by D. Vogt

Created on: February 15, 2010   Last Updated: April 30, 2010

The solar system consists of our planet Earth as well as all of the other objects which orbit around our Sun - the other terrestrial or "rocky" planets (Mercury, Venus, and Mars), the asteroid belt, the gas giants and ice giants (Jupiter, Saturn, Uranus, and Neptune), the Kuiper Belt objects (including Pluto and Eris), and, even further out, the small, frozen comets and debris of the scattered disk and Oort cloud. Just how large this system really is has been a matter of debate. Earth is 94 million miles from the Sun; Pluto, at the farthest point in its orbit, is 4.6 billion miles, and other Kuiper belt objects have known orbits twice as great as that.

THE SUN

At the centre of the solar system is our Sun, a stable yellow dwarf star which is approximately 5 billion years old, and probably has about that much time again left before it runs out of fuel and puffs up into a red giant. As with all stars, the Sun generates its massive quantities of heat and light through the fusion of hydrogen atoms into helium in its core. In one way or another, the climates and features of all other planets ultimately depend upon how much of this solar radiation they receive and absorb.

Despite this brief description, the Sun is actually extremely complex. All of the hydrogen fusion occurs in the core, a ball of plasma heated to many millions of degrees; almost a million miles farther out, the surface is actually comparatively cool and even prone to dark, cool areas, or "sunspots." A variety of scientific research missions track various facets of the life of this star, including NASA's Solar Dynamics Observatory, an orbiting probe launched in early 2010.

ROCKY ("TERRESTRIAL") PLANETS

The first four planets share one feature in common: they are mostly composed of rock. Mercury, Venus, Earth and Mars are also relatively close to the sun: Mars is 1.5 times Earth's distance from the Sun, but the next planet, Jupiter, is over 5 times the same distance. Beyond this, however, they vary considerably. Sometimes the rocky planets are referred to as terrestrial planets, referring to their similarity to Earth.

Mercury is so close to the Sun that it is tidally locked, the same process our own Moon has undergone relative to Earth (no matter where it is in its orbit, it always presents the same face to us, and Mercury always presents the same surface to the sun). Moreover, most of its marginal atmosphere has also been drawn away by the Sun, leaving its surface with only tenuous protection from

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