Planet

According to International Astronomical Union (IAU), a planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity (but not massive enough to cause thermonuclear fusion in its inner core), and has cleared its neighbouring region of planetesimals. There are eight bodies in the Solar Sysytem that meet this definition: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.

Again, according to that definition, bodies that don't obey the third condition, that is, orbiting the Sun, rounded by its own gravity but not cleared the neighbourhood, is called dwarf planet. One of them is Pluto, which is former considered as a planet. Other bodies that orbiting the Sun are generically called Small bodies of Solar System.

Another related term is exoplanet. Although there are no official defition for this term, it is often used to indicate bodies analogue to the planets, but that orbites other stars that not the Sun.

The Planets

The planets can be subdivided into two main groups. The inner 4 terrestrial planets have a rocky composition. They are relatively small in size. The outer 4 planets are known as Gas Giants and have a thick gaseous atmospheres. Some physical information is followed:

Name	Distance from the sun	Diameter	Mass	Date of Discovery
Mercury	57,910,000 km (0.38 AU)	4,880 km	3.30e23 kg	known in antiquity
Venus	108,200,000 km (0.72 AU)	12,103.6 km	4.869e24 kg	known in antiquity
Earth	149,600,000 km (1.00 AU)	12,756.3 km	5.972e24 kg
Mars	227,940,000 km (1.52 AU)	6,794 km	6.4219e23 kg	known in antiquity
Jupiter	778,330,000 km (5.20 AU)	142,984 km	1.900e27 kg	known in antiquity
Saturn	1,429,400,000 km (9.54 AU)	120,536 km (equatorial)	5.68e26 kg	known in antiquity
Uranus	2,870,990,000 km (19.218 AU)	51,118 km (equatorial)	8.683e25 kg	1781
Neptune	4,504,000,000 km (30.06 AU)	49,532 km	1.0247e26 kg	1846

Etimology and History

The word "planet" came from the greek πλανήτ- (planēt-), meaning "wanderers". There was, then, seven planets in the Sky, each one dedicated to a greek deitiy:

• Phainon, the shiner, dedicated to Cronus;
• Phaethon, brighter the first and below it, dedicated to Zeus;
• Pyroeis, "fiery", the red one, dedicated to Ares;
• Phosphoros, "light bringer", the brightest one, dedicated to Aphrodite;
• Stilbon, the gleamer, dedicated to Hermes;

added by our Sun and Moon, the bigest wanderers in the sky. Later, n the time of the philosophers, many models of universe was created, which the most famous for us is the Aristotle's one. In his system, the planets belong each one to one cristaline sphere, concentric to the others. Inner the seven planet's sphere stay the material shperes (grosso modo, the Earth); Outer them, the shpere of fixed stars.

In Roman Empire, the planets became knowed directly by the name of the associated deity - not the greek one, bet the latin correspondent god. So, Phainon became Saturnus; Phaethon was called Jupiter; Pyroeis was Mars; Phosphoros, Venus and Stilbon was turned to Mercurius. In medieval era, the future european people has herited the roman culture, and so these latinized version of planet's names remain in the vocabulary until present days. Also in latinian tratition (followed by the germanic one) the seven days of the week have the names of the seven planets.

After the Copernican Revolution, however, the idea of planet has changed: they are all the bodies that round the Sun - that therefore would be considered just a star like any other from the firmament. The Earth then turn into a planet, the Moon became an Earth's sattelite, and the so-called "Solar System" have then six planets. Later, three new bodies, that can't be seen with naked eye are baptized as a planet, following the latin-named tradition: Uranus, Neptunus and Pluto.

But many other small bodies are saying to orbit the Sun, such as the brights comets and the palid asteroids. The crisis in definition starts in 1980's and 1990's, when many bodies so big as - or, in few cases, bigger - Pluto start to be discovered above Neptune's orbit. Called Transneptunian objects, they opened the discussion about what could be a good and precise definition of planet. The discussion ends in August 24, 2006, when IAU voted the Resolution that defines officially the term.

Physical Formation

Planets are believed to form form a the disk of dust that surrounds a newly formed star. Gravitation forces pull the dust into lumps. The lumps gradually collide and join to make larger lumps or asteroids. These join further to make larger and larger planets until the planet has cleared it's orbit of objects.

Planets and Astrology

The orbits of the planets also have a bearing in Astrology. It is believed the the position of the planets within various constellations of stars can be an indicator of events on Earth. By carefully measuring the orbits and positions of the planets and calculating their future paths, it is believed that one can predict future events.