哪些行星需要衛星呢？（圖）
06-12
哪些行星需要衛星呢？（圖）
Earth"s long-term climate is stabilized by the moon. But even moonless terrestrial planets could be habitable, contrary to previous belief.地球長期的氣候條件是靠月球來穩定的。但是，沒有衛星的類地行星可能也適合居住，這一點與以往的觀點相反。
Who Needs a Moon
| BOSTON—The number of Earth-like extrasolar planets suitable for harboring advanced life could be 10 times higher than has been assumed until now, according to a new modeling study. The finding contradicts the prevailing notion that a terrestrial planet needs a large moon to stabilize the orientation of its axis and, hence, its climate. | 【波士頓消息】根據一項新的模擬研究，系外類地行星適合高級生命居住的可能性比此前一直所認為的情況要大10倍。該發現跟一種流行的觀念相抵觸，這種觀念認為類地行星需要一顆巨大的衛星來穩定行星軸的方向性，進而穩定其氣候。 |
| In 1993, French mathematicians Jacques Laskar and Philippe Robutel showed that Earth』s large moon has a stabilizing effect on our planet』s climate. Without the moon, gravitational perturbations from other planets, notably nearby Venus and massive Jupiter, would greatly disturb Earth』s axial tilt, with vast consequences for the planet』s climate. The steadily orbiting moon』s gravitational tug counteracts these disturbances, and Earth』s axial tilt never veers too far from the current value of 23.5°, where 0° would mean the axis was perpendicular to the plane of Earth』s orbit around the sun. | 1993年，法國數學家傑克斯·拉斯卡和菲利普·羅布特爾證明，地球的巨大衛星對地球的氣候起著一種穩定作用。如果沒有這顆衛星，來自其他行星的引力攝動，特別是來自附近的金星和木星的引力攝動就會在很大程度上干擾地軸的傾斜度，給地球的氣候造成嚴重的後果。在軌道上穩定運行的衛星發出的引力抵消了這些干擾，使地軸的傾斜度不會大幅度偏離目前的數值——23.5°，而0°意味著地軸垂直於地球環繞太陽的軌道平面。 |
| Indeed, Laskar and Robutel also showed that the axial tilt of Mars, which has only two tiny moons, has varied between 10° and 60° in the past, which caused huge climate variations that in turn could have contributed to the loss of most of the planet』s atmosphere, leaving Mars the bone-dry desert world that it is now. Since then, most astrobiologists have assumed that Earth-like planets in other solar systems would need a comparatively large moon to support complex life over long periods of time. | 的確如此，拉斯卡和羅布特爾還證明了：在過去，僅有兩個微型衛星的火星，軸傾斜度在10°和60°之間變化，結果引起了巨大的氣候變化，氣候變化反過來又造成火星絕大部分大氣的流失，最終使火星成為目前這個樣子——一顆極為乾燥的沙漠星球。自從那時起，多數天體生物學家一直認為，其他恆星系統中的類地行星都需要一顆相對大型的衛星，以便長期支持複雜的生命形式存在。 |
| Given the generally accepted idea of how Earth got its big moon—through an improbable, dramatic collision with a Mars-sized body that knocked a huge chunk of Earth loose—astronomers estimate that only 1% of all Earth-like planets in the universe might actually have such a hefty companion. That would mean that planets harboring complex life might be relatively rare. | 至於地球是如何得到這顆大型衛星的，一個被普遍接受的觀點是：地球曾經跟一顆火星大小的天體發生了不可思議的猛烈碰撞，撞擊使一大塊地球物質分離出去，形成了月球。鑒於月球的這種形成方式，天文學家們估計宇宙中只有1%的類地行星可能會真正擁有這樣一顆大型衛星。那意味著擁有複雜生命的行星可能會更加稀少。 |
| However, Jack Lissauer, a theoretical astrophysicist at NASA』s Ames Research Center in Moffet Field, California, is much more optimistic. Together with Jason Barnes, a physicist at the University of Idaho, Moscow, and John Chambers, a theoretical astrophysicist at the Carnegie Institution』s Department of Terrestrial Magnetism in Washington, D.C., he has carried out large numbers of detailed numerical simulations of "moon-less Earths," which show that the consequences are less dire than is generally assumed. | 然而，加州莫菲特場市美國國家航空航天管理局艾姆斯研究中心的理論天體物理學家傑克·利索爾對此要樂觀得多。利索爾和莫斯科市愛達荷大學的物理學家詹森·巴恩斯，以及華盛頓哥倫比亞特區卡內基研究院地磁研究所的理論天體物理學家約翰·錢伯斯一道，對「無衛星類地行星」進行了大量而詳盡的數字模擬，模擬顯示：跟平時的推測相比，結果並不那麼可怕。 |
| That』s because really big changes in a planet』s tilt would occur only after a very long time, so there would be more than enough time for the evolution of life, Lissauer reported yesterday here at the summer meeting of the American Astronomical Society. 「The variations in Earth』s axial tilt would indeed be substantially larger if there was no large moon,」 Lissauer says, 「but really big excursions from the current value would only occur on time scales of billions of years.」 That would leave ample time for advanced land life to evolve under relatively stable climatic conditions—although what would happen to such life during an axial shift remains unclear. | 昨天，在美國天文協會于波士頓舉行的夏季會議上，利索爾報道說：由於行星傾斜度的巨大變化只有在很長時間以後才會發生，因此對於生命的進化來說會有足夠的時間。「如果沒有大型衛星，地軸傾斜度的變化的確會大得多，」利索爾說，「以目前的傾斜度為基準，真正的大規模偏離只有在幾十億年的時間尺度上才會發生。」這樣，在相對穩定的氣候條件下高級陸地生命就會擁有足夠的進化時間。然而，至於在行星軸變化期間高級生命會遭遇什麼情況，這一點仍然不明。 |
| When a planet rotates in the opposite direction to its orbital motion (which happens to be the case for Venus), the effect of gravitational perturbations on its spin axis would be even smaller, the simulations indicate. And, of course, if a planetary system contains only one planet, there are no perturbations at all. Nobody knows how common such single-planet system might be. | 模擬顯示，如果正好像金星的情況——當行星的自轉方向與軌道運動方向相反時，那麼其轉軸的引力攝動效應將會更小。而且，如果一個行星系統只包含一顆行星，當然就根本不存在引力攝動。然而，沒有人知道只包含一顆行星的行星系統是否普遍存在。 |
| Not everybody is overwhelmed by the importance of the new results. 「I don』t think [changes in a planet』s axial tilt] would be a problem for the development of advanced life,」 as any type of life would adapt to changing circumstances anyway, says planetary scientist Sara Seager of the Massachusetts Institute of Technology in Cambridge. | 並不是每個人都能夠深刻地認識到該研究成果的重要性。「我認為，行星軸傾斜度的變化對於高級生命的進化來說不是問題，因為無論如何任何形式的生命都會適應不斷變化的環境，」劍橋市麻省理工學院的行星科學家薩拉·西格說。 |
| But Bill Borucki of NASA』s Ames Research Center, who is the principal investigator of the planet-hunting Kepler satellite mission, says he is 「surprised and delighted」 by Lissauer』s conclusions. 「Kepler is searching for Earth-like planets orbiting other stars,」 he says, 「and this means much more of them might be harboring complex life. It』s a wonderful result.」 | 「開普勒」號衛星的任務是搜尋行星。該衛星的首席研究員、美國國家航空航天管理局艾姆斯研究中心的比爾·博盧基表示：他對利索爾的結論感到驚訝和欣喜。「『開普勒』號衛星正在搜尋圍繞其他恆星旋轉的類地行星，」他說，「這意味著擁有複雜生命的類地行星可能要多得多。這是一項很棒的研究成果！」 |
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