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• 1 media/01 The Solar System PIA10231.jpg 2016-04-05T10:08:09+00:00 Orbits, Rotation and Temperature 10 image_header 2019-12-29T14:53:08+00:00 All planets orbit the Sun in the same sense:
  
  Each planet rotates and the axis and sense of rotation of these planets is very similar.  With the exception of Venus, Uranus and Pluto, the rotation axes of the planets are approximately parallel to the rotation axis of the sun and the sense of rotation is the same as illustrated in this image.
  
  As you might expect, planetary temperature decreases with increasing distance from the Sun.  The farther from the Sun the planet, the colder it is.  The big exception to that rule is Venus as illustrated in the graphic below (For comparison, the temperature to melt lead is 327.5˚C). We'll discuss that later. The temperatures in the graphic below of the gas and ice giants (Jupiter, Saturn, Uranus, and Neptune) are taken from a level in the atmosphere equal in pressure to sea level on Earth.
  There are also lots of objects besides planets in the solar system, as we'll see next.
• 1 2016-04-05T10:08:05+00:00 Composition of the solar system 2 plain 2016-04-08T07:14:24+00:00 There is another way in which we can examine the content of the solar system and that is by composition.  What was the mixture of elements from which the sun, the planets and well, us, were made?  Ours sun consumes about 600 million tons of hydrogen per second, so we need lots of H to keep it going.  As we will see in module 5 we life forms are primarily composed of Carbon, Hydrogen, Nitrogen, Oxygen, Phosphorus and Sulfur so we need an adequate amount of that.  And we wander about on the rocky surface of a planet with an iron rich core which means we also needed those ingredients. If the solar system had not possessed the right mixture of ingredients, we would not exist in our current form.  So what is that mixture?
  
  The technical term that is used for this mixture is the “Cosmic abundance”.  It is beyond the scope of this course to elaborate how it comes about, but if you interested check here. Most commonly this mixture is displayed as a relative abundance with increasing atomic number on one axis and a logarithmic scale (each tick is a factor of 10 more than the tick below it).  The reason for this presentation is that there is far more hydrogen and helium atoms than any other components in the solar system.  You will also notice that abundance generally decreases as atomic number increases.  There is relatively little of heavy elements such as Gold which may be one of the reasons why we value this shiny yellow metal so much.  There is also a relatively significant spike for Iron and that explains why we have iron-rich cores within our terrestrial planets.
  
  If you prefer a representation of the cosmic abundance as a percentage-based bar graph, you may prefer this graph of a few select elements and their classification as elements.  In module 5 we will see that several elements are essential for the formation of life and these are identified as well in this figure.