2.  A convenient unit of distance measurement within the solar system. Miles / kilometers are too small, and light years are too large.  One astronomical unit (AU) is defined as the average distance between the earth and the sun.  Pluto’s distance from the sun is 39.72 AU’s. This equates to 5.5 light-hours.

3.  Very accurate radioactive dating methods of lunar samples, terrestrial samples, and meteorites indicate that the solar system is approximately 4.6 billion years old.

4.  As Earth formed from the solar nebula, it heated and melted.  Differentiation took place, with heavier elements sinking to the center.  The least dense materials (gases) were burped out to form an atmosphere. This early primitive atmosphere was probably similar to present-day volcanic gases.

5.  UV light from the sun would have kept the surface sterile, until oxygen was released and formed a protective ozone layer in the upper atmosphere.

6.  Have caused extensive cratering on the Moon, Mercury and other bodies in the solar system.  Most craters on Earth have been obliterated by active recycling of the planet’s surface.  Some large impacts have had a dramatic effect on life on Earth, resulting in mass extinctions.