形成和遷徙

一組新的超級地球可能起初聚集在內太陽系。

地球和它鄰近的行星可能是在木星碰撞與摧毀這些在太陽附近的超級地球之後，從碎片中形成的。當木星遷徙至內太陽系，在理論家所謂的大遷徙假說，突然的引力推與拉，導致這些超級地球的軌道開始重疊，引發彼此間一系列的碰撞。天文學家已經發現500多個多行星系統，這些系統通常包括幾顆質量數倍於地球（超級地球）的行星，進到比水星更靠近太陽的距離，並且類似木星的氣體巨星也會很靠近它們的母恆星。看來，木星在太陽系的外側軌道上，是因為當它遷徙時， 土星拉著它往外移動。木星從內太陽系往外移動，可能給了內太陽系的行星，包括地球，可以形成的契機。2017年，來自美國勞倫斯利弗莫爾國家實驗室和德國明斯特大學的研究人員在分析來自小行星的隕鐵中鎢和鉬的同位素時發現，木星岩石內核可能在太陽系形成後的100萬年後就已經處在形成階段中，木星形成可能已有距今46億至50億年。

結構

木星主要由氣體和液體物質構成，它是太陽系中4顆巨行星中最大的，也是太陽系最大的行星。它的赤道直徑，密度1.326g/cm3，是巨行星中第二高的，但遠低於其它4顆類地行星。

成分

木星大氣層上層的成分以氣體分子的體積百分比大約88-92%是氫，8-12%是氦。因為氦的原子量是氫的4倍，當以質量描述組成時，不同原子量的元素就會有不同的比例。木星的大氣層大約75%的質量是氫，24%的質量是氦，剩餘的1%是其它的元素。內部包含密度較高的元素，大致是71%的氫，24%的氦，和5%其它的元素。大氣中含有微量的甲烷、水蒸氣、氨和矽基化合物。也有微量的碳、乙烷、硫化氫、氖、氧、磷化氫和硫，最外層的大氣含有結晶的氨。經由紅外線和紫外線的測量，也發現有微量的苯和其它的烴類。

大氣中氫和氦的比例接近理論上的原始太陽星雲組成。氖在大氣層上層僅佔百萬分之二十，大約是太陽中豐度的十分之一。氦也幾乎耗盡，大約只有太陽組成的80%左右。這種減少是這些元素沉降到行星內部的結果。較重的惰性氣體在木星大氣層中的豐度是太陽的2-3倍。

依據光譜，土星的組成被認為類似於木星，但其它的巨行星，天王星和海王星有著相對較少的氫與氦。由於缺乏直接深入大氣層的探測器，除了外層的大氣層外，缺乏內部更重元素豐度的精確數值。

質量和大小

木星的質量是太陽系其他行星質量總和的2.5倍，由於它的質量是如此巨大，因此太陽系的質心落在太陽的太陽表面之外，距離太陽中心1.068太陽半徑。雖然木星的直徑是地球的11倍，體積是地球的1,321倍，但是它的密度很低，質量只是地球的318倍。木星的半徑是太陽半徑的十分之一，質量是太陽質量的千分之一，所以兩者的密度是相近的。"木星質量"（MJ或MJup）通常被做為描述其它天體，特別是系外行星和棕矮星的質量單位。例如系外行星HD 209458 b的質量是0.69MJup，而仙女座κb的質量是12.8MJup。

理論模型顯示如果木星的質量比現在更大，而不是僅有目前的質量，它將會繼續收縮。質量上的些許改變，不會讓木星的半徑有明顯的變化，大約要在500地球質量（1.6MJup）才會有明顯的改變。儘管隨著質量的增加，內部會因為壓力的增加而縮小體積。結果是，木星被認為已經幾乎達到了行星結構和演化史所能決定的最大半徑。隨著質量的增加，收縮的過程會繼續下去，直到達到可察覺的恆星形成質量，大約是50MJup的高質量棕矮星。

然而，需要75倍的木星質量才能使氫穩定的融合成為一顆恆星。最小的紅矮星，半徑大約只是木星的30% 。儘管如此，木星仍然散發出大量能量。它接受來自太陽的能量，而內部產生的能量也幾乎和接受自太陽的總能量相等。這些額外的熱量是由開爾文-亥姆霍茲機制通過收縮產生的。這個過程造成木星每年縮小約2公分。當木星形成的時候，它比現在熱，直徑大約是現在的2倍。

內部結構

木星被認為有個由元素混合的緻密核心，被一層含有少量氦，主要是氫元素的液態金屬氫包覆著。除了這個基本的輪廓，不確定的成分還是相當多。核心經常被描述為岩石，但是其詳細的成分是未知的，而且在這種深度下的溫度、壓力、和材料的性質也都不清楚（見下文）。在1997年，有人建議用重力法測量是否存在著核心，顯示核心大約有12至45地球質量，約占木星總質量的4%至14%。

行星模型認為在行星形成的歷史上，木星至少有一段時間有個夠大的岩石或冰的核心，才可以從原始太陽星雲收集到足夠大量的氫和氦。假設它確實存在，它可能因為現存的熱液態金屬氫與地函混合的對流而萎縮，並且熔融在行星內部的較上層。核心現在可能完全消失，但由於重力測量仍不夠精確，還不能完全排除這種可能性。

模型的不確定性受限於測量參數的誤差：用來描述行星引力動量的一個自轉係數（J6）、木星的赤道半徑、在1帕壓力處的溫度。預期在2011年8月發射的朱諾號探測器將能獲得這些參數更好的數值，從而在核心的問題上取得進展。

核心區域被密集的金屬氫包圍著，向外延伸到大約行星半徑78%之處，通過這一層的氦和氖，像雨水滴般向下沉降，消耗掉這些元素在上層大氣的豐度。

在金屬氫上層是內層透明氫的大氣層。在這個深度，溫度是在臨界溫度之上，對氫而言只有33K。在此狀態下，沒有層次分明的液體和氣體位相 －氫可能是臨界的超流體狀態。在這層之上的，從雲層向下延伸至深度大約1,000公里的氫，順理成章的應該是氣體，而在更深的一層是流動的液體。在物理上，那裏沒有明確的邊界 －氣體很順利的變得更熱和更密集的下降。

由於開爾文-亥姆霍茲機制可知，木星內部的溫度和壓力在朝向核心地方向逐漸增加。在壓力為10帕的「表面」，溫度大約是。在氫相變的區域 －溫度達到臨界點－ 氫成為金屬，相信溫度是，壓力的200GPa。在核心邊界的溫度估計為，同時內部的壓力大約是3,000至4,500GPa。

大氣層

木星有著太陽系內最大的行星大氣層，跨越的高度超過 。由於木星沒有固體的表面，它的大氣層基礎通常被認為是大氣壓力等於，或十倍於地球表面壓力之處。

雲層

木星永遠被氨晶體和可能是氫硫化氨的烏雲籠罩著。對流層頂的雲，在不同緯度形成不同的區帶，最著名的是熱帶區。這些區帶分為亮色調的區（zones）和深色調的帶（belts）。這些模式互不相容環流間的交互作用導致風暴和湍流，風速達到100m/s（360Km/h）的緯向急流是很常見的。每一年，各區都有著不同的寬度、顏色和強度，但對天文學家而言，依然可以穩定的給予識別和指定。

雲層大約只有深，並且至少包含兩層覆蓋的雲：厚厚的下層和薄且清晰的區域。在氨雲層下面也有薄薄一層的水雲，有證據顯示木星的大氣層中也有閃爍的閃電。這是由水分子的極性造成的，它使得創造閃電所需要的電荷能夠分離。這些放電的強度達到地球上的一千倍。水雲可以形成雷暴，驅使熱量從內部不斷上升。

木星雲層的橙色和棕色是內部湧升的化合物暴露在紫外線下，引起顏色的改變造成的。確切的構成仍然不清楚，但被認為是含有磷、硫或可能是烴類。這些豐富多彩的混合物，稱為發色團，與下層較溫暖的雲層混合。 區是由上升的氨結晶對流胞形成的，在觀測上通常是較低層雲的掩蔽物。

木星的低轉軸傾角意味著兩極能接收到的太陽輻射遠遠的少於行星的赤道地區。行星內部的對流輸送大量的能量到極區，使雲層的溫度能夠平衡。

大紅斑和其它渦旋

木星最著名的特徵是大紅斑，這是比地球大的一個持久性反氣旋風暴，位置在赤道南方22°，至少在1831年以來，就已經知道它的存在，並且可能更提早至1665年。來自哈伯太空望遠鏡的影像顯示多達兩個紅斑毗鄰著大紅斑。這個風暴大得可以使用地基的小口徑或更大的望遠鏡看見。一些數學模型表明這個風暴是穩定的，可能是這顆行星上一個永久性的特徵。

鵝蛋形物體的自轉是逆時針方向，週期大約是六天。大紅斑的維度是24,000至40,000公里 X 12,000至14,000公里。它的直徑大到可以容得下2至3顆地球。這個風暴最大的高度比周圍的雲層高出約。

風暴通常都發生在巨行星大氣層的湍流內，木星也有白色和棕色的鵝蛋形風暴，但較小的那些風暴通常都不會被命名。白色的鵝蛋形風暴傾向於包含大氣層上層，相對較低溫的雲。棕色鵝蛋形風暴是較溫暖和位於普通雲層。這種風暴持續的時間可以只有幾個小時，也可以長達數個世紀。

在航海家證實大紅斑的特徵是一場風暴之前，因為它相對於周圍其餘的氣團有時快，有時慢的差異旋轉，已經是強有力的證據，表明大紅斑與行星表面或深處的地形特徵沒有關聯性。

在2000年，在南半球有一個外觀與大紅斑類似，但較小的大氣特徵出現。這是由幾個較小的白色鵝蛋形風暴合併成的一個特徵 －三個在1938年首度被觀測到的較小的鵝蛋形風暴。合併後的特徵被命名為鵝蛋形BA，並且因為它的強度增加，顏色由白轉紅，被暱稱為幼紅斑。

行星環

木星有個黯淡的行星環系統，約有6,500公里寬，但厚度不到10公里。由大量塵埃和黑色碎石組成，以大約7小時的週期圍繞木星旋轉。環由三個主要的部份組成：內側像花托，是由顆粒組成的暈環，中間是相對明亮的主環，還有外圈的薄紗環。這些環，看起來是由塵埃組成，而不像土星環是由冰組成。主環可能是從衛星阿德剌斯忒亞和梅蒂斯噴發的物質組成。正常應該落回衛星的物質由於受到木星強大引力的影響，被木星吸引住。這些材料轉變軌道的方向朝向木星，新的材料又因為碰撞影響而繼續被加入。以相同的方式，特貝和阿馬爾塞可能組成薄紗環塵土飛揚的兩個部分。也有證據顯示沿著阿馬爾塞的軌道可能有一連串與這顆衛星碰撞構成的岩石碎片。

磁層

木星的磁場強度是地球的14倍，範圍從赤道的4.2高斯（0.42mT）到極區的10至14高斯（1.0-1.4mT），是太陽系除太陽黑子以外最強的磁場源。這個場被認為是由渦流產生的，即木星內部渦旋運動的液態金屬氫。埃歐衛星上的火山釋放出大量的二氧化硫，形成沿著衛星軌道的氣體環。這些氣體在磁層內被電離，生成硫和氧的離子。它們與源自木星大氣層的氫離子，在木星的赤道平面形成電漿片。這些片狀的電漿與行星一起轉動，造成進入磁場平面的變形偶極磁場。在電漿片內的電流產生強大的無線電訊號，造成範圍在0.6至30MHz的爆發。

在距離木星大約75木星半徑之處，磁層與太陽風的交互作用生成弓形震波。環繞著木星磁層的是磁層頂，位於磁層鞘的內緣 －磁層頂和弓形震波之間的區域。太陽風與這些去的交互作用拉長了木星背風面的磁層，並且向外延伸至幾乎到達土星軌道的位置，而面向太陽方向也有數百萬公里厚。木星的四顆大衛星的軌道全都位於磁層內，受到保護而得以免受太陽風的侵襲，因此木星的衛星全都位於它的磁層之中。 伽利略號的大氣探測器在木星環與高層大氣之間新發現一個強輻射帶，類似地球的范艾倫輻射帶，但比范愛倫輻射帶強10倍左右，其中有高能的氦離子。

木星的磁層是其兩極地區激烈發送的電波輻射的源頭。木衛埃歐（見下文）劇烈的火山活動，噴發出的氣體進入木星的磁層，產生一個托環狀環繞著木星的微粒。當埃歐穿過這個托環時，相互作用生成的阿爾文波使游離的物質進入木星的極區。一個結果是，無線電波通過迴旋加速器的邁射機制，和能量沿著圓錐形的表面傳輸出去。當地球與這個錐面交會時，地球上探測到的木星發射的無線電波會強于太陽輸出的無線電波。

軌道和自轉

木星是行星中唯一與太陽的質心位於太陽本體之外的，但也只在太陽半徑之外7%。木星至太陽的平均距離是7億7800萬公里（大約是地球至太陽距離的5.2倍，或5.2天文單位），公轉太陽一週要11.8地球年。這是土星公轉週期的五分之二，也就是說太陽系最大的兩顆行星之間形成5：2的共振軌道週期。木星的橢圓軌道相對於地球軌道傾斜1.31°，因為離心率0.048，因此近日點和遠日點的距離相差7,500萬公里。木星的軌道傾角相較於地球和火星非常小，只有3.13°，因此沒有明顯的季節變化。

木星的自轉是太陽系所有行星中最快的，對其軸完成一次旋轉的時間少於10小時；這造成的赤道隆起，在地球以業餘的小望遠鏡就可以很容易看出來。這顆行星是顆扁球體，意思是他的赤道直徑比兩極之間的直徑長。木星的赤道直徑比通過兩極的直徑長。

因為木星不是固體，他的上層大氣有著較差自轉。木星極區大氣層的自轉週期比赤道的長約5分鐘，有三個系統做為參考框架，特別是在描繪大氣運動的特徵。系統I適用於緯度10°N至10°S的範圍，是最短的9h50m30.0s。系統II適用於從南至北所有的緯度，它的週期是9h55m40.6s。系統III最早是電波天文學定義的，對應於行星磁層的自轉，它的週期是木星的官方週期。

觀測

木星通常是天空中第四亮的天體（在太陽、月球和金星之後），但有時候火星會比木星亮。依據木星相對於地球的位置，可以表現出不同的視星等，在衝時最亮是-2.9等，在與太陽同向的合時，會降至-1.6等。木星的角直徑也會隨之改變，從50.1到29,8弧秒。木星在軌道上經過近日點附近時的衝最適宜觀賞，木星上次是在2011年3月經過近日點，所以在2010年和2011年9月的衝是最有利的。

地球每398.9日會在軌道上超越木星一次，這個時間稱為會合週期。每當會合之前，木星都會相對於背景的恆星出現明顯的逆行運動。這是木星似乎在夜空中向後（向西）移動一段，執行迴圈的運動。

木星接近12年的軌道週期對應於黃道的星宮。也就是，木星每一年約向東移動大約30°，約是一個星宮的寬度。

因為木星的軌在地球軌道之外，所以從木星看地球的相位角永遠不會超過11.5°。也就是，從地球用望遠鏡觀看木星時，它幾乎都是呈現滿月的姿態。只有當太空船飛近木星時，才會看見新月形的木星。通常，一架小望遠鏡就能看見木星的四顆伽利略衛星和跨越木星大氣層明顯的雲帶。當大紅斑面向地球時，小口徑的望遠鏡也有機會看得見。

研究和探測

望遠鏡發明之前的研究

對木星的觀測可以回溯至西元前7或8世紀的巴比倫天文學家。中國的歷史天文學家席澤宗宣稱中國天文學家甘德在西元前362年就以裸眼發現木星的衛星之一。如果此一說法正確的話，會比伽利略的發現早了近2000年。在西元2世紀的天文學大成，古希臘天文學家，地心說行星模型的先驅，托勒密以本輪和均輪來解釋行星相對於地球的運動，他給木星軌道環繞地球的週期是4332.38天，或11.86年。在西元499年，一位古典時代的印度數學家和天文學家，阿耶波多，也用地心說的模型估計出木星的週期是4332.2722天，或11.86年。

地基望遠鏡的研究

1610年，伽利略發現 木星的4顆大衛星 －埃歐、歐羅巴、佳利美德、和卡利斯多（現在稱為伽利略衛星－ 首度用望遠鏡發現不屬於地球的衛星。伽利略也是首度發現顯然不以地球為中心運動的天體。這是對哥白尼日心說最主要的支撐，伽利略直言不諱的支持哥白尼學說，使他被置於文字獄的威脅下。

1660年代。卡西尼使用一架新的望遠鏡發現木星的斑點和彩色的區帶，並且觀察到這顆行星出現扁平形；就是在兩極扁平。他也估計出這顆行星的自轉週期。在1690年，卡西尼發現大氣經歷較差自轉。

大紅斑是在木星南半球的一個顯著鵝蛋形特徵，可能早在1664年就被羅伯特·虎克和喬瓦尼·多梅尼科·卡西尼在1665年觀測過；雖然這仍有爭議。已知最早的繪圖來自藥劑師海因利希·史瓦貝，他在1831年顯示大紅斑詳細的資訊。

據傳說，大紅斑在1878年變得很顯眼前，在1665年至1708年曾經有多次從視線中消失的場合。它在1883年和20世紀初，再度被記錄到衰退。

Giovanni Borelli和卡西尼兩人都小心地做出木星衛星的運動表，可以預測這些衛星經過木星前方或背後的時間。在1670年代，人們觀測到當木星與地球在相對於太陽的兩側時，這些事件的發 會比預測的慢達17分鐘。奧勒·羅默推論視線看到的不是即時發生的事情（卡西尼在此之前曾經拒絕這樣的結論），而這個時間上的差異可以用來估計光速 。

1892年，愛德華·愛默生·巴納德在加利福尼亞州使用利克天文台 的折射望遠鏡觀察到木星的第5顆衛星。發現了這顆相對較小的衛星，證明了他敏銳的視力，使他很快的成名。這顆衛星後來被命名為阿馬爾塞。這是最後一顆以視覺發現的行星衛星。在1979年，航海家1號飛過木星之前，發現了額外的8顆衛星。

1932年，魯珀特·沃爾特根據木星的吸收光譜確定木星大氣中含有甲烷和氨。

1938年，觀察到3個長壽的白色鵝蛋形反氣旋特徵。幾十年來，它們是獨立存在木星大氣層的特徵，有時會互相靠近，但永遠不會合併。最後，兩個在1998年合併，並在2000年吸收了第三個，被稱為長圓形BA。

電波望遠鏡的研究

在1955年，巴納德柏克和肯尼斯·佛蘭克林偵測到來自木星的22.2MHz的無線電信號爆發。這些爆發與木星的自轉週期匹配，也能夠用這些資訊來改進自轉速率。發現來自木星的無線電爆發有兩種形式：長達數秒的長爆發（L爆發），和持續時間短於百分之一秒的短爆發（S爆發）。

科學家發現來自木星的無線電訊號有三種傳輸的形式：

• 隨著木星旋轉的十米無線電爆發（波長10米的無線電波），並且受到埃歐與木星磁場交互作用的影響。

• 公分無線電輻射（波長為公分的無線電波）於1959年首度由弗蘭克·德雷克和Hein Hvatum觀測到。這個信號起源於木星赤道附近的圓環帶狀，是由木星磁場中被加速電子引起的迴旋輻射。

• 輻射熱是由大氣中的熱產生的。

太空探索與探測

自1973年以來，有數艘自動化的太空船拜訪過木星，最引人注目的是先鋒10號太空船。它是第一艘足夠接近木星，並發送回有關這顆太陽系最大行星的屬性和現象的太空船。飛往太陽系內其他行星的太空船完全依賴能量的價值，太空船速度的淨變化或ΔV。從地球的低地球軌道進入到木星的霍曼轉移軌道只需要6.3Km/s的ΔV，這媲美於要進入低地球軌道的9.7Km/s的ΔV。幸運的是，重力助推可以用來減少抵達木星所需要的能量，然而，這也很明顯的需要較長的飛行時間。

飛越任務

從1973年開始，數艘太空船在執行探測其他行星的任務時，有計畫的從可以觀測木星的範圍內飛越。先鋒計畫最先觀測到木星大氣層和幾顆衛星的特寫影像。它們發現這顆行星的輻射場遠遠超出預期，但這兩艘太空船在這種環境下都依然存活。這些太空船的運動軌跡被用來更精確地估計木星系統質量。行星的無線電掩星結果得到更好的木星質和和兩極扁平的數值。

六年後，航海家計畫任務極大地提高了對伽利略衛星的認識，並且發現了木星環。它們還證實大紅斑是反氣旋，比較影像顯示大紅斑已經改變了形狀和顏色，從先鋒任務的橙色轉變成暗褐色。此外，這一計劃還發現電離的原子沿著埃歐的軌道構成環形，和發現這顆衛星表面的火山，其中有一些還在噴發的過程中。當太空船從木星的背後飛過時，還觀察到夜晚大氣中的閃電。

隨後探測木星的是尤利西斯太陽探測器，以執行繞行太陽的極軌道任務。在接近木星的階段中，進行對木星磁層的研究。由於尤利西斯沒有照相機，所以沒有獲取影像，第二次是在六年後以更遠的距離飛越。

在2000年，卡西尼探測器在前往土星的途中飛越木星，並提供了一些有史以來最高解析度的木星影像。在2000年12月9日，太空船拍攝到衛星希瑪利亞的影像，但是解析力太低，無法顯示表面的細節。

新視野號探測器在途中，於2007年2月28日達到最接近木星的位置，藉由飛越木星時的重力助推前往冥王星 。這艘探測器的照相機測量從埃歐的火山噴發出的電漿，並且以細的研究全部4顆的伽利略衛星，以及遠距離的觀測外圍的希瑪利亞和伊拉拉。從2006年9月4日就開始拍攝木星系統的影像。

伽利略任務

伽利略號是第一艘在軌道上環繞木星的太空船。它於1995年12月7日進入軌道，環繞這顆行星7年之久，並飛越過所有的伽利略衛星和阿馬爾塞。這艘太空船在接近木星的途中，對1994年舒梅克－李維九號彗星撞木星的事件進行了觀測，見證了此一撞擊事件的影響。雖然伽利略號廣泛的收集了大量木星系統的資訊，但因為高增益無線電發射天線的布署失敗，使原設計的能力大為減損。

一個340公斤的鈦金屬製的大氣探針，於1995年12月7日從伽利略號釋放進入木星大氣層。它以大約2,575公里（1,600英里）的時速，在大氣層中下降了約，在它被壓力和高溫（23倍地球大氣壓，153℃）摧毀之前，蒐集了57.6分鐘的資料，而這個探針可能被熔解和蒸發了。伽利略軌道器本身也遭遇了同樣的命運，經過刻意操作在2003年9月21日以超過50Km/s的速度撞進木星的大氣層，以避免它撞上歐羅巴而可能造成的汙染——這顆衛星已被假設可能是生命的避風港。

來自此一任務的資料揭露氫在木星大氣層佔90% 。在探針汽化前，溫度資料紀錄超過了300℃（>570℉），風速測量超過644km/h（>400mph）。

朱諾任務

美國國家航空暨太空總署的太空船朱諾號在2016年7月4日抵達木星，預計未來的20個月將在軌道上繞行木星37圈。這次任務將以繞極軌道仔細的研究這顆行星。在2016年8月27日，朱諾號完成其第一次的低空飛越木星，並且送回木星北極的第一張圖像。

未來的探測

歐洲太空總署的木星冰月探測器（JUICE）預計在2022年發射。接下來是NASA在2025年的歐羅巴帆船任務。

取消的任務

由於木星的衛星歐羅巴、佳利美德、和卡利斯多的地表下可能有液體的海洋，因此對詳細研究冰衛星非常感興趣。但資金的困難拖延了進度，NASA的木星冰月軌道器（JIMO，Jupiter Icy Moons Orbiter）於2005年被取消。隨後提案由NASA和ESA共同執行的任務，EJSM/Laplace臨時決定預計在2020年研製而成。EJSM/Laplace將有NASA主導的木星歐羅巴軌道器和ESA主導的木星佳利美德軌道器。然而，在2011年4月，ESA因為預算的原因結束與NASA的任務夥伴關係。取而代之的是ESA計畫以只有歐洲參與的L1宇宙願景任務來在競爭和超越。

衛星

木星有79顆衛星。木星是人類迄今為止發現的天然衛星第二多的行星 (僅次於土星) ，儼然一個小型的太陽系：木星系。1610年1月，意大利天文學家伽利略最早以望遠鏡發現木星最亮的四顆衛星，並被後人稱為伽利略衛星。它們環繞在離木星40～190萬千米的軌道帶上，由內而外依次為木衛一、木衛二、木衛三、木衛四，然而近年中國有天文史學家提出在公元前364年，甘德以肉眼發現木衛三，但直至現時還未被公認。在1892年巴納德以望遠鏡肉眼觀測發現木衛五後，木星的其他衛星皆透過照相觀測或行星際探測器的相片發現。

在以後的幾個世紀中（至1950年代），人們又接連發現了12顆較大的衛星，使木星衛星的總數達到了16顆。直至1979年美國旅行者一號及1995年伽利略號等飛臨木星系的時候，又發現了許多更細小的、離木星更遠的天然衛星，使人類所知的木星系衛星總數達到67個，成為太陽系擁有最多天然衛星的行星，這數字還很有可能繼續增加。2017年，卡內基科學研究所在追蹤第九行星時意外發現多12顆衛星，並在2018年7月正式確認，因此至今已確認的木星衛星總數達到79個。

伽利略衛星

埃歐、歐羅巴和佳利美德，這些在太陽系中最大的衛星，軌道的形成拉普拉斯共振的模式；埃歐每繞木星運轉4圈，歐羅巴也很精確的繞著木星轉2圈，佳利美德則很精確的繞木星轉一圈。因為每顆衛星都在軌道上相同的點受到相鄰衛星額外的拖曳，這種共振造成的引力效應使它們的軌道被扭曲成橢圓的形狀。另一方面，來自木星的潮汐力致力於將它們的軌道弄成圓形。

它們的軌道離心率造成當木星的引力拉扯它們接近時，這三顆衛星的形狀規律的扭曲；而當他們遠離時，又會回復到比較接近球體的形狀。這種潮汐的扭曲使衛星的內部摩擦生熱，最顯而易見的是最內側的埃歐（受到最強的潮汐力）異於平常的火山活動；和程度較輕的歐羅巴表面年輕的地質（暗示衛星的外觀最近重新鋪過）。

衛星的分類

在航海家任務之前，基於它們整齊排列共通的軌道要素，木星的4顆衛星被分成4個群組。之後，大量新的小衛星使這個畫面變得複雜起來。現在被認為有六個主要的群組，還有一些特立獨行，與其它的衛星顯然有所不同。

基本的子群是8顆在內側的週期性衛星，它們有著在木星赤道平面附近，接近圓形的軌道，並且被認為是與木星同時形成的。其它的衛星，包括數目不詳的不規則小衛星，有著橢圓與傾斜的軌道，被認為是被捕獲的小行星或是被捕或小行星的碎片。屬於同一群的不規則衛星共用相似的軌道要素，因而可能有著共同的起源，或許是一顆大衛星或是碎裂的一個天體。

與太陽系的交互作用

伴隨著太陽，木星的引力影響與幫助塑造了太陽系。在主小行星帶的柯克伍德空隙主要是由木星造成的，而且這顆行星可能也要對內太陽系歷史上的後期重轟炸期負責。

和它的衛星，木星的引力場控制了無數被安頓在拉格朗日點的小行星。這些小行星在木星之前或跟隨在木星之後一起繞著太陽公轉。它們被稱為特洛伊小行星，並且分為希臘營和特洛伊營，以紀念伊利亞特。第一顆是馬克斯·沃夫在1906年發現的(588) 阿基里斯，自此之後，迄今已經發現了數千顆，其中最大的是(624) 赫克特。

大多數短週期彗星屬於木星族 －定義為軌道半長軸比木星小的彗星。木星族彗星被認為起源於海王星軌道之外的古柏帶。在接近木星時，軌道受到攝動進入較短的週期，然後在木星和太陽的引力交互作用下，規律地環繞著太陽。

撞擊

由於其巨大的重力井和鄰近內太陽系，木星被稱為太陽系的真空吸塵器。它是太陽系內最頻繁接受到彗星撞擊的行星。它被認為是保護內太陽系的行星得以免受彗星的轟擊。最近的電腦模擬顯示，木星重力的攝動雖然可以改變進入內太陽系彗星的軌道，將它們吸積或彈出，但並未減少進入內太陽系的彗星數量。這仍然是天文學家爭議的主題，有些人相信它會將zh-hans:柯伊伯带;zh-hant:古柏帶的彗星拉近地球，而另一些人認為木星保護地球免於受到被宣稱來自zh-hans:奥尔特云;zh-hant:歐特雲的彗星撞擊。木星被小行星和彗星撞擊的經驗是地球的200倍。

在1997年，對歷史上的天文圖繪的調查認為喬瓦尼·多梅尼科·卡西尼可能在1690年紀錄了一次木星被撞擊的疤痕。調查也確認其它8個候選的觀測可能性太低或不是撞擊事件。在1979年3月，航海家1號在與木星相遇時拍到一顆火球。在1994年7月16日至7月22日這段期間，超過20顆舒梅克－李維九號彗星（SL-9，正式的名稱是D/1993 F2）的碎片撞擊在木星的南半球，首次提供了直接觀測太陽系內兩個天體的碰撞。這種撞擊對木星大氣的成分提供了有用的資料。

在2009年7月19日，在系統2的經度216度之處發現被撞擊的位置。這個撞擊在木星的大氣層留下一個與長圓形BA的大小相似的黑點。紅外線的觀測顯示在撞擊點上有一個亮點，意味著撞擊造成南極地區低層區域大氣層的溫度升高。

在2010年6月3日，澳洲的業餘天文學家 Anthony Wesley觀測到一顆火球的撞擊，造成小於以前觀測到的事件。稍後，另一位菲律賓的業餘天文學家也錄影捕捉到這次事件。2010年8月20日又有人見到一顆火球。

2012年9月19日，又檢測到另一顆火球。

生命的可能

在1953年，米勒-尤里實驗證明了閃電和存在於原始地球大氣中的化合物組合可以形成有機物（包括胺基酸），可以做為生命的基石。這模擬的大氣成分為水、甲烷、氨和氫分子；所有的這些物質都在現今的木星大氣層中被發現。木星的大氣層有強大的垂直空氣流動，運載這些化合物進入較低的地區。 但在木星的內部有更高的溫度，會分解這些化學物，會妨礙類似地球生命的形成。

在木星，因為大氣層中只有少量的水，還有任何的固體表面都在深處壓力極大的地區，因此被認為不可能存在任何類似地球的生命。在1976年，在航海家任務之前，曾經假設基於氨與水的生命可能在木星大氣層的上層進化。這一假設是基於地球的海洋態環境，頂層有簡單的光合作用浮游生物，低層的魚可以餵食這些生物，而肉食的海洋生物可以獵食這些魚。

在木星的一些衛星，地表之下可能有海洋存在，導致這些衛星更可能有生物存在的猜測。

神話

木星，因為在夜晚以肉眼很容易就看見它，當太陽的位置很低時，偶爾也能在白天看見，因此自古以來就為人所知。在巴比倫，這個天體代表他們的神馬爾杜克（Marduk）。他們用木星軌道大約12年繞行黃道一週來定義它們生肖的星宮。

羅馬人依據神話將它命名為朱庇特（Iuppiter, Iūpiter，也稱為Jova），是羅馬神話中主要的神，它的名字來自原始印歐語系的呼格合成*Dyēu-pəter（主格：*Dyēus-pətēr，意思是, "O 天神之父"或"O 日神之父"）。相對而言，木星對應於希臘神話是 宙斯（Ζεύς），也被稱為Dias (Δίας)，其中的行星名稱仍然保留在現代的希臘語中。

木星的天文符號，是以風格化表示的閃電符號。原始希臘神Zeus的字根是zeno-，用於和木星相關的語詞，例如：zenographic。

Jovian是從Jupiter轉成的形容詞，古老的形容詞是jovial，是中世紀的占星家使用的詞彙，原來的意思是"幸福"或"聖誕快樂"，是占星學中木星對情緒的影響。

在中、日、韓語系中，基於中國的五行，這顆行星被稱為木星。中國的道教它擬人化成為福星，希臘人稱之為Φαέθων,；法厄同（Phaethon）、"創新（blazing）"。在吠陀占星，木星被稱為祭主仙人（Brihaspati），是啟發靈性的宗教導師，通常稱為上師（Guru），字面的意思是"重人"。

在英語，週四（Thursday）是源自"雷神日"（Thor's day），是出在日耳曼神話。相較於羅馬神話就是朱庇特。羅馬星期的Jovis也重新命名為Thursday。

在突厥神話，木星稱為"Erendiz/Erentüz"，這意味著"eren(?)+ yultuz(star)"，而關於"eren"有許多有意義的理論。同樣的，它們也算出木星的軌道週期是11年又300天。他們認為一些社會和自然的事件連結到在天上運行的。

相關條目

• 熱木星

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• 朱諾號

• 虛構作品中的木星

• 新視野號

• 太空探索

• 太陽系探測器列表

• 太陽系探索時間線

• 先驅者10號

• 航海家號：1號、2號

• 伽利略號

• 卡西尼號

註解

Formation and migration

Jupiter is most likely the oldest planet in the Solar System. Current models of Solar System formation suggest that Jupiter formed at or beyond the snow line; a distance from the early Sun where the temperature is sufficiently cold for volatiles such as water to condense into solids. It first assembled a large solid core before accumulating its gaseous atmosphere. As a consequence, the core must have formed before the solar nebula began to dissipate after 10 million years. Formation models suggest Jupiter grew to 20 times the mass of the Earth in under a million years. The orbiting mass created a gap in the disk, thereafter slowly increasing to 50 Earth masses in 3–4 million years.

According to the "grand tack hypothesis", Jupiter would have begun to form at a distance of roughly 3.5 AU. As the young planet accreted mass, interaction with the gas disk orbiting the Sun and orbital resonances with Saturn caused it to migrate inward. This would have upset the orbits of what are believed to be super-Earths orbiting closer to the Sun, causing them to collide destructively. Saturn would later have begun to migrate inwards too, much faster than Jupiter, leading to the two planets becoming locked in a 3:2 mean motion resonance at approximately 1.5 AU. This in turn would have changed the direction of migration, causing them to migrate away from the Sun and out of the inner system to their current locations. These migrations would have occurred over an 800,000 year time period, with all of this happening over a time period of up to 6 million years after Jupiter began to form (3 million being a more likely figure). This departure would have allowed the formation of the inner planets from the rubble, including Earth.

However, the formation timescales of terrestrial planets resulting from the grand tack hypothesis appear inconsistent with the measured terrestrial composition. Moreover, the likelihood that the outward migration actually occurred in the solar nebula is very low. In fact, some models predict the formation of Jupiter's analogues whose properties are close to those of the planet at the current epoch.

Other models have Jupiter forming at distances much further out, such as 18 AU. In fact, based on Jupiter's composition, researchers have made the case for an initial formation outside the molecular nitrogen (N2) snowline, which is estimated at 20-30 AU, and possibly even outside the argon snowline, which may be as far as 40 AU. Having formed at one of these extreme distances, Jupiter would then have migrated inwards to its current location. This inward migration would have occurred over a roughly 700,000 year time period, during an epoch approximately 2–3 million years after the planet began to form. Saturn, Uranus and Neptune would have formed even further out than Jupiter, and Saturn would also have migrated inwards.

Physical characteristics

Jupiter is one of the four gas giants, being primarily composed of gas and liquid rather than solid matter. It is the largest planet in the Solar System, with a diameter of at its equator. The average density of Jupiter, 1.326 g/cm3, is the second highest of the giant planets, but lower than those of the four terrestrial planets.

Composition

Jupiter's upper atmosphere is about 90% hydrogen and 10% helium by volume. Since helium atoms are more massive than hydrogen atoms, Jupiter's atmosphere is approximately 75% hydrogen and 24% helium by mass, with the remaining one percent consisting of other elements. 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.

The atmospheric proportions of hydrogen and helium are close to the theoretical composition of the primordial solar nebula. Neon in the upper atmosphere only consists of 20 parts per million by mass, which is about a tenth as abundant as in the Sun. Helium is also depleted to about 80% of the Sun's helium composition. This depletion is a result of precipitation of these elements as helium-rich droplets deep in the interior of the planet.

Based on spectroscopy, Saturn is thought to be similar in composition to Jupiter, but the other giant planets Uranus and Neptune have relatively less hydrogen and helium and relatively more of the next most abundant elements, including oxygen, carbon, nitrogen, and sulfur. As their volatile compounds are mainly in ice form, they are called ice giants.

Mass and size

Jupiter's mass is 2.5 times that of all the other planets in the Solar System combined—this is so massive that its barycentre with the Sun lies above the Sun's surface at 1.068 solar radii from the Sun's centre. Jupiter is much larger than Earth and considerably less dense: its volume is that of about 1,321 Earths, but it is only 318 times as massive. Jupiter's radius is about one tenth the radius of the Sun, and its mass is one thousandth the mass of the Sun, so the densities of the two bodies are similar. A "Jupiter mass" ( or ) is often used as a unit to describe masses of other objects, particularly extrasolar planets and brown dwarfs. For example, the extrasolar planet HD 209458 b has a mass of , while Kappa Andromedae b has a mass of .

Theoretical models indicate that if Jupiter had much more mass than it does at present, it would shrink. For small changes in mass, the radius would not change appreciably, and above 160% of the current mass the interior would become so much more compressed under the increased pressure that its volume would decrease despite the increasing amount of matter. As a result, Jupiter is thought to have about as large a diameter as a planet of its composition and evolutionary history can achieve. The process of further shrinkage with increasing mass would continue until appreciable stellar ignition was achieved, as in high-mass brown dwarfs having around 50 Jupiter masses.

Although Jupiter would need to be about 75 times more massive to fuse hydrogen and become a star, the smallest red dwarf is only about 30 percent larger in radius than Jupiter. Despite this, Jupiter still radiates more heat than it receives from the Sun; the amount of heat produced inside it is similar to the total solar radiation it receives. This additional heat is generated by the Kelvin–Helmholtz mechanism through contraction. This process causes Jupiter to shrink by about 1 mm/yr. When formed, Jupiter was hotter and was about twice its current diameter.

Internal structure

Before the early 21st century, most scientists expected Jupiter to either consist of a dense core, a surrounding layer of liquid metallic hydrogen (with some helium) extending outward to about 80% of the radius of the planet, and an outer atmosphere consisting predominantly of molecular hydrogen, or perhaps to have no core at all, consisting instead of denser and denser fluid (predominantly molecular and metallic hydrogen) all the way to the center, depending on whether the planet accreted first as a solid body or collapsed directly from the gaseous protoplanetary disk. When the Juno mission arrived in July 2016, it found that Jupiter has a very diffuse core that mixes into its mantle. A possible cause is an impact from a planet of about ten Earth masses a few million years after Jupiter's formation, which would have disrupted an originally solid Jovian core. It is estimated that the core is 30–50% of the planet's radius, and contains heavy elements 7–25 times the mass of Earth.

Above the layer of metallic hydrogen lies a transparent interior atmosphere of hydrogen. At this depth, the pressure and temperature are above molecular hydrogen's critical pressure of 1.3 MPa and critical temperature of only 33 K. In this state, there are no distinct liquid and gas phases—hydrogen is said to be in a supercritical fluid state. It is convenient to treat hydrogen as gas extending downward from the cloud layer to a depth of about 1,000 km, and as liquid in deeper layers. Physically, there is no clear boundary—the gas smoothly becomes hotter and denser as depth increases. Rain-like droplets of helium and neon precipitate downward through the lower atmosphere, depleting the abundance of these elements in the upper atmosphere. Calculations suggest that helium drops separate from metalic hydrogen at a radius of 60,000 km (11,000 km below the cloudtops) and merge again at 50,000 km (22,000 km beneath the clouds). Rainfalls of diamonds have been suggested to occur, as well as on Saturn and the ice giants Uranus and Neptune.

The temperature and pressure inside Jupiter increase steadily inward, this is observed in microwave emission and required because the heat of formation can only escape by convection. At the pressure level of 10 bars (1 MPa), the temperature is around . The hydrogen is always supercritical (that is, it never encounters a first-order phase transition) even as it changes gradually from a molecular fluid to a metallic fluid at around 100–200 GPa, where the temperature is perhaps . The temperature of Jupiter's diluted core is estimated at around or more with an estimated pressure of around 4,500 GPa.

Atmosphere

Jupiter has the deepest planetary atmosphere in the Solar System, spanning over in altitude.

Cloud layers

Jupiter is perpetually covered with clouds composed of ammonia crystals, and possibly ammonium hydrosulfide. The clouds are in the tropopause and are in bands of different latitudes, known as tropical regions. These are subdivided into lighter-hued zones and darker belts. The interactions of these conflicting circulation patterns cause storms and turbulence. Wind speeds of are common in zonal jet streams. The zones have been observed to vary in width, colour and intensity from year to year, but they have remained sufficiently stable for scientists to name them.

The cloud layer is about deep, and consists of at least two decks of clouds: a thick lower deck and a thin clearer region. There may also be a thin layer of water clouds underlying the ammonia layer. Supporting the presence of water clouds are the flashes of lightning detected in the atmosphere of Jupiter. These electrical discharges can be up to a thousand times as powerful as lightning on Earth. The water clouds are assumed to generate thunderstorms in the same way as terrestrial thunderstorms, driven by the heat rising from the interior. The Juno mission revealed the presence of "shallow lightning" which originates from ammonia-water clouds relatively high in the atmosphere. These discharges carry "mushballs" of water-ammonia slushes covered in ice, which fall deep into the atmosphere. Upper-atmospheric lightning has been observed in Jupiter's upper atmosphere, bright flashes of light that last around 1.4 milliseconds. These are known as "elves" or "sprites" and appear blue or pink due to the hydrogen.

The orange and brown colours in the clouds of Jupiter are caused by upwelling compounds that change colour when they are exposed to ultraviolet light from the Sun. The exact makeup remains uncertain, but the substances are thought to be phosphorus, sulfur or possibly hydrocarbons. These colourful compounds, known as chromophores, mix with the warmer lower deck of clouds. The zones are formed when rising convection cells form crystallising ammonia that masks out these lower clouds from view.

Jupiter's low axial tilt means that the poles always receive less solar radiation than the planet's equatorial region. Convection within the interior of the planet transports energy to the poles, balancing out the temperatures at the cloud layer.

Great Red Spot and other vortices

The best known feature of Jupiter is the Great Red Spot, a persistent anticyclonic storm located 22° south of the equator. It is known to have existed since at least 1831, and possibly since 1665. Images by the Hubble Space Telescope have shown as many as two "red spots" adjacent to the Great Red Spot. The storm is visible through Earth-based telescopes with an aperture of 12 cm or larger. The oval object rotates counterclockwise, with a period of about six days. The maximum altitude of this storm is about above the surrounding cloudtops. The Spot's composition and the source of its red color remain uncertain, although photodissociated ammonia reacting with acetylene is a robust candidate to explain the coloration.

The Great Red Spot is larger than the Earth. Mathematical models suggest that the storm is stable and will be a permanent feature of the planet. However, it has significantly decreased in size since its discovery. Initial observations in the late 1800s showed it to be approximately across. By the time of the Voyager flybys in 1979, the storm had a length of and a width of approximately . Hubble observations in 1995 showed it had decreased in size to , and observations in 2009 showed the size to be . , the storm was measured at approximately , and was decreasing in length by about per year.

Juno missions show that there are several polar cyclone groups at Jupiter's poles. The northern group contains nine cyclones, with a large one in the center and eight others around it, while its southern counterpart also consists of a center vortex but is surrounded by five large storms and a single smaller one. These polar structures are caused by the turbulence in Jupiter's atmosphere and can be compared with the hexagon at Saturn's north pole.

In 2000, an atmospheric feature formed in the southern hemisphere that is similar in appearance to the Great Red Spot, but smaller. This was created when smaller, white oval-shaped storms merged to form a single feature—these three smaller white ovals were first observed in 1938. The merged feature was named Oval BA and has been nicknamed "Red Spot Junior." It has since increased in intensity and changed from white to red.

In April 2017, a "Great Cold Spot" was discovered in Jupiter's thermosphere at its north pole. This feature is across, wide, and cooler than surrounding material. While this spot changes form and intensity over the short term, it has maintained its general position in the atmosphere for more than 15 years. It may be a giant vortex similar to the Great Red Spot, and appears to be quasi-stable like the vortices in Earth's thermosphere. Interactions between charged particles generated from Io and the planet's strong magnetic field likely resulted in redistribution of heat flow, forming the Spot.

Magnetosphere

Jupiter's magnetic field is fourteen times stronger than Earth's, ranging from 4.2 gauss (0.42 mT) at the equator to 10–14 gauss (1.0–1.4 mT) at the poles, making it the strongest in the Solar System (except for sunspots). This field is thought to be generated by eddy currents—swirling movements of conducting materials—within the liquid metallic hydrogen core. The volcanoes on the moon Io emit large amounts of sulfur dioxide, forming a gas torus along the moon's orbit. The gas is ionised in the magnetosphere, producing sulfur and oxygen ions. They, together with hydrogen ions originating from the atmosphere of Jupiter, form a plasma sheet in Jupiter's equatorial plane. The plasma in the sheet co-rotates with the planet, causing deformation of the dipole magnetic field into that of a magnetodisk. Electrons within the plasma sheet generate a strong radio signature that produces bursts in the range of 0.6–30 MHz which are detectable from Earth with consumer-grade shortwave radio receivers.

At about 75 Jupiter radii from the planet, the interaction of the magnetosphere with the solar wind generates a bow shock. Surrounding Jupiter's magnetosphere is a magnetopause, located at the inner edge of a magnetosheath—a region between it and the bow shock. The solar wind interacts with these regions, elongating the magnetosphere on Jupiter's lee side and extending it outward until it nearly reaches the orbit of Saturn. The four largest moons of Jupiter all orbit within the magnetosphere, which protects them from the solar wind.

The magnetosphere of Jupiter is responsible for intense episodes of radio emission from the planet's polar regions. Volcanic activity on Jupiter's moon Io injects gas into Jupiter's magnetosphere, producing a torus of particles about the planet. As Io moves through this torus, the interaction generates Alfvén waves that carry ionised matter into the polar regions of Jupiter. As a result, radio waves are generated through a cyclotron maser mechanism, and the energy is transmitted out along a cone-shaped surface. When Earth intersects this cone, the radio emissions from Jupiter can exceed the solar radio output.

Orbit and rotation

Jupiter is the only planet whose barycentre with the Sun lies outside the volume of the Sun, though by only 7% of the Sun's radius. The average distance between Jupiter and the Sun is 778 million km (about 5.2 times the average distance between Earth and the Sun, or 5.2 AU) and it completes an orbit every 11.86 years. This is approximately two-fifths the orbital period of Saturn, forming a near orbital resonance. The orbital plane of Jupiter is inclined 1.31° compared to Earth. Because the eccentricity of its orbit is 0.048, Jupiter is slightly over 75 million km nearer the Sun at perihelion than aphelion.

The axial tilt of Jupiter is relatively small, only 3.13°, so its seasons are insignificant compared to those of Earth and Mars.

Jupiter's rotation is the fastest of all the Solar System's planets, completing a rotation on its axis in slightly less than ten hours; this creates an equatorial bulge easily seen through an amateur telescope. The planet is an oblate spheroid, meaning that the diameter across its equator is longer than the diameter measured between its poles. On Jupiter, the equatorial diameter is longer than the polar diameter.

Because Jupiter is not a solid body, its upper atmosphere undergoes differential rotation. The rotation of Jupiter's polar atmosphere is about 5 minutes longer than that of the equatorial atmosphere; three systems are used as frames of reference, particularly when graphing the motion of atmospheric features. System I applies to latitudes from 10° N to 10° S; its period is the planet's shortest, at 9h 50m 30.0s. System II applies at all latitudes north and south of these; its period is 9h 55m 40.6s. System III was defined by radio astronomers and corresponds to the rotation of the planet's magnetosphere; its period is Jupiter's official rotation.

Observation

Jupiter is usually the fourth brightest object in the sky (after the Sun, the Moon, and Venus); at opposition Mars can appear brighter than Jupiter. Depending on Jupiter's position with respect to the Earth, it can vary in visual magnitude from as bright as −2.94 at opposition down to −1.66 during conjunction with the Sun. The mean apparent magnitude is −2.20 with a standard deviation of 0.33. The angular diameter of Jupiter likewise varies from 50.1 to 29.8 arc seconds. Favorable oppositions occur when Jupiter is passing through perihelion, an event that occurs once per orbit.

Because the orbit of Jupiter is outside that of Earth, the phase angle of Jupiter as viewed from Earth never exceeds 11.5°; thus, Jupiter always appears nearly fully illuminated when viewed through Earth-based telescopes. It was only during spacecraft missions to Jupiter that crescent views of the planet were obtained. A small telescope will usually show Jupiter's four Galilean moons and the prominent cloud belts across Jupiter's atmosphere. A large telescope will show Jupiter's Great Red Spot when it faces Earth.

History of research and exploration

Pre-telescopic research

Observation of Jupiter dates back to at least the Babylonian astronomers of the 7th or 8th century BC. The ancient Chinese knew Jupiter as the "Suì Star" ( 歲星) and established their cycle of 12 earthly branches based on its approximate number of years; the Chinese language still uses its name (simplified as 岁) when referring to years of age. By the 4th century BC, these observations had developed into the Chinese zodiac, with each year associated with a Tai Sui star and god controlling the region of the heavens opposite Jupiter's position in the night sky; these beliefs survive in some Taoist religious practices and in the East Asian zodiac's twelve animals, now often popularly assumed to be related to the arrival of the animals before Buddha. The Chinese historian Xi Zezong has claimed that Gan De, an ancient Chinese astronomer, reported a small star "in alliance" with the planet, which may indicate a sighting of one of Jupiter's moons with the unaided eye. If true, this would predate Galileo's discovery by nearly two millennia.

A 2016 paper reports that trapezoidal rule was used by Babylonians before 50 BCE for integrating the velocity of Jupiter along the ecliptic. In his 2nd century work the Almagest, the Hellenistic astronomer Claudius Ptolemaeus constructed a geocentric planetary model based on deferents and epicycles to explain Jupiter's motion relative to Earth, giving its orbital period around Earth as 4332.38 days, or 11.86 years.

Ground-based telescope research

In 1610, Italian polymath Galileo Galilei discovered the four largest moons of Jupiter (now known as the Galilean moons) using a telescope; thought to be the first telescopic observation of moons other than Earth's. One day after Galileo, Simon Marius independently discovered moons around Jupiter, though he did not publish his discovery in a book until 1614. It was Marius's names for the major moons, however, that stuck: Io, Europa, Ganymede, and Callisto. These findings were the first discovery of celestial motion not apparently centred on Earth. The discovery was a major point in favor of Copernicus' heliocentric theory of the motions of the planets; Galileo's outspoken support of the Copernican theory led to him being tried and condemned by the Inquisition.

During the 1660s, Giovanni Cassini used a new telescope to discover spots and colourful bands, observe that the planet appeared oblate, and estimate the planet's rotation period. In 1690 Cassini noticed that the atmosphere undergoes differential rotation.

The Great Red Spot may have been observed as early as 1664 by Robert Hooke and in 1665 by Cassini, although this is disputed. The pharmacist Heinrich Schwabe produced the earliest known drawing to show details of the Great Red Spot in 1831. The Red Spot was reportedly lost from sight on several occasions between 1665 and 1708 before becoming quite conspicuous in 1878. It was recorded as fading again in 1883 and at the start of the 20th century.

Both Giovanni Borelli and Cassini made careful tables of the motions of Jupiter's moons, allowing predictions of when the moons would pass before or behind the planet. By the 1670s, it was observed that when Jupiter was on the opposite side of the Sun from Earth, these events would occur about 17 minutes later than expected. Ole Rømer deduced that light does not travel instantaneously (a conclusion that Cassini had earlier rejected), and this timing discrepancy was used to estimate the speed of light.

In 1892, E. E. Barnard observed a fifth satellite of Jupiter with the refractor at Lick Observatory in California. This moon was later named Amalthea. It was the last planetary moon to be discovered directly by visual observation. An additional eight satellites were discovered before the flyby of the Voyager 1 probe in 1979.

In 1932, Rupert Wildt identified absorption bands of ammonia and methane in the spectra of Jupiter.

Three long-lived anticyclonic features termed white ovals were observed in 1938. For several decades they remained as separate features in the atmosphere, sometimes approaching each other but never merging. Finally, two of the ovals merged in 1998, then absorbed the third in 2000, becoming Oval BA.

Radiotelescope research

In 1955, Bernard Burke and Kenneth Franklin detected bursts of radio signals coming from Jupiter at 22.2 MHz. The period of these bursts matched the rotation of the planet, and they used this information to refine the rotation rate. Radio bursts from Jupiter were found to come in two forms: long bursts (or L-bursts) lasting up to several seconds, and short bursts (or S-bursts) lasting less than a hundredth of a second.

Scientists discovered that there are three forms of radio signals transmitted from Jupiter:

• Decametric radio bursts (with a wavelength of tens of metres) vary with the rotation of Jupiter, and are influenced by the interaction of Io with Jupiter's magnetic field.

• Decimetric radio emission (with wavelengths measured in centimetres) was first observed by Frank Drake and Hein Hvatum in 1959. The origin of this signal was a torus-shaped belt around Jupiter's equator. This signal is caused by cyclotron radiation from electrons that are accelerated in Jupiter's magnetic field.

• Thermal radiation is produced by heat in the atmosphere of Jupiter.

Exploration

Since 1973, a number of automated spacecraft have visited Jupiter, most notably the Pioneer 10 space probe, the first spacecraft to get close enough to Jupiter to send back revelations about its properties and phenomena. Flights to planets within the Solar System are accomplished at a cost in energy, which is described by the net change in velocity of the spacecraft, or delta-v. Entering a Hohmann transfer orbit from Earth to Jupiter from low Earth orbit requires a delta-v of 6.3 km/s, which is comparable to the 9.7 km/s delta-v needed to reach low Earth orbit. Gravity assists through planetary flybys can be used to reduce the energy required to reach Jupiter, albeit at the cost of a significantly longer flight duration.

Flyby missions

Beginning in 1973, several spacecraft have performed planetary flyby maneuvers that brought them within observation range of Jupiter. The Pioneer missions obtained the first close-up images of Jupiter's atmosphere and several of its moons. They discovered that the radiation fields near the planet were much stronger than expected, but both spacecraft managed to survive in that environment. The trajectories of these spacecraft were used to refine the mass estimates of the Jovian system. Radio occultations by the planet resulted in better measurements of Jupiter's diameter and the amount of polar flattening.

Six years later, the Voyager missions vastly improved the understanding of the Galilean moons and discovered Jupiter's rings. They also confirmed that the Great Red Spot was anticyclonic. Comparison of images showed that the Red Spot had changed hue since the Pioneer missions, turning from orange to dark brown. A torus of ionised atoms was discovered along Io's orbital path, and volcanoes were found on the moon's surface, some in the process of erupting. As the spacecraft passed behind the planet, it observed flashes of lightning in the night side atmosphere.

The next mission to encounter Jupiter was the Ulysses solar probe. It performed a flyby maneuver to attain a polar orbit around the Sun. During this pass, the spacecraft studied Jupiter's magnetosphere. Ulysses has no cameras so no images were taken. A second flyby six years later was at a much greater distance.

In 2000, the Cassini probe flew by Jupiter on its way to Saturn, and provided higher-resolution images.

The New Horizons probe flew by Jupiter in 2007 for a gravity assist en route to Pluto. The probe's cameras measured plasma output from volcanoes on Io and studied all four Galilean moons in detail, as well as making long-distance observations of the outer moons Himalia and Elara.

Galileo mission

The first spacecraft to orbit Jupiter was the Galileo probe, which entered orbit on December 7, 1995. It orbited the planet for over seven years, conducting multiple flybys of all the Galilean moons and Amalthea. The spacecraft also witnessed the impact of Comet Shoemaker–Levy 9 as it approached Jupiter in 1994, giving a unique vantage point for the event. Its originally designed capacity was limited by the failed deployment of its high-gain radio antenna, although extensive information was still gained about the Jovian system from Galileo.

A 340-kilogram titanium atmospheric probe was released from the spacecraft in July 1995, entering Jupiter's atmosphere on December 7. It parachuted through of the atmosphere at a speed of about 2,575 km/h (1600 mph) and collected data for 57.6 minutes before the signal was lost at a pressure of about 23 atmospheres and a temperature of 153 °C. It melted thereafter, and possibly vapourised. The Galileo orbiter itself experienced a more rapid version of the same fate when it was deliberately steered into the planet on September 21, 2003, at a speed of over 50 km/s to avoid any possibility of it crashing into and possibly contaminating the moon Europa, which may harbor life.

Data from this mission revealed that hydrogen composes up to 90% of Jupiter's atmosphere. The recorded temperature was more than 300 °C (570 °F) and the windspeed measured more than 644 km/h (>400 mph) before the probes vapourised.

Juno mission

NASA's Juno mission arrived at Jupiter on July 4, 2016, and was expected to complete thirty-seven orbits over the next twenty months. The mission plan called for Juno to study the planet in detail from a polar orbit. On August 27, 2016, the spacecraft completed its first fly-by of Jupiter and sent back the first ever images of Jupiter's north pole. Juno would complete 12 science orbits before the end of its budgeted mission plan, ending July 2018. In June of that year, NASA extended the mission operations plan to July 2021, and in January of that year the mission was extended to September 2025 with four lunar flybys: one of Ganymede, one of Europa, and two of Io. When Juno reaches the end of the mission, it will perform a controlled deorbit and disintegrate into Jupiter's atmosphere. During the mission, the spacecraft will be exposed to high levels of radiation from Jupiter's magnetosphere, which may cause future failure of certain instruments and risk collision with Jupiter's moons.

Canceled missions and future plans

There has been great interest in studying Jupiter's icy moons in detail because of the possibility of subsurface liquid oceans on Europa, Ganymede, and Callisto. Funding difficulties have delayed progress. NASA's JIMO (Jupiter Icy Moons Orbiter) was cancelled in 2005. A subsequent proposal was developed for a joint NASA/ESA mission called EJSM/Laplace, with a provisional launch date around 2020. EJSM/Laplace would have consisted of the NASA-led Jupiter Europa Orbiter and the ESA-led Jupiter Ganymede Orbiter. However, ESA had formally ended the partnership by April 2011, citing budget issues at NASA and the consequences on the mission timetable. Instead, ESA planned to go ahead with a European-only mission to compete in its L1 Cosmic Vision selection.

These plans were realized as the European Space Agency's Jupiter Icy Moon Explorer (JUICE), due to launch in 2022, followed by NASA's Europa Clipper mission, scheduled for launch in 2024. Other proposed missions include the Chinese National Space Administration's Interstellar Express, a pair of probes to launch in 2024 that would use Jupiter's gravity to explore either end of the heliosphere, and NASA's Trident, which would launch in 2025 and use Jupiter's gravity to bend the spacecraft on a path to explore Neptune's moon Triton.

Moons

Jupiter has 79 known natural satellites. Of these, 60 are less than 10 km in diameter. The four largest moons are Io, Europa, Ganymede, and Callisto, collectively known as the "Galilean moons", and are visible from Earth with binoculars on a clear night.

Galilean moons

The moons discovered by Galileo—Io, Europa, Ganymede, and Callisto—are among the largest in the Solar System. The orbits of three of them (Io, Europa, and Ganymede) form a pattern known as a Laplace resonance; for every four orbits that Io makes around Jupiter, Europa makes exactly two orbits and Ganymede makes exactly one. This resonance causes the gravitational effects of the three large moons to distort their orbits into elliptical shapes, because each moon receives an extra tug from its neighbors at the same point in every orbit it makes. The tidal force from Jupiter, on the other hand, works to circularise their orbits.

The eccentricity of their orbits causes regular flexing of the three moons' shapes, with Jupiter's gravity stretching them out as they approach it and allowing them to spring back to more spherical shapes as they swing away. This tidal flexing heats the moons' interiors by friction. This is seen most dramatically in the volcanic activity of Io (which is subject to the strongest tidal forces), and to a lesser degree in the geological youth of Europa's surface, which indicates recent resurfacing of the moon's exterior.

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| style="font-size:0.9em; text-align:center;" | The Galilean moons Io, Europa, Ganymede, and Callisto (in order of increasing distance from Jupiter)

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Classification

Jupiter's moons were traditionally classified into four groups of four, based on commonality of their orbital elements. This picture has been complicated by the discovery of numerous small outer moons since 1999. Jupiter's moons are currently divided into several different groups, although there are several moons which are not part of any group.

The eight innermost regular moons, which have nearly circular orbits near the plane of Jupiter's equator, are thought to have formed alongside Jupiter, whilst the remainder are irregular moons and are thought to be captured asteroids or fragments of captured asteroids. Irregular moons that belong to a group share similar orbital elements and thus may have a common origin, perhaps as a larger moon or captured body that broke up.

Planetary rings

Jupiter has a faint planetary ring system composed of three main segments: an inner torus of particles known as the halo, a relatively bright main ring, and an outer gossamer ring. These rings appear to be made of dust, rather than ice as with Saturn's rings. The main ring is probably made of material ejected from the satellites Adrastea and Metis. Material that would normally fall back to the moon is pulled into Jupiter because of its strong gravitational influence. The orbit of the material veers towards Jupiter and new material is added by additional impacts. In a similar way, the moons Thebe and Amalthea probably produce the two distinct components of the dusty gossamer ring. There is also evidence of a rocky ring strung along Amalthea's orbit which may consist of collisional debris from that moon.

Interaction with the Solar System

Along with the Sun, the gravitational influence of Jupiter has helped shape the Solar System. The orbits of most of the system's planets lie closer to Jupiter's orbital plane than the Sun's equatorial plane (Mercury is the only planet that is closer to the Sun's equator in orbital tilt). The Kirkwood gaps in the asteroid belt are mostly caused by Jupiter, and the planet may have been responsible for the Late Heavy Bombardment event in the inner Solar System's history.

In addition to its moons, Jupiter's gravitational field controls numerous asteroids that have settled into the regions of the Lagrangian points preceding and following Jupiter in its orbit around the Sun. These are known as the Trojan asteroids, and are divided into Greek and Trojan "camps" to commemorate the Iliad. The first of these, 588 Achilles, was discovered by Max Wolf in 1906; since then more than two thousand have been discovered. The largest is 624 Hektor.

Most short-period comets belong to the Jupiter family—defined as comets with semi-major axes smaller than Jupiter's. Jupiter family comets are thought to form in the Kuiper belt outside the orbit of Neptune. During close encounters with Jupiter their orbits are perturbed into a smaller period and then circularised by regular gravitational interaction with the Sun and Jupiter.

Due to the magnitude of Jupiter's mass, the centre of gravity between it and the Sun lies just above the Sun's surface, the only planet in the Solar System for which this is true.

Impacts

Jupiter has been called the Solar System's vacuum cleaner because of its immense gravity well and location near the inner Solar System there are more impacts on Jupiter, such as comets, than on the Solar System's other planets. It was thought that Jupiter partially shielded the inner system from cometary bombardment. However, recent computer simulations suggest that Jupiter does not cause a net decrease in the number of comets that pass through the inner Solar System, as its gravity perturbs their orbits inward roughly as often as it accretes or ejects them. This topic remains controversial among scientists, as some think it draws comets towards Earth from the Kuiper belt while others think that Jupiter protects Earth from the Oort cloud. Jupiter experiences about 200 times more asteroid and comet impacts than Earth.

A 1997 survey of early astronomical records and drawings suggested that a certain dark surface feature discovered by astronomer Giovanni Cassini in 1690 may have been an impact scar. The survey initially produced eight more candidate sites as potential impact observations that he and others had recorded between 1664 and 1839. It was later determined, however, that these candidate sites had little or no possibility of being the results of the proposed impacts.

Mythology

The planet Jupiter has been known since ancient times. It is visible to the naked eye in the night sky and can occasionally be seen in the daytime when the Sun is low. To the Babylonians, this object represented their god Marduk. They used Jupiter's roughly 12-year orbit along the ecliptic to define the constellations of their zodiac.

The Romans called it "the star of Jupiter" (Iuppiter Stella), as they believed it to be sacred to the principal god of Roman mythology, whose name comes from the Proto-Indo-European vocative compound *Dyēu-pəter (nominative: *Dyēus-pətēr, meaning "Father Sky-God", or "Father Day-God"). In turn, Jupiter was the counterpart to the mythical Greek Zeus (Ζεύς), also referred to as Dias (Δίας), the planetary name of which is retained in modern Greek. The ancient Greeks knew the planet as Phaethon (Φαέθων|label=none), meaning "shining one" or "blazing star". As supreme god of the Roman pantheon, Jupiter was the god of thunder, lightning, and storms, and appropriately called the god of light and sky.

The astronomical symbol for the planet, , is a stylised representation of the god's lightning bolt. The original Greek deity Zeus supplies the root zeno-, used to form some Jupiter-related words, such as zenographic. Jovian is the adjectival form of Jupiter. The older adjectival form jovial, employed by astrologers in the Middle Ages, has come to mean "happy" or "merry", moods ascribed to Jupiter's astrological influence. In Germanic mythology, Jupiter is equated to Thor, whence the English name Thursday for the Roman dies Jovis.

In Vedic astrology, Hindu astrologers named the planet after Brihaspati, the religious teacher of the gods, and often called it "Guru", which literally means the "Heavy One". In Central Asian Turkic myths, Jupiter is called Erendiz or Erentüz, from eren (of uncertain meaning) and yultuz ("star"). There are many theories about the meaning of eren. These peoples calculated the period of the orbit of Jupiter as 11 years and 300 days. They believed that some social and natural events connected to Erentüz's movements on the sky. The Chinese, Vietnamese, Koreans, and Japanese called it the "wood star" (木星 mùxīng), based on the Chinese Five Elements.

Gallery

File:Gemini North Infrared View of Jupiter.jpg|The tempestuous atmosphere of Jupiter, captured by the Wide Field Camera 3 on the Hubble Space Telescope in infrared.

File:Hubble Visible View of Jupiter.jpg|Hubble Visible View of Jupiter

File:Hubble Ultraviolet View of Jupiter.jpg|Hubble Ultraviolet View of Jupiter