http://hubblesite.org/newscenter/archive/releases/2012/44/full/
Solar systems with life-bearing planets may be rare if they are dependent on the presence of asteroid belts of just the right mass, according to a study by Rebecca Martin, a NASA Sagan Fellow from the University of Colorado in Boulder, and astronomer Mario Livio of the Space Telescope Science Institute in Baltimore, Md.
由美国宇航局萨根研究员（NASA Sagan Fellow）、马里兰州巴尔的摩市太空望远镜科研所（STScI）天文学家、巨砺市科罗拉多州立大学瑞贝卡•马丁（Rebecca Martin）领导的团队，今天宣布了他们的研究成果：如果恒星系统中生命宜居行星存在，与其小行星带的质量、大小相关的话， 那么地球将是极为稀少的。
They suggest that the size and location of an asteroid belt, shaped by the evolution of the Sun's protoplanetary disk and by the gravitational influence of a nearby giant Jupiter-like planet, may determine whether complex life will evolve on an Earth-like planet.
他们的研究推论：恒星系统中小行星带的宽度和位置，取决于原行星盘的演化和近邻类木气态巨行星引力的影响，也许决定着类地行星上能否出现复杂的生命形态。
This might sound surprising because asteroids are considered a nuisance due to their potential to impact the Earth and trigger mass extinctions. But an emerging view proposes that asteroid collisions with planets may provide a boost to the birth and evolution of complex life.
这个初听起来令人惊讶：小行星因它们可能撞击地球并引发生物大灭绝而声名狼藉。但一项新研究却认为，小行星撞击可能促进了复杂生命的诞生和演化。
Asteroids may have delivered water and organic compounds to the early Earth. According to the theory of punctuated equilibrium, occasional asteroid impacts might accelerate the rate of biological evolution by disrupting a planet's environment to the point where species must try new adaptation strategies.
小行星可能给地球带来水和原始有机物。根据间断平衡理论（反达尔文渐变进化的假说——译注），小行星撞击改变了地球的环境，迫使物种必须适应新环境，从而加速了生命进化。
The astronomers based their conclusion on an analysis of theoretical models and archival observations of extrasolar Jupiter-sized planets and debris disks around young stars. "Our study shows that only a tiny fraction of planetary systems observed to date seem to have giant planets in the right location to produce an asteroid belt of the appropriate size, offering the potential for life on a nearby rocky planet," said Martin, the study's lead author. "Our study suggests that our solar system may be rather special."
天文学家的这一结论基于对理论模型的分析和年轻恒星周围尘埃盘与类木系外行星的观测档案资料。报告首席作者马丁介绍说：“我们的研究显示，观测数据中仅有极少的行星系统，能够使气态巨行星处于恰到好处的位置，形成一个刚刚好的小行星带，为其附近的岩态行星提供可能的生命条件。我们的研究表明，太阳系极为特殊。”
The findings will appear today in the Monthly Notices of the Royal Astronomical Society: Letters (published by Oxford University Press).
团队研究成果发布在牛津大学出版社于今天出版的《皇家天文学会每月通讯》上。
Martin and Livio suggest that the location of an asteroid belt relative to a Jupiter-like planet is not an accident. The asteroid belt in our solar system, located between Mars and Jupiter, is a region of millions of space rocks that sits near the "snow line," which marks the border of a cold region where volatile material such as water ice are far enough from the Sun to remain intact. At the time when the giant planets in our solar system were forming, the region just beyond the snow line contained a dense mix of ices, rock, and metals that provided enough material to build giant planets like Jupiter.
马丁和利维奥(Livio)推测：小行星带与类木行星为伴并非意外事件。在太阳系中，小行星带位于木星和火星之间，那里有数百万的石块在“雪线”附近运行——那里离太阳足够远，水等挥发性物质能够以固体存在。当太阳系中的巨行星形成时，雪线外的区域正充满了水冰、岩石、和金属等物质，足以形成木星这样的巨行星。
When Jupiter formed just beyond the snow line, its powerful gravity prevented nearby material inside its orbit from coalescing and building planets. Instead, Jupiter's influence caused the material to collide and break apart. These fragmented rocks settled into an asteroid belt around the Sun.
木星刚在雪线形成，它的强大引力就会把附近的物质从聚合形成行星的位置吸引到自己的轨道中。结果木星的影响就是使剩余的物质相互碰撞并分裂，这些碎石块最终形成了围绕太阳的小行星带。
"To have such ideal conditions you need a giant planet like Jupiter that is just outside the asteroid belt [and] that migrated a little bit, but not through the belt," Livio explained. "If a large planet like Jupiter migrates through the belt, it would scatter the material. If, on the other hand, a large planet did not migrate at all, that, too, is not good because the asteroid belt would be too massive. There would be so much bombardment from asteroids that life may never evolve."
Livio解释道：“要达到如此的理想化条件，你需要木星这样的巨行星恰好位于小行星带外，并移动范围不大——不能穿过小行星带。假如巨行星穿过了此带，将把其中的物质四处散射一空。另外，假如巨行星一点没不移动，那也不行——小行星带回变得过于稠密，地球等将饱受持续的轰击，生命演化就别指望了。”
In fact, during the solar system's infancy, the asteroid belt probably had enough material to make another Earth, but Jupiter's presence and its small migration towards the Sun caused some of the material to scatter. Today, the asteroid belt contains less than one percent of its original mass. Using our solar system as a model, Martin and Livio proposed that asteroid belts in other solar systems would always be located approximately at the snow line. To test their proposal, Martin and Livio created models of protoplanetary disks around young stars and calculated the location of the snow line in those disks based on the mass of the central star.
事实上，在太阳系早期，小行星带的物质可能足以形成另一个地球，但木星向太阳一定的迁移运动散射了小行星带的一部分物质，今天的小行星带物质量还不到初期的1%。根据我们的太阳系模型马丁和利维奥推测，其他恒星系统的小行星带总是在雪线附近。为了测试该推断，他们创建了围绕年轻恒星的原行星盘模型，并根据中央恒星的质量分析雪线的位置。
They then looked at all the existing space-based infrared observations from NASA's Spitzer Space Telescope of 90 stars having warm dust, which could indicate the presence of an asteroid belt-like structure. The temperature of the warm dust was consistent with that of the snow line. "The warm dust falls right onto our calculated snow lines, so the observations are consistent with our predictions," Martin said.
接着他们分析了NASA斯皮策空间望远镜的红外数据，在现存的所有资料中，有90个恒星的周围存在温暖尘埃带——表面它们存在类似小行星带的结构。而该区域的温度与雪线相符。马丁说：“温暖的尘埃带恰好位于我们分析的雪线位置，因此我们的推论与观测相符。”
The duo then studied observations of the 520 giant planets found outside our solar system. Only 19 of them reside outside the snow line, suggesting that most of the giant planets that may have formed outside the snowline have migrated too far inward to preserve the kind of slightly-dispersed asteroid belt needed to foster enhanced evolution of life on an Earth-like planet near the belt. Apparently, less than four percent of the observed systems may actually harbor such a compact asteroid belt.
两位接着研究了已经发现的520个系外巨行星的分布情况。结果仅有19个位于雪线之外，推测它们中的大部分都已经移到了很大距离，穿过了尘埃带；仅保留了极少小行星带被略微散射，并为其附近的类地行星提供了生命演化的支持。显然，观测数据中仅有4%以下的系统，恰好庇护着这样紧凑而简洁的小行星带。
"Based on our scenario, we should concentrate our efforts to look for complex life in systems that have a giant planet outside of the snow line," Livio said.
利维奥总结道：“根据我们的方案，要寻找复杂生命，就应该集中注意力在巨行星位于雪线外的那些系统。”
CONTACT
联系人
Donna Weaver / Ray Villard
Space Telescope Science Institute, Baltimore, Md.
马里兰州巴尔的摩市，太空望远镜科研所
410-338-4493 / 410-338-4514
dweaver@stsci.edu / villard@stsci.edu
Rebecca Martin
University of Colorado, Boulder, Colo.
科罗拉多州巨砺市，科罗拉多州立大学
rebecca.martin@jila.colorado.edu
Mario Livio
Space Telescope Science Institute, Baltimore, Md.
马里兰州巴尔的摩市，太空望远镜科研所
410-338-4439
mlivio@stsci.edu |
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