Scientists have detected an exotic planet in another solar system where the weather forecast is always dire — a 100 percent chance of the most outrageous rain imaginable, with droplets of scaldingly hot liquid iron.
The researchers said on Wednesday they used the planet-hunting instrument ESPRESSO, short for “Echelle Spectrograph for Rocky Exoplanets and Stable Spectroscopic Observations,” on the European Southern Observatory’s Very Large Telescope in Chile to observe a planet called WASP-76b located about 640 light years from Earth. It is nearly twice the size of Jupiter, our solar system’s largest planet.
Planets discovered outside our solar system are called exoplanets, and WASP-76b is one of the most extreme in terms of climate and chemistry. It is a member of a family of exoplanets spotted in recent years called “ultra-hot” gas giants.
Photo: Reuters
照片：路透
It resides outstandingly close to its parent star, which is almost twice as big as the sun. WASP-76b orbits at only three times the radius of that star, much closer than our solar system’s innermost planet Mercury orbits the sun. The same side of WASP-76b always faces its star, much as the same side of our moon always faces Earth.
WASP-76b receives 4,000 times the solar radiation that Earth gets from the sun, and its “dayside” is baked, broiled and barbecued, reaching 2,400°C. This ferocious heat vaporizes metals present in the planet, with strong winds then carrying iron vapor to the planet’s cooler night side where it condenses into liquid iron droplets.
Molten iron rain may be a unique feature of these “ultra-hot” exoplanets, according to University of Geneva astronomer David Ehrenreich, lead author of the study published in the journal Nature. WASP-76b illustrates the exotic nature of some exoplanets and shows that much remains to be learned about alien planetary systems.
“The extreme atmospheric conditions met in WASP-76b and its siblings, other ‘ultra-hot’ gas giants, are not found anywhere in our solar system and would be very difficult to reproduce in a lab,” Ehrenreich said. “Therefore, these exotic objects are unique laboratories to crash-test our climate models and understand the most extreme forms of atmospheric evolution.” “The ‘zoo’ of planetary systems is far beyond our expectations,” added astrophysicist and study co-author Maria Rosa Zapatero Osorio of the Center for Astrobiology in Madrid, coordinator of the ESPRESSO science team.
WASP-76b appears to be the only planet orbiting its star. While its diameter of approximately 266,000km is about 1.9 times bigger than Jupiter, WASP-76b is actually slightly less massive than Jupiter. This may be the result of perpetually being heated up by the star or never having the chance to cool down and shrink after the planet’s formation because of its position so close to the star. “In other words, it is very much puffed up,” Ehrenreich said.
(Reuters)
科學家近日在另一個太陽系中觀測到一顆奇異的行星，當地的天氣預報每天都讓人絕望：下雨機率百分之百，而且下著最駭人的雨──滾燙炙熱的「液態鐵雨滴」。
研究人員於週三表示，他們利用行星探測儀「ESPRESSO」，觀察到一顆名為WASP-76b的行星。該儀器設置於歐洲南方天文台在智利建造的「極大望遠鏡」上，全名為「岩質系外行星與穩定光譜觀測階梯光柵攝譜儀」。WASP-76b距離地球約六百四十光年，大小幾乎是木星──我們太陽系中體積最大的行星──的兩倍。
在我們太陽系外發現的行星被稱為「系外行星」。就氣候和化學的層面而言，WASP-76b可說是最極端的系外行星之一。該星球屬於科學家近年發現、稱作「超熱」氣態巨行星的系外行星家族。
這顆行星座落在非常靠近其宿主恆星的位置，而且該恆星幾乎是我們太陽的兩倍大。WASP-76b公轉的軌道半徑僅是恆星本體半徑的三倍，比起我們太陽系最內側的行星──水星──繞太陽公轉的距離還要更靠近。WASP-76b的同一側固定面向其宿主恆星，就像是我們的月亮總是以同一側面對地球。
這顆行星接收到的太陽輻射量是地球的四千倍，而永遠是白晝的那一面可說是烘了又烤、再遭到炙燒，溫度達到攝氏兩千四百度。如此兇猛的高溫讓行星上的金屬都遭到氣化，強風再將「鐵蒸氣」吹向行星表面較為涼爽的永夜側，鐵蒸氣就在那裡凝結成液態的「鐵雨滴」。
該研究日前發表於期刊《自然》，主要作者為日內瓦大學的天文學家大衛‧艾倫萊赫。根據他的說法，熔化的鐵雨可能是這類「超熱」系外行星的特色。WASP-76b也展現出某些系外行星的特異性質，顯示我們對於陌生的行星系統仍有許多需要學習的地方。
艾倫萊赫指出：「在WASP-76b和其他同類型『超熱』氣態巨行星上遇到的極端大氣條件，不曾出現於我們的太陽系中，也很難在實驗室重現。」艾倫萊赫進一步表示：「也因此，這些特異的星球可說是獨特的實驗室，讓我們能夠對氣候模式進行撞擊性實驗，藉此了解最極端的大氣演化型態。」研究共同作者瑪莉亞‧蘿莎‧薩帕特洛‧奧索里歐則補充說：「行星系統的混亂狀態遠遠超出我們預期。」奧索里歐是馬德里天體生物學中心的天體物理學家，同時也是「ESPRESSO」科學團隊的協調專員。
WASP-76b似乎是唯一一顆繞著其宿主恆星公轉的行星。儘管其直徑約為二十六萬六千公里，約比木星大一點九倍，WASP-76b實際上的質量比木星還要小一點。這也許是因為該行星永遠都處於被恆星加熱的狀態，或是因為距離恆星太近，因此自從形成後一直都沒有機會冷卻收縮。艾倫萊赫表示：「換句話說，這顆行星很大一部份是膨起來的。」
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