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Planetary mass is a measure of the mass of a planet-like object. Within the Solar System, planets are usually measured in the astronomical system of units, where the unit of mass is the solar mass (), the mass of the Sun. In the study of extrasolar planets, the unit of measure is typically the mass of Jupiter () for large gas giant planets, and the mass of Earth () for smaller rocky terrestrial planets. The mass of a planet within the Solar System is an adjusted parameter in the preparation of ephemerides. There are three variations of how planetary mass can be calculated: * If the planet has natural satellites, its mass can be calculated using Newton's law of universal gravitation to derive a generalization of Kepler's third law that includes the mass of the planet and its moon. This permitted an early measurement of Jupiter's mass, as measured in units of the solar mass. * The mass of a planet can be inferred from its effect on the orbits of other planets. In 1931-1948 flawed applications of this method led to incorrect calculations of the mass of Pluto. * Data from influence collected from the orbits of space probes can be used. Examples include ''Voyager'' probes to the outer planets and the MESSENGER spacecraft to Mercury. * Also, there are numerous other methods can give reasonable approximations. For instance, potential dwarf planet, known as Varuna, rotates very quickly upon its axis, as does the dwarf planet Haumea. Haumea, for example, has to be very dense in order to not rip apart due to its rotation. Through some calculations, one could place a limit on the object's density. Thus, if the object's size is known, a limit on the mass can be determined. See the links in the aforementioned articles for more details on this. == Choice of units == The choice of solar mass, , as the basic unit for planetary mass comes directly from the calculations used to determine planetary mass. In the most precise case, that of the Earth itself, the mass is known in terms of solar masses to twelve significant figures: the same mass, in terms of kilograms or other Earth-based units, is only known to five significant figures, which is less than a millionth as precise.〔"(2009 Selected Astronomical Constants )" in .〕 The difference comes from the way in which planetary masses are calculated. It is impossible to "weigh" a planet, and much less the Sun, against the sort of mass standards which are used in the laboratory. On the other hand, the orbits of the planets give a great range of observational data as to the relative positions of each body, and these positions can be compared to their relative masses using Newton's law of universal gravitation (with small corrections for General Relativity where necessary). To convert these relative masses to Earth-based units such as the kilogram, it is necessary to know the value of the Newtonian gravitational constant, ''G''. This constant is remarkably difficult to measure in practice, and its value is only known to a precision of one part in ten-thousand.〔.〕 The solar mass is quite a large unit on the scale of the Solar System: 1.9884(2) kg.〔 The largest planet, Jupiter, is 0.09% the mass of the Sun, while the Earth is about three millionths (0.000003%) of the mass of the Sun. Various different conventions are used in the literature to overcome this problem: for example, inverting the ratio so that one quotes the planetary mass in the 'number of planets' it would take to make up one Sun.〔 Here, we have chosen to list all planetary masses in 'microSuns' – that is the mass of the Earth is just over three 'microSuns', or three millionths of the mass of the Sun – unless they are specifically quoted in kilograms. When comparing the planets among themselves, it is often convenient to use the mass of the Earth (''M''E or ) as a standard, particularly for the terrestrial planets. For the mass of gas giants, and also for most extrasolar planets and brown dwarfs, the mass of Jupiter () is a convenient comparison. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「planetary mass」の詳細全文を読む スポンサード リンク
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