Planets

Planets are astronomical objects that orbit a star and are massive enough to be rounded by their own gravity, but not massive enough to cause thermonuclear fusion. In our Solar System, there are eight recognized planets, which are, in order of their distance from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

Definition

The definition of a planet was historically based on cultural and mythological factors, but in 2006, the International Astronomical Union (IAU) provided a clear definition. According to the IAU, for an object to be classified as a planet, it must meet three criteria:

1. It must orbit the Sun.
2. It must be spherical in shape, having achieved hydrostatic equilibrium.
3. It must have cleared its orbit of other debris.

This definition excludes dwarf planets like Pluto, which has not cleared its orbit.

Types of Planets

Planets can be categorized into different types based on their physical characteristics and orbital positions:

Terrestrial Planets

Terrestrial or rocky planets are those that are primarily composed of silicate rocks or metals. In the Solar System, the terrestrial planets are Mercury, Venus, Earth, and Mars. These planets have solid surfaces and are closer to the Sun.

Gas Giants

Gas giants are large planets that have a substantial atmosphere of hydrogen and helium. These include Jupiter and Saturn in our Solar System. They lack a well-defined solid surface.

Ice Giants

Ice giants are a subclass of gas giants, characterized by their significant composition of icy materials such as water, methane, and ammonia. Uranus and Neptune are the ice giants in our Solar System.

Extrasolar Planets

Beyond our Solar System, there are thousands of extrasolar planets, or exoplanets, which orbit other stars. These planets vary widely in size, composition, and temperature. They are often detected through methods such as the transit method and the radial velocity method.

Cultural Impact

Planets have had a profound impact on human culture, influencing everything from our calendar systems to art and literature. The discovery of planets and their movements has played a crucial role in the development of astronomy and our understanding of the universe.

See Also

• Dwarf planet
• Exoplanet
• Solar System
• Star