Jupiter ♃

Jupiter is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, while being slightly less than one-thousandth the mass of the Sun. Jupiter is the third brightest natural object in the Earth’s night sky after the Moon and Venus, and it has been observed since prehistoric times. It was named after Jupiter, the chief deity of ancient Roman religion.

Jupiter is primarily composed of hydrogen, but helium constitutes one-quarter of its mass and one-tenth of its volume. It probably has a rocky core of heavier elements, but (like the Solar System’s other giant planets) lacks a well-defined solid surface. The ongoing contraction of Jupiter’s interior generates more heat than the planet receives from the Sun. Because of its rapid rotation, the planet’s shape is an oblate spheroid, having a slight but noticeable bulge around the equator. The outer atmosphere is divided into a series of latitudinal bands, with turbulence and storms along their interacting boundaries.

Jupiter’s upper atmosphere is about 90% hydrogen and 10% helium by volume. Since helium atoms are more massive than hydrogen molecules, Jupiter’s atmosphere is approximately 24% helium by mass. The atmosphere contains trace amounts of methane, water vapour, ammonia, and silicon-based compounds. There are also fractional amounts of carbon, ethane, hydrogen sulfide, neon, oxygen, phosphine, and sulfur. The outermost layer of the atmosphere contains crystals of frozen ammonia. Through infrared and ultraviolet measurements, trace amounts of benzene and other hydrocarbons have also been found. The interior of Jupiter contains denser materials — by mass it is roughly 71% hydrogen, 24% helium, and 5% other elements.

古代中國則稱木星歲星太岁,取其繞行天球一周約為12年,與地支相同之故,且產生了歲星紀年法。到西漢時期,《史記‧天官書》作者天文學家司馬遷從實際觀測發現歲星呈青色,與「五行」學說聯繫在一起,正式把它命名為木星。木星是顆巨行星,質量是太陽的千分之一,但卻是太陽系其他行星質量總和的2.5倍。太陽系的行星中,木星和土星是氣體巨星(天王星和海王星是冰巨星)。從地球看木星,它的視星等可以達到 -2.94等,已經可以照出陰影,並使它成為繼月球和金星之後,是夜空平均第三亮的天體(火星在其軌道的特定點上時能短暫與木星的亮度相比)。