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According to the IAU, "planets and dwarf planets are two distinct classes of objects" – in other words, "dwarf planets" are not planets. A non-satellite body fulfilling only the first criterion is termed a small Solar System body (SSSB). An alternate proposal included dwarf planets as a subcategory of planets, but IAU members voted against ...
Mercury is the first planet from the Sun and the smallest in the Solar System.In English, it is named after the ancient Roman god Mercurius (), god of commerce and communication, and the messenger of the gods.
A planet or moon's interior heat is produced from the collisions that created the body, by the decay of radioactive materials (e.g. uranium, thorium, and 26 Al), or tidal heating caused by interactions with other bodies. Some planets and moons accumulate enough heat to drive geologic processes such as volcanism and tectonics.
The outer planets' orbits are chaotic over longer timescales, with a Lyapunov time in the range of 2–230 million years. [105] In all cases, this means that the position of a planet along its orbit ultimately becomes impossible to predict with any certainty (so, for example, the timing of winter and summer becomes uncertain).
A terrestrial planet, tellurian planet, telluric planet, or rocky planet, is a planet that is composed primarily of silicate, rocks or metals. Within the Solar System , the terrestrial planets accepted by the IAU are the inner planets closest to the Sun : Mercury , Venus , Earth and Mars .
In many cases it is not possible to have an exact value, and an estimated range is instead provided. The coldest and oldest planet directly imaged is Epsilon Indi Ab, which has six times Jupiter's mass, an effective temperature of 275 K, and an age of about 3.5 Ga. This list includes the four members of the multi-planet system that orbit HR 8799.
Terrestrial planets have numerous similarities to dwarf planets (objects like Pluto), which also have a solid surface, but are primarily composed of icy materials. During the formation of the Solar System, there were probably many more ( planetesimals ), but they have all merged with or been destroyed by the four remaining worlds in the solar ...
[f] Six planets, seven dwarf planets, and other bodies have orbiting natural satellites, which are commonly called 'moons'. The Solar System is constantly flooded by the Sun's charged particles, the solar wind, forming the heliosphere. Around 75–90 astronomical units from the Sun, [g] the solar wind is halted, resulting in the heliopause.