Traditional Definition
Etymology: Middle English planete, from Old French, from Late Latin planeta, modification of Greek planEt-, planEs, literally, wanderer, from planasthai to wander.
1. any of the seven celestial bodies: Sun, Moon, Venus, Jupiter, Mars, Mercury, and Saturn that in ancient belief have motions of their own among the fixed stars.
2. any of the large bodies that revolve around the Sun in the solar system. [see IAU resolutions below]
3. a similar body associated with another star.
EARTH -- usually used with "the".
4. a celestial body held to influence the fate of human beings
5. a person or thing of great importance : LUMINARY

- plan·et·like /-"lIk/ adjective
Merriam-Webster Online Dictionary

* The International Astronomical Union [IAU] officially classifies planets.

STATUS February 2, 2006

"The IAU notes the very rapid pace of discovery of bodies within the Solar system over the last decade and so our understanding of the Trans-Neptunian Region is therefore still evolving very rapidly. This is in serious contrast to the situation when Pluto was discovered. As a consequence, The IAU has established a Working Group to consider the definition of a minimum size for a Planet. Until the report of this Working Group is received, all objects discovered at a distance from the Sun greater than 40 AU will continue to be regarded as part of the Trans-Neptunian population."

UPDATE August 24, 2006

The IAU therefore resolves that planets and other bodies in our Solar System be defined into three distinct categories in the following way:

1. A planet1 is a celestial body that
(a) is 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.

2. A dwarf planet is a celestial body that
(a) is 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) shape2,
(c) has not cleared the neighbourhood around its orbit, and
(d) is not a satellite.

3. All other objects3 orbiting the Sun shall be referred to collectively as "Small Solar System Bodies".

1The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
2An IAU process will be established to assign borderline objects into either dwarf planet and other categories.
3These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies.

Relative sizes of the planets and stars by Dave Jarvis.
Smallest Planets
Smallest Planets
Largest Planets
Largest Planets
The Planets to the Sun
Planets and Sun
The Sun to Arcturus
Sun to Arcturus
Betelgeuse and Antares
Betelgeuse and Antares

Messenger of the gods.

Closest planet to the Sun and the eighth largest.
    mass: 3.30 x 1023 kg
    orbit: 57,910,000 km (0.38 AU) from Sun
    diameter: 4,880 km

Mercury is always very near the Sun and difficult to see in the early sunset or pre-dawn. It is best pursued with small telescopes and binoculars when at its greatest eastern or western elongation, that is when its angular deviations are at a maximum with the Sun as it appears to swing back and forth from the morning to the evening skies.

Mercury has no known moons.

Goddess of love
and beauty.

Second planet from the Sun and the sixth largest.
    mass: 4.869 x 1024 kg
    orbit: 108,200,000 km (0.72 AU) from Sun
    diameter: 12,103.6 km

Venus shows phases when viewed with a telescope from the perspective of Earth because it is closer to the Sun than the Earth. When seeing part of its unlit side it presents a crescent or gibbous shape, much like our Moon. The reflected sunlight from Venus is very bright. On a dark moon-less night you may see your shadow from the light of Venus.

Venus has no known moons.

Old English word.
The only planet not
named after a god.

Third planet from the Sun and the fifth largest.
    mass: 5.972 x 1024 kg
    orbit: 149,600,000 km (1.00 AU) from Sun
    diameter: 12,756.3 km

It was not until the 16th century that the idea began to spread and be accepted that the Earth was a planet in the same sense as the other planets that orbit the Sun.

Earth has only one moon, which is unlike any of the other satellite bodies in the solar system. It is extremely large yet relatively distant from its parent planet. Other moons of the solar system tend to be classified as natural satellites or captured planetoids. The naturals are mostly large with circular orbits, very near the parent planet, aligned with the planet's equator. Captured planetoids tend to be small with eccentric orbits inclined to the equator and relatively distant from the parent planet. The Earth's moon does not fit into either of these categories very well.

Thousands of artificial satellites have been placed in orbit around the Earth.

God of war.

Fourth planet from the Sun and the seventh largest.
    mass: 6.4219 x 1023 kg
    orbit: 227,940,000 km (1.52 AU) from Sun
    diameter: 6,794 km

In an amateur telecope Mars varies greatly depending on its orbital relationship with Earth. When it is close, it is easy to see a white polar cap and dark features on the lower latitudes.

Mars has two very small moons which are very close to the surface of the planet. Their names are Phobos and Deimos (Fear and Terror).

King of the gods.

Fifth planet from the Sun and the largest.
    mass: 1.900 x 1027 kg
    orbit: 778,330,000 km (5.20 AU) from Sun
    diameter: 142,984 km (at the equator)

Viewing the surface features depends greatly on seeing conditions. Usually a 6" or 8" diameter telescope with reveal the Great Red Spot, which is a giant cyclonic storm about three times the size of Earth. The lateral bands on the surface are alternately colored: The darker ones ranging from brick red to light peach and the lighter ones ranging from aqua to light grey. Sometime you will see a black dot slowly travelling across the surface of the planet; a shadow from an intervening moon.

Jupiter has more than 60 known moons including four that were discovered in 1610 when Galileo was the first to view it through a telescope. Their names are Io, Europa, Ganymede and Callisto. They can be seen easily through binoculars or a small telescope.

God of agriculture,
father of Jupiter.

Sixth planet from the Sun and the second largest.
    mass: 5.68 x 1026 kg
    orbit: 1,429,400,000 km (9.54 AU) from Sun
    diameter: 120,536 km (at the equator)

Saturn's rings are beautiful even in small amateur telescopes, especially when the planet is at its greatest tilt to the Earth, exposing the maximum surface area of the ring system. The rings are extremely thin compared to the width. They are about 250,000 km or so in diameter, but they're less than one kilometer thick. The ring particles are composed primarily of water ice, with some rocky particles with icy coatings.

Saturn has more than 30 moons. The largest, Titan, is prominent in views through a small telescope.

God of the sky,
father of Saturn.

Seventh planet from the Sun and the third largest.
    mass: 8.683 x 1025 kg
    orbit: 2,870,990,000 km (19.218 AU) from Sun
    diameter: 51,118 km (at the equator)

It is difficult for beginners with small telescopes to find Uranus. It may appear as a bluish-green star. In slightly larger scopes it will look more like a disk and less like a point.

An interesting thing about this planet is that unlike most of the planets, which spin on an axis nearly perpendicular to the plane of the ecliptic, Uranus' axis is almost parallel to the ecliptic. It seems tipped on its side.

Uranus has at least 27 moons. The largest are named after characters in Shakespeare plays. Others are named after characters in writings by Alexander Pope.

God of the sea.

Eighth planet from the Sun and the fourth largest.
    mass: 1.0247 x 1026 kg
    orbit: 4,504,000,000 km (30.06 AU) from Sun
    diameter: 49,532 km (at the equator)

Like Uranus it is difficult for beginners with small telescopes to find it. It usually appears simply as a bluish-green star. In slightly larger scopes it will look more like a disk and less like a point of light.

Neptune has at least 13 moons. The biggest is named Triton.

God of the underworld.

Now officially classified as a "dwarf planet," Pluto crosses inside the orbit of Neptune for part of its orbit. It is smaller by far than the planets. Pluto is smaller than seven of the solar system's moons.
    mass: 1.27 x 1022 kg
    orbit: 5,913,520,000 km (39.5 AU) from Sun (average)
    diameter: 2274 km (at the equator)

In earth-bound telescopes Pluto looks like a star. To know if you are observing it, make a drawing of the star field when you think you have found Pluto. Come back in a couple of weeks and find the field again. If it was Pluto it will have moved slightly in relation to the backdrop of stars.

Pluto has a moon named Charon which is named after the mythical ferry boat operator who take souls across the River Acheron to Hades.