Astronomers have pinpointed what appears to be the first moon detected outside our solar system, a large gaseous world the size of Neptune that is unlike any other known moon and orbits a gas planet much more massive than Jupiter.
The discovery, detailed by researchers on Oct. 3, was a surprise, and not because it showed that moons exist elsewhere — they felt it was only a matter of time for one to be found in another star system. They were amazed instead by how different this moon was from the roughly 180 known in our solar system.
“It’s big and weird by solar system standards,” Columbia University astronomy professor David Kipping said of the moon, known as an exomoon because it is outside our solar system.
Photo: Reuters
照片：路透
Our solar system’s moons all are rocky or icy objects. The newly discovered exomoon and the planet it orbits, estimated to be several times the mass of our solar system’s largest planet Jupiter, are both gaseous, an unexpected pairing. They are located 8,000 light years from Earth.
Kipping and study co-author Alex Teachey, a Columbia graduate student, said their observations using NASA’s Hubble Space Telescope and Kepler Space Telescope provided the first clear evidence of an exomoon, but further Hubble observations next May must be used to confirm the finding.
The exomoon is exponentially larger than our solar system’s biggest moon. Jupiter’s moon Ganymede has a diameter of about 5,260km. The exomoon is estimated to be roughly the size of Neptune, the smallest of our solar system’s four gas planets, with a diameter of about 49,000km.
The exomoon and its planet, Kepler-1625b, orbit Kepler-1625, a star similar in temperature to our sun but about 70 percent larger. The exomoon orbits roughly 3 million km from its planet. The exomoon’s mass is about 1.5 percent that of its planet.
Kipping and Teachey relied on the “transit” method already used by researchers to discover nearly 4,000 planets outside our solar system, called exoplanets. They observed a dip in Kepler-1625’s brightness when the planet and then the exomoon passed in front of it.
The size and gaseous composition of the exomoon challenge current moon formation theories. “Because larger objects are easier to detect than smaller ones, this is really the lowest-hanging fruit, so it might not be wholly unexpected that the first exomoon detection would be among the largest possible,” Teachey said.
The findings were published in the Oct. 3 edition of the journal Science Advances.(Reuters)
天文學家日前確認出第一顆在太陽系外被偵測到的「月球」精確位置。這顆大小如同海王星的大型氣態天體，迥異於目前所知的任何一顆衛星，並且繞行著一顆比木星還要巨大許多的氣態行星。
研究人員十月三日公布相關細節的這項發現可說是一大驚喜，並不是因為月球也存在於其他地方—學者認為，在另一個星系發現月球只不過是時間早晚的問題。他們驚訝之處在於，這顆月球是如何不同於我們在太陽系內所知的一百八十多顆衛星。
這顆月球因為存在於我們的太陽系外，所以被稱為「系外衛星」。哥倫比亞大學的天文學教授大衛‧齊平表示，「就太陽系的標準而言，它實在是又大又怪異」。
我們太陽系內的衛星全部都是岩石或結冰體。新發現的這顆系外衛星，以及它繞行的行星—據估計質量超過太陽系內最大行星木星好幾倍—皆為氣態，是一個讓人意想不到的組合。這兩顆星體距離地球約八千光年遠。
齊平與研究共同作者、現為哥倫比亞大學研究生的艾立克斯‧提采表示，他們使用美國太空總署哈伯太空望遠鏡和克卜勒太空望遠鏡獲得的觀察結果，首次提供明確的系外衛星證據，不過明年五月還要再次使用哈伯望遠鏡觀察，以確認這項發現。
這顆系外衛星以等比級數的倍率大於我們太陽系中的最大衛星，也就是木星的衛星「蓋尼米德」，後者直徑約為五千兩百六十公里。據估計，這顆系外衛星的大小約等同於海王星，也就是太陽系四顆氣態行星中最小的一顆，直徑約為四萬九千公里。
這顆系外衛星與它的行星繞行著「克卜勒1625」，後者是一顆溫度與我們的太陽相近，但體積約大出百分之七十的恆星。這顆系外衛星以大約三百萬公里的距離繞行著它的行星，質量約為其行星的百分之一點五。
齊平與提采運用的「凌日」現象觀測法，已被許多研究學者用來找到將近四千顆太陽系外的行星，也就是所謂的「系外行星」。兩位研究者發現，當行星與系外衛星繞行經過克卜勒1625前方時，這顆恆星的亮度稍微降低了一些。
這顆系外衛星的大小與氣態組成顛覆了當今的衛星形成理論。提采也表示：「因為較大的天體較小的更容易被觀測到，所以這項發現可說是唾手可得。第一次探測的結果就找到很可能是最大的系外衛星，或許並不讓人感到意外。」
這份研究刊登於十月三日出版的期刊《科學前緣》。
(台北時報章厚明譯)
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