Solar System Inventory
· 1 Sun
· 9 Planets
· 40 Satellites
· minor planets/asteroids, interplanetary medium (gas, dust, ice, etc.)

Distribution of Mass

	Wt. %
Sun	99.86
Planets	0.135
Satellites	.00004

 

Composition of the Sun:
More than 65 elements have been detected in the Sun's optical spectrum, including:

	Volume %
hydrogen	81.76
helium	18.17
all others	00.07

mean density  =  1.41 gm/cm3

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• All planets lie virtually in the same plane.  Two exceptions include Mercury, which is 7 degrees out and Pluto, which is 17 degrees out.
• All planets revolve around the Sun in the same direction (counter clockwise).
• With the exception of Venus and Uranus, all planets rotate in the same direction.
• When the asteroid belt is included, the orbits of the planets are regularly spaced outward from the Sun, with each planet being located approximately twice as far from the Sun as its innermost neighbor planet.  This is described by the Titus-Bode Rule.
• The planets form two very distinctive groups based on size and density.  These include the small, inner terrestrial (earth-like) planets characterized by densities between 4.0 and 5.5 gm/cc, and the outer giant (Jovian) planets with densities between 0.7 and 1.7 gm/cc.   This observation implies that the inner planets have chemical compositions similar to that of Earth, whereas compositions of the outer planets are more like that of the Sun.
• Radiometric ages of the oldest rocks on Earth, the Moon and meteorites (including those identified as coming from Mars) have similar ages, and suggest a common time of planetary formation at 4.7 billion years ago (4.7 Ga).

The Nebular Hypothesis
 Current hypotheses link the origin of the planets to that of the solar system itself.  The modern theories actually date back to ideas originally presented by the German philosopher Kant and the French mathematician Laplace in the late 1700's!
 

Modern theories are based on the observation that some areas of interstellar  space contain concentrations of gas (mostly H and He) as well as dust of composition similar to terrestrial rocks.  These areas from what are known as nebulae.  These slowly rotating clouds of matter are dominated by forces of gravitational attraction and the laws governing angular momemtum.  Figure A illustrates such a spherical  cloud.  Under the influence of gravity, matter in the nubula will begin to be concentrated in the center, and the nebula as a whole will contract.  This central area will become what is known as a proto-Sun.  As the nebula contracts due to gravity, conservation of angular momentum requires that the rate of rotation must increase.  As the rotation increases, a flat, rapidly rotating disk forms around the proto-Sun, as shown in Figure B.  At this stage, this might look similar to the current situation of the ring around Saturn.   At the same time, contraction of the material in the proto-Sun results in an increase of both temperature (T) and pressure (P). At some stage of gravitational contraction, T and P become high enough that nuclear fusion of Hydrogen begins and the Sun starts to "burn".  Meanwhile, gravitational attraction of material in the rotating disk (ring) will result in concentration of matter within regularly spaced "orbits" within the disk (Figure C.  The gravitationally concentrated material within the now rapidly rotating disk will coalesce to form the planets by a process known as accretion (see below).  By this time, the proto-Sun has achieved the T and P necessary to sustain nuclear fusion.  [GIF image from Press and Siever (1994), Understanding Earth, Freeman Publishing]

 

N.B.  Proof that modern-day disks of matter around proto-stars can been seen in this telescopic image

Formation of the Planets

All of the planets are believed to have formed by a process known as accretion.  This term means "to grow by being added upon".  It is a process whereby gravitational attraction pulls smaller particles toward a larger (higher mass body).  Accretion can best be understood by realizing that it goes on to this day.  The Earth's gravitational attraction causes small bodies within the vicinity of the planet to gravitate toward the Earth.  We witness this phenomenon as meteors that today are pulled to the Earth.  Today the space around Earth is relatively sparce, so meteorite impacts are relatively infrequent.  However, at  4.7 Ga, the space around Earth was very cluttered, and impacts were both frequent and significant in size.  This is the process of accretion.

When the Earth (and other planets) was forming from the material in the solar disk, the rate of meteorite impact must have been very high, as depicted in Figure A. (above).

Meteorite impact is NOT a passive event, but one that releases large amounts of energy.  As an example, an iron meteorite with a volume of 1 cubic meter has a mass of 10^6 grams (15,000 lbs).  Such a body impacting on Earth would have a velocity of 15 km/sec ((32,000 mph). Upon impact, it would release kinetic energy (k.e. = 0.5mv^2) of 1.9 x 10^11 calories or the equivalent of 52 tons of TNT explosive! The net result of numerous impacts is that the surface of the growing earth becomes very HOT.