|the Earth System|
A planet is a big ball floating through space, but the same holds for stars and certain moons and asteroids. The difference between a star and a planet is that a star generates energy in its center through nuclear fusion, and a planet does not. A star never has a night (dark) side, but planets almost always do. The boundary between stars and planets is at about 3000 times the mass of the Earth (which is about 10 times the mass of Jupiter, the most massive planet of our Solar System, and about one tenth of the mass of the Sun). This corresponds to a diameter of about 300,000 km or 200,000 miles. There are therefore no planets larger than about 300,000 km.
There isn't a fundamental physical difference between a spherical moon (such as the Earth's Moon) and a planet. If there's more than one large sphere in a planetary system, then the largest one is usually much larger than the other ones, and that largest one is then called the planet, and the other ones the moons. If the largest two are almost equal in size (such as for Pluto and Charon), then those are sometimes referred to as double planets.
A planet is round because it has so much mass that its own gravity is strong enough to pull down bits that stick out, until everything is as close as possible to the center of the planet. If a celestial object is a small enough mass, then it can keep a non-round shape. The boundary is at a diameter of about 1000 km or 600 miles. There are therefore no planets smaller than about 1000 km.
Jovian planets are large planets that have an atmosphere that forms a large part of the mass of the planet. The atmosphere is mostly hydrogen and helium gas, just like for the Sun. In our Solar System, Jupiter, Saturn, Uranus and Neptune are the jovian planets. It might be that jovian planets have a rocky core deep below their atmospheres, but that isn't certain.
Terrestrial planets have a rocky core and no atmosphere at all, or a thin atmosphere that forms only a very small fraction of the total mass of the planet. In our Solar System, Mercury, Venus, the Earth, and Mars are terrestrial planets. In the atmospheres of terrestrial planets, hydrogen and helium play at most a very small part.
Pluto is too small to be a jovian planet, and has a much lower density than terrestrial planets, so it's probably mostly a frozen clump of ice. It appears that there are many more such relatively small celestial object floating around at the large distances from the Sun where Pluto roams, of which Pluto can be regarded as the largest one. That's why since 2006 the IAU no longer regards Pluto as a planet, but instead as a dwarf planet.
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Last updated: 2016−02−07