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The term Hot Jupiter (HJ) is widely used, to indicate discovered gas extra-solar planets roughly the size of Jupiter or larger, that are orbiting close to their host star, such as less than half an AU. Examples with less than 0.1 AU are known, extremely close ones having very short orbital periods, i.e., their "years" can be less than an Earth day. Also, some orbit retrograde. The term inflated gas giant is also used for such planets. Being hot from their short distance from the star, their gas expands, thus a larger volume and lower density than otherwise. Being heavy, close to the star, and having short orbital periods, hot jupiters are the easiest planets to find: the star's radial velocity is much affected and other signs of an exoplanet are also more pronounced, and first exoplanet detected around a main sequence star (i.e., currently undergoing fusion) was a hot Jupiter, 51 Pegasi b. An interesting statistic has been that the higher the metallicity of the star, the more likely it is to have a hot Jupiter, leading to theories both on their formation and on mechanisms that could bring them close to the star.
A question that arose as soon as they were discovered is how they came to be so close to the star, since gas planets had been presumed to form much further out, beyond the snow line. Theories can be divided into the categories of in situ planet formation, planetary migration, or tidal migration.