Quick Reference
ASTRONOMY - THE SOLAR SYSTEM
The Solar System | Star Formation | Celestial Bodies | Spectral Classes
The solar system is about 4.6 million years old. It began as a spinning disc of hydrogen and helium which, over a period of 25 million years, formed into the sun and surrounding planets.
THE SUN
The Sun is a nuclear reactor generating energy by the conversion of hydrogen into helium. The rate of conversion is relatively slow, there is enough hydrogen for another 15 billion years.
The granular surface (photosphere) is constantly changing, the main features of which are sunspots and erupting jets of gas (spicules). Above the surface is a hydrogen chromosphere and beyond this is the corona. The Sun is classified as a G2 yellow dwarf.
THE TERRESTRIAL PLANETS
The inner four planets all have an iron core, rock mantle and a crust of varying thickness.
MERCURY
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Mercury, the innermost planet, is heavily cratered,
very hot and has virtually no atmosphere. It spins very slowly,
only three times in every two orbits of the sun. |
VENUS
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Venus is often the brightest object in the night
sky due to the reflective cloud layer, the result of a hot carbon-dioxide
atmosphere. The rocky surface is relatively flat with two pronounced
highland regions, a temperature of over 800 C and prone to violent
volcanic activity. |
EARTH
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The third planet, Earth, is so far the only one
known to support life. Two thirds of the surface is covered by water
and the atmosphere is mostly nitrogen-oxygen. See previous chapters
for more information. |
MARS
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Mars: the red planet. The surface is barren and
cratered with massive volcanos, deep chasms and two small polar
ice caps. The red colour comes from the iron rich sand that is literally
rusty. Mars has a thin carbon-dioxide atmosphere. |
THE GAS GIANTS
So called because of their gaseous surfaces (albeit frozen), all the gas giants have ring systems of dust and ice fragments. In some cases the rings are 'shepherded' by small satellites.
JUPITER
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The largest planet in the solar system. Jupiter
has a rocky core and a hydrogen/helium atmosphere the inner layers
of which are very dense. The atmosphere is distorted by the high
rotational speed causing the equator to budge. The circulating winds
form coloured belts that include a huge storm called the Great Red
Spot. The outermost satellites are probably captured asteroids. |
SATURN
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Very similar to Jupiter in composition. Saturn
has a distinctive ring system split into 7 main bands each containing
hundreds of separate rings. |
URANUS
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Uranus, the seventh planet, has a rocky core, a
frozen shell of water, methane and ammonia, and an atmosphere of
hydrogen, helium and methane. Uranus has nine rings and the axis
is horizontal: it rolls rather than spins round the sun. |
NEPTUNE
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The structure of Neptune is similar to that of
Uranus. The atmosphere contains distinctive, blue, fast moving clouds
and the Great Dark Spot; a storm the size of Earth about which a
smaller storm, the Scooter, orbits. |
PLUTO
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Pluto is a small rocky planet with a thick ice
mantle that orbits high out of the elliptic and, until 1999, was
inside the orbit of Neptune. |
1. The orbital (sidereal) period is measured in "Earth" units. For example:
1 Mercury year = 88 Earth days.
2. The masses of the planets are given relative to the mass of the Earth
which is 6 × 1021 tonnes.
