| 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. |
【波士顿消息】根据一项新的模拟研究，系外类地行星适合高级生命居住的可能性比此前一直所认为的情况要大10倍。该发现跟一种流行的观念相抵触，这种观念认为类地行星需要一颗巨大的卫星来稳定行星轴的方向性，进而稳定其气候。 |
| 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，法国数学家杰克斯·拉斯卡和菲利普·罗布特尔证明，地球的巨大卫星对地球的气候起着一种稳定作用。如果没有这颗卫星，来自其他行星的引力摄动，特别是来自附近的金星和木星的引力摄动就会在很大程度上干扰地轴的倾斜度，给地球的气候造成严重的后果。在轨道上稳定运行的卫星发出的引力抵消了这些干扰，使地轴的倾斜度不会大幅度偏离目前的数值——23.5°，而0°意味着地轴垂直于地球环绕太阳的轨道平面。 |
| 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. |
的确如此，拉斯卡和罗布特尔还证明了：在过去，仅有两个微型卫星的火星，轴倾斜度在10°和60°之间变化，结果引起了巨大的气候变化，气候变化反过来又造成火星绝大部分大气的流失，最终使火星成为目前这个样子——一颗极为干燥的沙漠星球。自从那时起，多数天体生物学家一直认为，其他恒星系统中的类地行星都需要一颗相对大型的卫星，以便长期支持复杂的生命形式存在。 |
| 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. |
至于地球是如何得到这颗大型卫星的，一个被普遍接受的观点是：地球曾经跟一颗火星大小的天体发生了不可思议的猛烈碰撞，撞击使一大块地球物质分离出去，形成了月球。鉴于月球的这种形成方式，天文学家们估计宇宙中只有1%的类地行星可能会真正拥有这样一颗大型卫星。那意味着拥有复杂生命的行星可能会更加稀少。 |
| 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，加州莫菲特场市美国国家航空航天管理局艾姆斯研究中心的理论天体物理学家杰克·利索尔对此要乐观得多。利索尔和莫斯科市爱达荷大学的物理学家詹森·巴恩斯，以及华盛顿哥伦比亚特区卡内基研究院地磁研究所的理论天体物理学家约翰·钱伯斯一道，对“无卫星类地行星”进行了大量而详尽的数字模拟，模拟显示：跟平时的推测相比，结果并不那么可怕。 |
| 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，在美国天文协会于波士顿举行的夏季会议上，利索尔报道说：由于行星倾斜度的巨大变化只有在很长时间以后才会发生，因此对于生命的进化来说会有足够的时间。“如果没有大型卫星，地轴倾斜度的变化的确会大得多，”利索尔说，“以目前的倾斜度为基准，真正的大规模偏离只有在几十亿年的时间尺度上才会发生。”这样，在相对稳定的气候条件下高级陆地生命就会拥有足够的进化时间。然而，至于在行星轴变化期间高级生命会遭遇什么情况，这一点仍然不明。 |
| 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. |
模拟显示，如果正好像金星的情况——当行星的自转方向与轨道运动方向相反时，那么其转轴的引力摄动效应将会更小。而且，如果一个行星系统只包含一颗行星，当然就根本不存在引力摄动。然而，没有人知道只包含一颗行星的行星系统是否普遍存在。 |
| 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. |
并不是每个人都能够深刻地认识到该研究成果的重要性。“我认为，行星轴倾斜度的变化对于高级生命的进化来说不是问题，因为无论如何任何形式的生命都会适应不断变化的环境，”剑桥市麻省理工学院的行星科学家萨拉·西格说。 |
| 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.” |
“开普勒”号卫星的任务是搜寻行星。该卫星的首席研究员、美国国家航空航天管理局艾姆斯研究中心的比尔·博卢基表示：他对利索尔的结论感到惊讶和欣喜。“‘开普勒’号卫星正在搜寻围绕其他恒星旋转的类地行星，”他说，“这意味着拥有复杂生命的类地行星可能要多得多。这是一项很棒的研究成果！” |
