Planets is your place to read, learn, share and discuss the solar system and beyond! Planets.org.uk is regularly updated with the latest planetary news, technology and discoveries as well as detailed factual content regarding all eight planets in our solar system. Here you can discuss the new and old areas of science and interact and discuss with other users on the site. Begin your interplanetary journey by clicking on a planet above and discover more about the solar system we live in.
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The eight planets were defined in 2006 by the International Astronomical Union (IAU) as Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
Total mass of gaseous planets: 2.66 x 1027 kg
Mass of Sun: 1.98 x 1030 kg
Distances between planets: View matrix
The eight planets are the dominant bodies in the Solar System after the Sun. Ancient peoples could identify the planets in the sky and saw them as emissaries of the gods. In 2006, the International Astronomical Union (IAU) recognised Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune as planets. A planet was defined as a celestial body that orbits the Sun. It must have enough mass for its self-gravity to surmount rigid body forces. In this way, it acquires a hydrostatic equilibrium, meaning that it has an almost round shape. In addition, it must have a clear path in its orbit around the Sun.
The planets are divided into two groups: terrestrial planets and gas giants.
Terrestrial planets are those that are similar to Earth. In addition to the Earth, these planets are Mercury, Venus and Mars. They are mostly composed of rock and are sealed within a hard external crust. Denser material, such as iron and nickel, sinks into the centre – the core – of the planet. The interior of the planet becomes differentiated, or separated, into a molten core, a semi-plastic or liquid mantle and a hard, outer crust. Mercury is the smallest terrestrial planet and the smallest planet in the solar system. Earth is the largest terrestrial planet.
The gas giants, or Jovians (named after the planet Jupiter), are composed of gaseous material. They are thought to have some rock at their core. Their mantles are composed of mostly iced water, ammonia and methane in various proportions. These planets are Jupiter, Saturn, Uranus and Neptune. Jupiter, which is 318 times the mass of the Earth, is the largest gaseous planet and the largest planet in the solar system. Uranus is the smallest gaseous planet.
All of the planets have atmospheres. Mercury has the thinnest atmosphere. Energy radiated from the Sun heats these atmospheres and creates weather systems such as storms and cyclones.
The orbits of the planets around the Sun are nearly circular and in a thin disc. The eccentricity of an orbit is a measure of the elongation of this circular path into an elliptical shape. The planets revolve around their own axes. These axes lie at an angle – the axial tilt – to the Sun’s equator.