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八大行星環繞太陽運轉已是不爭事實。那麼太陽系以外的恒星是否亦有其他個別的行星系？在過去的20年裡(截至2009年10月尾)，天文學家已發現403顆圍繞其他恒星運行的行星。通常天文學家都是通過測量行星對母星的引力攝動而推斷行星的存在。不過，哈勃太空望遠鏡終於2004年拍得南魚座主星(北落獅門)旁的一個小光點，經過分隔21個月的兩次觀測記錄(參看小圖)，證明光點是環繞母星公轉的行星。這顆行星被名為北落獅門b。北落獅門距地球只有25光年，是一顆相當於2億歲的年輕恒星。北落獅門b距母星119天文單位，相當於木星與太陽距離的22倍，它的質量比木星還要大3倍，同樣屬一顆氣態行星。由於這顆行星異常光亮，天文學家相信它可能如土星般擁有光環系統。
Extrasolar Planets
Just like our Earth orbiting the Sun, there could be planets revolving other stars. Over the past 20 years, astronomers have found 403(up to the end of October 2009)such extrasolar planets. These objects are usually detected indirectly by measuring the slight wobbles of their parent stars. Then in 2004 the Hubble Space Telescope discovered a speck of light very close to the bright star, Fomalhaut. Fomalhaut, located 25 light years away in the constellation Piscis Australis, is relatively young, only 200 million years old. Observations taken 21 months apart(see the inset)show that the speck of light, dubbed Fomalhaut b, is a planet orbiting Fomalhaut at a distance of approximately 119 AU. This is equivalent to 22 times the distance between Jupiter and the Sun. Just like Jupiter, Fomalhaut b is a gas-giant world but 3 times as massive. Because of its unusual brightness, astronomers suspect it might have a ring system. ■甄枝強 資深天文愛好者 圖片鳴謝：NASA/ESA/STScI
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