Planet

According to the 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 System that meet this definition: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.

Bodies that orbit the Sun and are rounded by their own gravity, but have not cleared the neighbourhood, are called "dwarf planets." One of them is Pluto, which was formerly considered a planet. Other bodies orbiting the Sun are generically called "small bodies of the Solar System."

Another related term is "exoplanet." Although there is no official defition for this term, it is often used to indicate bodies analogous to the planets, but that orbit stars other than the Sun.

The planets of the Solar System

The Sun's planets can be subdivided into two main groups. The inner four terrestrial planets have a rocky composition. They are relatively small in size. The outer four planets are known as gas giants, being considerably larger and having thick gaseous atmospheres.

Table of the planets

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

Etymology and history

The word "planet" came from the greek πλανήτ- (planēt-), meaning "wanderers." Before the invention of the telescope, there were seven planets visible in the sky; the ancient Greeks considered each one to be dedicated to a deity:

• Phainon, "the shiner," dedicated to Cronos;
• Phaethon, also meaning "shining," brighter than the first, 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;

as well as the Sun and Earth's Moon, the biggest wanderers in the sky. Philosophers in antiquity created many models of the universe, of which the best-known is Aristotle's. In his system, each planet belongs to one crystalline sphere, concentric with the others. Inner the seven planets' spheres are the material spheres (grosso modo, the Earth); outside, the "sphere of fixed stars."

In the Roman Empire, the planets became known directly by the name of the associated deity - not the Greek one, but the corresponding Latin god. So, Phainon became Saturnus; Phaethon, Jupiter; Pyroeis, Mars; Phosphoros, Venus; and Stilbon, Mercurius. These Latinized names remain current in European languages. Also in the Latin tradition, as well as the Germanic, the seven days of the week have the names of the seven planets.

After the Copernican Revolution, however, the definition of a planet changed: they are all the bodies that orbit the Sun; the Earth thus turns into a planet, the Moon becomes Earth's satellite instead of being a planet, and the so-called "Solar System" then has six planets. Later, three new bodies, unseen by the naked eye, were baptized as planets, following the Latin-name tradition: Uranus, Neptune and Pluto.

But many other small bodies orbit the Sun, such as the bright comets and the pallid asteroids. The crisis in definition started in the 1980's and 1990's, when many bodies as big as -- or, in a few cases, bigger than -- Pluto started to be discovered beyond Neptune's orbit. Called Transneptunian objects, they sparked a discussion about what could be a good and precise definition of "planet." The discussion ended on August 24, 2006, when the IAU approved a resolution that officially defines the term.

Physical formation

Planets are believed to form from the disk of dust that surrounds a newly formed star. Gravitational 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 its orbit of objects.

Planets and astrology

The orbits of the planets also have a bearing on astrology, believers in which claim that 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, one can predict future events.