Chapter 2: The Sun, the Solar System, and the Planets

Our Sun

Our sun is just one of uncountable stars (more than 100 billion) in a single (disk shaped) galaxy known to us as the Milky Way, so named because it forms a bright ribbon of abundant stars across the night sky.  We see it as such because we ourselves are in it  and thus have an on-edge view of it.  The sun is a comparatively small and young star.  The solar system is probably no more than about 4.5 to 5 billion years old (we will discuss later how we know this), and its average chemical composition is pretty much the same as that of many other suns and galaxies.
    Yet, because it is so conspicuous and because its importance was readily realized by long gone primitive societies, it has been the object of veneration by many religions.

Picture at left is a depiction of the Egyptian sun god Ra.

Mass Distribution in the Solar System
The Sun contains 99.85% of all the matter in the Solar System. The planets, which condensed out of the same disk of material that formed the Sun, contain only 0.135% of the mass of the solar system. Jupiter contains more than twice the matter of all the other planets combined. Satellites of the planets, comets, asteroids, meteoroids, and the interplanetary medium constitute the remaining 0.015%. The following table is a list of the mass distribution within our Solar System.

Sun: 99.85%
Planets: 0.135%
Comets: 0.01% ?
Satellites: 0.00005%
Minor Planets: 0.0000002% ?
Meteoroids: 0.0000001% ?
Interplanetary Medium: 0.0000001% ?
sundiag.jpg (11744 bytes)

Because the sun is still in the business of converting hydrogen to helium, it is obvious from these figures that just as the universe, the composition of the solar system is dominated by hydrogen and helium as well. Considering the low abundance of the elements we ourselves are made of (carbon, oxygen, nitrogen, etc.), we are truly made of rare stuff that has been handed down from billions of years before us due to the death of ancestral stars (the ultimate cosmic rebirth?).

The Sun is by far the largest object in the  solar system and contains most of the mass.  It has a diameter of 1,390,000 km's (as compared to 12,756 km's for the Earth), and its outer visible layer is called the photosphere and has a temperature of 6,000C (11,000F). This layer has a mottled appearance due to the turbulent eruptions of energy at the surface.  A link to Soho, a space probe that has been monitoring solar activity since 1995.

Solar energy is created deep within the core of the Sun. It is here that the temperature (15,000,000 C; 27,000,000 F) and pressure (340 billion times Earth's air pressure at sea level) is so intense that nuclear reactions take place.  The energy is carried to the surface of the Sun through a process known as convection.  This convection causes the sun to churn and bubble on the surface, something that is reflected in observation of sunspots and magnetic field fluctuations.  That the surface of the sun behaves essentially as a (very hot and dense) liquid can be seen when major eruptions produce quakes and surface waves.  The energy of the sun is released as light and heat. Energy generated in the Sun's core takes a million years to reach its surface. Every second 700 million tons of hydrogen are converted into helium ashes. In the process 5 million tons of pure energy is released; therefore, as time goes on the Sun is becoming lighter.

The chromosphere is above the photosphere. Solar energy passes through this region on its way out from the center of the Sun. Solar flares arise in the chromosphere as bright filaments of hot gas emerging from sunspot regions. The latter are dark depressions on the photosphere with a typical temperature of  4,000C (7,000F).

The corona is the outer part of the Sun's atmosphere. It is in this region that prominences appears. Prominences are immense clouds of glowing gas that erupt from the upper chromosphere. The outer region of the corona stretches far into space and consists of particles traveling slowly away from the Sun. The corona can only be seen during total solar eclipses eclips94.jpg (3385 bytes).
The temperature of the corona is about 300 times higher than the temperature of the surface of the sun, a mystery for many years.  Recent detail observations of so called coronal loops, immense coils of hot, electrified gas, have given scientists new clues as to the underlying causes.  New data from NASA's Transition Region and Coronal Explorer (TRACE) spacecraft.

The Sun appears to have been active for about 4.6 billion years and has enough fuel to go on for another five billion years or so. At the end of its life, the Sun will start to fuse helium into heavier elements and begin to swell up, ultimately growing so large that it will swallow the Earth. After a billion years as a red giant, it will suddenly collapse into a white dwarf -- the final end product of a star like ours. It may take a trillion years to cool off completely.  An alternative view.

As pointed out earlier, we imagine that vortexes and irregular matter distribution in the solar nebula gave rise to subsidiary accretionary bodies that finally became our planets.

solar-sy-evol.jpg (42510 bytes)

As we go from the upper left to the lower right corner of this drawing we see the history contraction from an irregular gas cloud, to a rotating gas/dust disk (protolyd), gradual sweeping up of matter by our protoplanets, the heating up of the nuclear furnace, the sweeping out of remaining matter by solar wind and the reaction of the planets to the increasing solar radiation (volatiles/gases are "boiled out" and the protoplanets have glowing gas tails like comets), and finally we arrive a a steadily "burning" sun and planets that have equilibrated with solar radiation. Some of the earliest solar system condensates may have been found in a meteorite that fell to earth.

The Solar System

The solar system consists of the Sun; the nine planets, sixty six (66) satellites of the planets, a large number of small bodies (the comets and asteroids), and the interplanetary medium (gas and dust). The inner solar system is subdivided into an inner portion that contains the Sun, Mercury, Venus, Earth and Mars (the so called terrestrial planets), and an outer portion that contains Jupiter, Saturn, Uranus, Neptune and Pluto (the so called jovian planets).  Links for orbital data and physical properties of solar system objects.  A link to the NASA site on solar system exploration.

The orbits of the planets are ellipses with the Sun at one focus, though all except Mercury and Pluto are very nearly circular. The orbits of the planets are all more or less in the same plane (called the ecliptic and defined by the plane of the Earth's orbit). The ecliptic is inclined only 7 degrees from the plane of the Sun's equator. Pluto's orbit deviates the most from the plane of the ecliptic with an inclination of 17 degrees. The above diagrams show the relative sizes of the orbits of the nine planets from a perspective somewhat above the ecliptic (hence their non-circular appearance). They all orbit in the same direction (counter-clockwise looking down from above the Sun's north pole); all but Venus and Uranus also rotate in that same sense.   The fact that the planetary orbits fall more or less in the same plane and move around the sun in the same direction agrees well with the idea that the solar system originated from a rotating gas cloud.

As is obvious from our orbital and physical data, the size, mass, and density of planets varies widely.  There are, however, some general rules that can be recognized.  The inner/terrestrial planets are typically small and dense, and are close to the sun, whereas the outer/jovian planets are much larger, of lower density, and far away from the sun.   This peculiarity can also be explained as a consequence of our general model for solar system formation.   If we assume that all protoplanets sucked up matter as they orbited in the contracting nebula, they should have accreted to pretty much the same composition, and most likely should have contained abundant gaseous matter.  Those, however, that were close to the sun were heated too severely to be able to hold on to these gases (gases reached escape velocity), whereas those that were far away (and stayed cold) could keep most of their gas content in the condensed state, and may even have picked up some more from the material that was vaporized from the inner planets.

Music inspired by the planets.

One way to help visualize the relative sizes in the solar system is to imagine a model in which it is reduced in size by a factor of a billion (1e9). Then the Earth is about 1.3 cm in diameter (the size of a grape). The Moon orbits about a foot away. The Sun is 1.5 meters in diameter (about the height of a man) and 150 meters (about a city block) from the Earth. Jupiter is 15 cm in diameter (the size of a large grapefruit) and 5 blocks away from the Sun. Saturn (the size of an orange) is 10 blocks away; Uranus and Neptune (lemons) are 20 and 30 blocks away. A human on this scale is the size of an atom; the nearest star would be over 40000 km  away.

ss2.gif (32488 bytes) A composite of the planets at approximately correct relative size.

The Terrestrial (inner) Planets

merc.gif (2124 bytes) Mercury is the planet closest to the Sun and the eighth largest (see tables for orbital and physical data).  


venus.gif (1648 bytes) Venus is the second planet from the Sun and the sixth largest (see tables for orbital and physical data). A link to data from the Magellan mission, a spacecraft that mapped the surface of Venus by radar (to penetrate cloud cover). 

earth.gif (2137 bytes) Earth is the third planet from the Sun and the fifth largest. (see tables for orbital and physical data). 

mars.gif (2464 bytes) Mars is the fourth planet from the Sun and the seventh largest.  (see tables for orbital and physical data). Here is a link to NASA's Mars Exploration Program.   and a link to the two Mars Exploration Rovers that landed in January of 2004.

The Jovian (outer) Planets

jupiter.gif (3227 bytes) Jupiter is the fifth planet from the Sun, is by far the largest, and more than twice as massive as all the other planets combined.   (see tables for orbital and physical data). Here is a link to the Cassini spacecraft flyby data.  A link to Galileo, a space probe that has been in orbit around Jupiter since 1995. The first direct observation of Jupiter was made in 1973 by the Pioneer 10 space craft, a probe that since then has left the solar system but still is in touch with Earth.

saturn.gif (1938 bytes) Saturn is the sixth planet from the Sun and the second largest.  (see tables for orbital and physical data).  Here is a link to Cassini-Huygens Mission to Saturn and one of its moons, Titan.

uranus.gif (1553 bytes) Uranus is the seventh planet from the Sun and the third largest (by diameter; by mass it is smaller than Neptune).  (see tables for orbital and physical data).

neptune.gif (1570 bytes) Neptune is the eighth planet from the Sun and the fourth largest (by diameter; by mass it is larger than Uranus).  (see tables for orbital and physical data).

pluto.gif (859 bytes) Pluto is the farthest planet from the Sun (usually) and by far the smallest. It is even smaller than seven of the solar system's moons (the Moon, Io, Europa, Ganymede, Callisto, Titan and Triton). (see tables for orbital and physical data).

We have not discussed so far the numerous smaller bodies that inhabit the solar system, the large number of asteroids (small rocky bodies) orbiting the Sun, mostly between Mars and Jupiter but also elsewhere; and the comets (small icy bodies) which come and go from the inner parts of the solar system in highly elongated orbits and at random orientations to the ecliptic. With a few exceptions, the planetary satellites orbit in the same sense as the planets and approximately in the plane of the ecliptic but this is not generally true for comets and asteroids.

Other Participants in the Solar System Ballet

There are thousands of known asteroids and comets and undoubtedly many more unknown ones. Most asteroids orbit between Mars and Jupiter (scientists speculate that the material in the so called asteroids belt. A few (e.g. 2060 Chiron) are farther out. There are also some asteroids whose orbits carry them closer to the Sun than the Earth (Aten, Icarus, Hephaistos). Most comets have highly elliptical orbits which spend most of their time in the outer reaches of the solar system with only brief passages close to the Sun.  Here is a link to a web site about Deep Space 1, a probe that is testing new technologies and visits asteroids.

The first successful landing of a spacecraft on an asteroid was accomplished by the NEAR space probe on February 12th, 2001.  See some pictures and movies taken by NEAR during its orbit and descent onto asteroid Eros.

Finally, the space between the planets is not empty at all. It contains a great deal of microscopic dust and gas as well as radiation and magnetic fields.

Chapter 3

"The woods are lovely, dark and deep.
But I have promises to keep,
And miles to go before I sleep,
And miles to go before I sleep."
- Robert Frost (1874-1963)