequence of all this, (1) The pre-main sequence proto-Sun was brighter than it is now; and (2) The pre-main sequence proto-Sun would have been very "active," in the stellar-flare and sunspot sense.

This is important for it explains how most of the light gases got ejected from the inner part of the Solar System. Think of the photons of light as little particles. When these photons collide with something, like an atom, they impart momentum. This is radiation pressure. Now think of sunspots and solar flares. When this activity occurs, the Sun throws off into space many high energy particles, mostly protons and helium nuclei (called alpha particles). This is called the solar wind. Radiation pressure and the solar wind in the early Solar System essentially blew all the excess gas out of the inner part of the Solar System. That is why the terrestrial planets are so small - the light gases were blown away.

Once the major planets were formed, the excess debris (extra proto-planets, asteroids, comets, and so on) couldn't stick around long. It turns out that while two bodies can go around each other in nice, stable orbits, the orbits of more than two bodies are invariably chaotic. A body, say, in the orbit between Jupiter and Saturn, would occasionally pass (or be passed) by these planets (due to Kepler's and Newton's laws) and its orbit would be perturbed. After many years (or centuries, or millenia), the orbits would be affected so much that they can become highly elliptical, crossing the paths of other bodies. Eventually these objects will either crash into another planet, or be ejected completely from the Solar System. Up until about 3 billion years ago, impacts in the Solar System were common. However, as time went on, fewer and fewer of the excess bodies remained, and the rate of impacts decreased significantly.

There is no easy explanation for Bode's law, other than to say that computer simulations of the evolution of the Solar System typically create a similar distribution of planets. Clearly, a small body can't exist in an orbit near Jupiter: Jupiter's gravitational pull would contort its orbit until it was ejected from the Solar System. (This is probably the same reason that no planet ever formed in the asteroid belt - Jupiter was there to keep it from happening.) Given the distribution of masses in the Solar System, it seems that Bode's law gives the minimum separation planets can have without affecting each other's orbits too much.

While all this is going on, the terrestrial planets were forming. In the beginning all of these objects were molten. Almost continual impacts of the debris from the Solar System ensured this, and, if this weren't enough, the inside of these planets would have been heated to molten temperatures by the energy produced by the nuclear fission of radioactive materials. In a molten environment, differentiation occurs, where heavy metals sink, and lighter silicates (rocks) rise. That's why, for example, the Earth's core is mostly iron and nickel, while the surface is made of rocker substances.

As the crust of the terrestrial planets cooled, the age of cratering continued, as the Solar System slowly cleaned itself out. This was followed by flooding, not necessarily by water, but by molten lava from inside. (In time, the interior cools, as the amount of radioactivite materials on hand decreases, but at an age of 1-2 billion years, this source of energy was still very important.)

At the same time as this cratering and flooding was occuring, the planet was outgassing. Gases trapped inside the planet during formation, or formed from radioactive decay, burst out and form an atmosphere. Included in these gases are water vapor, carbon dioxide, hydrogen, helium, and probably methane and ammonia. As the planet cooled, the water vapor started to liquify and fall as rain. The first oceans were formed.

Finally, the processes of surface erosion began. Included here are simple erosion due to water and the atmosphere, plus plate techtonics and geologic motions.