Astropedia Textbook

Hannes Alfvén

Nobel laureate Hannes Alfven, who spent years researching the Solar System's origin, once said: "To trace the origin of the solar system is archaeology, not physics." In other words, instead of working with direct observations, we have to search for clues from ancient materials to help us reconstruct the events that occurred long ago. The most important clues to the formation of the Solar System are the properties that cannot be explained by present-day conditions but must have arisen as the solar system formed. These properties do not reflect ordinary geological evolution over 4.5 billion years, but rather they reflect the primordial, or original, conditions. These clues help us piece together the origin of the Solar System.

Sometimes our way of thinking about a problem affects our ability to come up with the answer. For a long time, scientists viewed the formation of the Solar System as a catastrophic event. Such a view implies that the creation of planets is rare, random, and difficult to predict. The discovery of extrasolar planets around other stars indicates that the formation of planets is a "normal" process, a common byproduct of star formation.

Here are the clues, or pieces of evidence, that a successful theory of the formation of the solar system must explain:

Scale model of the diameter of the planets.

• Most of the mass of the solar system is contained in the Sun; the planets contribute only 0.2%.

• All of the planets' orbits lie roughly in a single plane.

• The Sun's rotational equator lies nearly in this plane.

• The planets and the Sun all orbit in the same west-to-east direction, called prograde (or direct) revolution.

• Planetary orbits are nearly circular.

• Planets differ in composition.

• The composition of planets varies roughly with distance from the Sun: dense, metal-rich planets lie in the inner system, whereas giant, hydrogen-rich planets lie in the outer system.

• Meteorites differ in geological properties from all known planetary and lunar rocks.

Artist concept of a dusty star disk

• The Sun and all the planets except Venus and Uranus rotate on their axes in the same direction (prograde rotation).

• Obliquity (tilt between equatorial and orbital planes) is generally small.

• Planets and most asteroids rotate with rather similar periods, about 5 to 10 hours, except where obvious tidal forces have slowed their rotation (as in the Earth's case).

• Distances between planets for the most part conform to a simple rule, Bode's rule, which is a roughly geometric spacing with distance from the Sun.

• Outer planets are more massive than inner planets, and most of the mass of the outer planets is hydrogen and helium.

• Planet-satellite systems resemble the Solar System in miniature.

• Impact craters are common throughout the Solar System, and the cratering rate was much higher in the first few hundred million years of solar system history.

• As a group, most comets' orbits define a large, almost spherical swarm around the Solar System (the Oort cloud). Other comets reside in the Kuiper belt, near Pluto and just beyond it.

• The planets have much more angular momentum than the Sun.

This list of seemingly unrelated facts can be organized into several themes:

Clues from Geometry. Most of the mass of the Solar System is contained in a central spherical object: the Sun. The planets are not distributed randomly around the Sun. All the orbits lie roughly in a plane. The distances of the planets from the Sun obey a roughly geometric spacing. However, some components of the Solar System are distributed differently in space. In particular, the swarm of comets called the Oort cloud fills an enormous sphere centered on the Sun and the planets.

Artist concept of a protoplanetary disk

Clues from Motions. We must first distinguish clearly between two important motions. A revolution is the motion of a planet around the Sun. A rotation is the spin of a planet on its axis. The planets all revolve around the Sun in a prograde direction, in orbits that are nearly circular. In addition, the Sun and all but two of the planets rotate on their axes in the same direction. In other words, the planets spin in the same sense that they travel around the Sun. The spin axes of the planets are in most cases perpendicular to the plane of the orbits.

Clues from Composition. Planets near the Sun are small, dense, and rich in metals. Planets far from the Sun are large, low density, and rich in hydrogen and helium. Each planet differs slightly in composition. Meteorites give us our only direct evidence of the composition of interplanetary bodies — they differ in geological properties from all known terrestrial and lunar rocks.

Clues from Morphology. Planet-satellite systems resemble the Solar System in miniature; satellites orbit in the equatorial plane of the planet and in the same sense as the planet's rotation. Ring systems behave the same way. Rings seem to be evidence of a failed planet (in the case of the main asteroid belt) or a failed satellite (in the case of the ring systems of the four outer gas giants). Impact craters occur throughout the solar system, but most of them were created in the first 10% of Solar System history.

Imagine you are a great detective and you have just arrived at a crime scene. The body is cold; the house is empty. The murder took place hours before, but the room around you contains clues as to what happened. Some clues are obvious, others are subtle, but all clues are important! You may find the body in a particular position. Objects in the room may be out of place. There may be chemical traces on the carpet or a distinctive odor in the air. In the same way, the inhabitants of planet Earth are faced with a mystery. The events that led to our existence took place billions of years ago. Yet all around us are telltale signs of the formation process.