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The three laws state that: [1] [2] The orbit of a planet is an ellipse with the Sun at one of the two foci. A line segment joining a planet and the Sun sweeps out equal areas during equal intervals of time. The square of a planet's orbital period is proportional to the cube of the length of the semi-major axis of its orbit.
The sizes and masses of many of the moons of Jupiter and Saturn are fairly well known due to numerous observations and interactions of the Galileo and Cassini orbiters; however, many of the moons with a radius less than ~100 km, such as Jupiter's Himalia, have far less certain masses. [5]
Alan Stern calls these satellite planets, although the term major moon is more common. The smallest natural satellite that is gravitationally rounded is Saturn I Mimas (radius 198.2 ± 0.4 km). This is smaller than the largest natural satellite that is known not to be gravitationally rounded, Neptune VIII Proteus (radius 210 ± 7 km).
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Of the Solar System's eight planets and its nine most likely dwarf planets, six planets and seven dwarf planets are known to be orbited by at least 300 natural satellites, or moons. At least 19 of them are large enough to be gravitationally rounded; of these, all are covered by a crust of ice except for Earth's Moon and Jupiter's Io . [ 1 ]
Astronomers have found a massive planet orbiting a small, cool star, and planet formation theories struggle to explain its existence.
On 21 December 2012, the Cassini–Huygens probe, in orbit around Saturn, observed the planet Venus transiting the Sun. [3] On 3 June 2014, the Mars rover Curiosity observed the planet Mercury transiting the Sun, marking the first time a planetary transit has been observed from a celestial body besides Earth.
Saturn is known as "Shani" and judges everyone based on the good and bad deeds performed in life. [139] [141] Ancient Chinese and Japanese culture designated the planet Saturn as the "earth star" (土星). This was based on Five Elements which were traditionally used to classify natural elements. [142] [143] [144]