Welcome to The Planets
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The Planets

What is a planet? A planet is generally considered a massive spherical natural object that orbits around a central star. Planets are part of our solar system. There is sometimes a bit of a controversy as whether an object is planet. For example Pluto is sometimes not considered a planet. Most recently Pluto has been designated a dwarf planet, so it is a kind of planet but not in the same sense as the other larger main planets. It is now widely believed from good astronomical evidence that there are planets around other stars. Although many of these planets are very large, sometimes 2-3 times bigger than Jupiter. These large planets are sometimes called failed stars or brown dwarfs because they do not produce their own fusion reactions like typical stars. What does "officially" mean? It typically means voting decisions by the International Astronomical Union

The word "planet" comes from the Greek word that means 'to wander". So planets become " wanders" or  wanders of the night sky. The officially recognised planets include: Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Dwarf planets - Ceres, Pluto and 2003 UB313(Eris) Defining when is a planet a planet? The definition of planet is often difficult and seems to depend on scientific and cultural acceptance of things that are planets. Officially a planet is a celestial body that is : (a) in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit. If an object only meets the first two criteria it is considered a dwarf planet.

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Why are the planets important? The Planets are important to us for a wide range of reasons. These include: Technologically - Planets provide us with a goal. Just trying to get to planet inspires us to come up with some very innovative technologies. Without planets we would have less of an interest in coming up with space exploration. Scientifically - discovering things about planets eventually helps us to understand some of the process on Earth. We may also discover new natural processes on other planets. Coming up with theories for the formation of planets is also a scientific goal. Culturally - Planets have an important impact on our creative spirit. They have had a profound impact on the development of ancient religions. The names of the planets themselves reflect the names of ancient gods.

The planets have inspired poets, musicians and artists for thousands of years. For example the composer Gustav Holst (1874-1934) wrote an entire symphony called "The Planets".

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News about planets Google News search for planets  provides some very recent headlines. Nature & Science News ClearlyExplained.Com often provides news about discoveries about planets.

Some reliable sources of information about the planets The Nine Planets Planet -Wikipedia The Planets - NASA-Jet Propulsion Laboratories Lives of the Planets : A Natural History of the Solar System (book)

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How are the planets studied/classified? The planets can be classified or studied in various ways: Nature of the planets ClearlyExplained.Com The nature and science of the planets is about how biology, chemistry and physics can be used to describe and explain these objects out in space The characteristics of each planet, such as where they are , how far away, their temperature and composition. Technology of planetary exploration ClearlyExplained.Com This is the practical application of science to help explore planets such as The space probes that have or are currently visiting the planets in our solar system The development of technologies for planetary exploration for use here on earth. Culture of aspects of the planets ClearlyExplained.Com This is the social, & creative humanistic context of planets that relate to: Art/Movies inspired by planets Music inspired by themes of planets or planetary exploration.

Planets are typically classified by their physical structure.  Rocky/terrestrial Planets and dwarf planets Gaseous (gas giants) planets Plutonian Dwarf planets  1.Mercury - 2. Venus - 3. Earth - 4. Mars (from top to bottom) Just after Mars there is a dwarf planet called Ceres. 5. Jupiter - 6. Saturn -7. Uranus - 8. Neptune Gas giants are to scale against part of the sun. Beyond the orbit of Neptune lie objects called Dwarf planets sometimes called Plutonians of which Pluto is the prime example. To scale to each other. How do the planets move? The planets move according Newtons laws of motion. Typically they move in an ellipse (an oval shape). The movement of most planets are described by Kepler's laws ( (1571-1630). But the orbit of Mercury does not quite fit Newtons laws because it is so close to the sun instead Einstein's theory of general relativity explained the slight difference in Mercury's orbit.

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What is the  history of the planets? The planets were one of the first star like objects in the night sky that were noticed to move in a regular pattern accross the night sky. Some of the first hunter gather societies probably noticed planets while sitting by their camp fires and wonder about these objects with regular paths across the night sky.

Cultural history of the planets Typically when we think of the history of planets we think about who discovered them first. This usually applies to the planets furthest away. The largest planets such as Jupiter and Saturn have been known since ancient times. The natural history of the planets The history of the planets would also consider the origin of the planets. How did they form? how long did it take, what forces were needed? image credit: NASA/Caltech Pyle (SSC) Generally it is thought that the planets were created from recycled star dust, and interstellar hydrogen that collapsed in under its own gravity. It formed something called an accretion disk. The denser materials tended to stay closer to the sun and lighter materials produced the gas giants.

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The future of planets Will the planets continue into the future? Planets are dynamic, they move and are influenced by the sun and even by the milky way galaxy that it travels around. The look of the various planets will change over time mainly due to asteroid impacts. These may be small or large and devestating enough to change the orbits of planets. Some gas giant planets have a surface that changes all the time. For example jupiter with its storms the size of inner planets. This storm may eventually fade away after a few thousand years. When the Comet Shoemaker Levy-9 hit Jupiter in 1994 it left a large scar in the atmosphere that lasted several months. The impact of a comet on Jupiter from July 18 - August 24 1994. image: Hubble Space telescope, NASA

The planets will not last forever. The sun will likely turn into a red giant and consume most of the inner planets in the inner solar system. reference: Planetary Nebulae and the future of the solar system, Washington Univeristy

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