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  • 行星形成

    Protostars spin up as they are contracting. Look again at the   . The flattened dark rings are a consequence of gravity and conservation of angular momentum and  contain  the leftover debris from star formation.  Remarkably, star formation theorists expected that stars would form flattened protoplanetary disks, long before  astronomers were able to obtain  these images.  Planetary bodies are thought to form through a method called core accretion. Dust grains accrete to form rocky aggregates called planetesimals that range from kilometer size to the size of Earth's moon. Planetesimals interact with each other through gravity, colliding and building up to form planetary embryos that are Moon- to Mars-sized bodies.
    ::质子星随着它们正在萎缩而旋转。 再看一看。平坦的暗环系是重力和角动力保护的结果,并含有恒星形成后留下的碎片。 引人注目的是,恒星形成理论家预期恒星在天文学家获得这些图像之前就将形成扁形原行星磁盘。 行星体被认为是通过一种叫做核心弧形的方法形成的。 尘粒渐渐接近于形成岩质集合, 叫做从千米大小到地球月球大小的行星群。 行星体通过引力相互作用, 碰撞和形成行星胚胎, 形成月球到火星大小的行星胚胎。

    Scientific theory has to agree with existing observations and the only example of a known planetary system in the 1980s was our own solar system.  Any theory about how the planets formed had to explain some basic observations:
    ::科学理论必须与现有的观测结果一致,而1980年代已知行星系统的唯一例子是我们自己的太阳系。 任何关于行星是如何形成的理论都必须解释一些基本观测结果:

    1. All of the planets  revolve around the Sun in the same direction, and most of their moons  orbit their planets in that same direction; the rotation ("spin") of the Sun has the same direction as the orbital rotation of planets.    
      ::所有行星环绕太阳向同一方向旋转,大多数卫星的行星向同一方向旋转;太阳的旋转(“斯宾”)方向与行星的轨道旋转方向相同。
    2. The orbital planes of the planets are inclined by less than six degrees with   respect to each other. In other words, planetary orbits are nearly co-planar.
      ::行星的轨道平面彼此倾角小于6度,换句话说,行星轨道几乎是共平轨道。
    3. There are two types of planets: small rocky planets reside in the inner part of the planetary disk and gas giants in the outer part of the disk.
      ::行星有两种类型:小岩石行星位于行星磁盘的内部,气体巨行星位于磁盘外部。
    4. The solar system contains large   numbers of asteroids and comets that have specific distributions in the disk.
      ::太阳系含有大量小行星和彗星,在磁盘上有特定的分布。
    5. There is some randomness in the solar system that a good model should accommodate: e.g., the spin axes of some planets are not well aligned with the spin axis of the Sun or the orbital plane; some planets have moons while others do not. 
      ::太阳系中有一些随机性,一个好模型应该能够适应:例如,一些行星的旋轴与太阳的旋轴或轨道平面不完全吻合;有些行星有月亮,而另一些行星则没有。

    The solar nebula theory assumed that the same material that makes up the Sun was uniformly present throughout the   . Because material close to the star will be hotter, there are     and pressure in the protoplanetary disk. As a result, condensation of refractory elements (metals and silicates) into to grains and pebbles mainly occurs in the hydrogen-depleted inner part of the disk. Volatile elements (ices, waters and gases) would not survive in the inner disk because it is too hot there, but at larger distances, beyond the so-called frost line (roughly the location of Mars), the volatiles could be incorporated into low-density gas-rich planets like Jupiter. The solar nebula model posited that grains in the disk began to aggregate into larger bolder-sized objects called planetesimals. As the planetesimals plowed through the disk, they accreted more mass.
    ::太阳星云理论假定,太阳星云的构成材料在整个磁盘中均匀存在。由于接近恒星的物质会更热,原行星磁盘中存在并且有压力。结果,将相残元素(金属和硅酸盐)凝结成颗粒和碎石,主要发生在磁盘中氢耗尽的内部部分。挥发元素(冰、水和气体)不会在内盘中生存,因为那里太热,但是在更大的距离上,在所谓的冻状线(几乎是火星的位置)之外,挥发性可以被吸收到像木星这样的低密度气体富集的行星中。太阳星云模型假设磁盘中的颗粒开始聚集到更大的果状物体中,称为行星。随着行星动物在磁盘中游动,它们会聚集得更多。

     

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    Artistic impression of the protoplanetary disk. Accretion of particles through collisions led to the formation of planetesimals, which then grew into planets. How does this model accommodate the existence of the two types of planets in our solar system?
    ::原行星磁盘的艺术印象:碰撞中粒子的积聚导致行星动物的形成,然后形成行星。 这个模型如何适应太阳系中两种行星的存在?

     

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    This cartoon sketch of the protoplanetary disk illustrates that rocks and metals condense in the inner part of the disk where the temperatures are high. What is the approximate distance to the frost line in our solar system?
    ::原行星磁盘的卡通草图表明,岩石和金属在温度高的磁盘内部凝结。 离太阳系冰冻线的近距离是多少?

    The solar nebula model helps to  explain why planets should be expected to   orbit in the same direction as the spin of the star - conservation of angular momentum results in a disk, and since planets accrete in this disk, they will inherit that same rotation. The planetary orbits  would also be constrained to the plane of the protoplanetary disk, explaining the co-planarity that is observed in the solar system.
    ::太阳星云模型有助于解释为什么行星应该按恒星旋转的同一方向运行 — — 保护角动向会形成一个磁盘,而且由于这个磁盘中的行星在扩张,它们将继承同样的旋转。行星轨道也将限制在原行星磁盘的平面上,解释太阳系观测到的共同计划性。

    The solar nebula model rather neatly explains the two types of planets we have: rocky planets are built from refractory material that can survive at the higher temperatures closer to the Sun, while hydrogen-dominated gas giant planets would necessarily form in the cool outer reaches of the disk. The large number of asteroids and comets?  Those are left over crumbs that were not swept up into a planet or the central star.  The random features that we observe, such as the tilt of Uranus, could be the result of collisions during the planet-building phase.  The solar nebula model enjoyed widespread acceptance.  The real test for this theory would happen when exoplanets were discovered: would they show the same attributes listed above for our solar system?
    ::太阳星云模型相当清楚地解释了我们拥有的两类行星:岩质行星是用在离太阳较近的较高温度下能够生存下来的易碎材料建造的,而氢占主导地位的气体巨行星必然会形成在磁盘的冷层外端。大量小行星和彗星?这些小行星和彗星是落在未冲入行星或中星的碎屑之上的。我们观察到的天王星的倾斜等随机特征可能是行星建设阶段相撞的结果。太阳星云模型得到广泛接受。当发现外行星时,这一理论的真正测试将会发生:它们是否显示上面列出的太阳系的特性?

    One unusual attribute of the Earth is that for our size, we have a very big moon. Our moon is similar in size to the moons of Jupiter, even though Jupiter is ~300 times the mass of the Earth. Such a large moon-to-Earth mass ratio results in ocean tides. Through c onservation of   angular momentum, t he gravitational tug of the tides that  cause  the orbit of the moon  to expand   also slows   the spin of the Earth.  T he Moon is slowly drifting away from the Earth and by tidal interactions, the spin of the Earth is decreasing - good news for those of us who need more hours in the day.    Reversing the arrow of time, this means that long ago, the moon was much closer to the Earth, and the Earth used to complete one day-night spin on its axis in roughly 5 hours. There  would   have been shorter lectures when the day was only 2.5 hours long.
    ::地球的一个非同寻常的属性是,对于我们的大小,我们有一个非常大的月亮。我们的月亮大小与木星的月亮类似,尽管木星的大小是地球质量的300倍。如此巨大的月对地质量比例导致海洋潮流。通过保护角动力,导致月球轨道扩张的潮汐引力拉力也减缓了地球的旋转速度。月球正在缓慢地从地球漂移,潮汐相互作用,地球的旋转正在减少——对于我们这些需要更多时间的人来说是好消息。扭转时间的箭头,这意味着很久以前,月球离地球要远得多,而地球在大约5小时的时间里在轴上完成一个晚上的旋转。当一天只有2.5小时时,讲座会缩短。

    The leading theory for the formation of the moon is that it formed when a Mars-sized object collided with a glancing blow to the Earth. The early solar system would have had a lot of debris that was not swept up in planet formation, so at first blush, this is plausible .  This model for the formation of the moon is further supported by  two quantitative lines of evidence. First, the Moon is compositionally the same as the Earth mantle. In particular the isotopic fractions of oxygen are the same on the Earth and the moon. This  suggests that the collision  must  have occurred some tens of millions of years after the Earth formed, after iron and other heavy elements were already settling into the core.    

    ::形成月球的主要理论是,当火星大小的物体与地球相撞时,月球会形成。早期的太阳系会有很多碎片,这些碎片没有在行星形成过程中被扫过,因此在一开始,这看起来是有道理的。形成月球的模型还得到了两个定量证据的进一步支持。首先,月球的构成与地球地壳一样。特别是,地球和月球的氧气同位素部分是相同的。这说明碰撞必定发生在地球形成后的数千万年后,在铁和其他重元素已经沉入核心之后。

    One of the conundrums with the impact theory for the formation of the moon was that we were only able to  establish a lower  limit in time for the collision. Because time was needed for differentiation of heavy elements, the collision  likely happened about 50 Myr after the formation of the Earth... but,  could the moon-forming impact have  occurred 200 Myr after the formation of the Earth?  This turns out to be an important consideration for the origin of life on Earth. The energy from the moon-forming collision would have melted the surface of the Earth, raising the temperature to about 2000 K and  sterilizing  the surface of the planet.  In a paper published in 2017, researchers    how they have dated the age of the Moon using zircon crystals.  This give an extremely precise age of 4.5 Gyr (with an uncertainty of only  ± 10 Myr). Since the age of the Earth is about 4.56 Gyr, this finding supports the theory that a moon-forming collision occurred 60 Myr after the formation of the Earth. This is an exciting result because it allows us to calculate a cooling time for the Earth that is needed in estimating the earliest times for the rise of life on the planet .
    ::月球形成时的撞击理论的难题之一是,我们只能为碰撞设定一个更低的时间限制。由于需要时间来区分重元素,碰撞在地球形成后可能发生约50米尔...但是,月亮成形的撞击在地球形成后是否已经发生200米尔?这被证明是地球生命起源的一个重要考虑因素。月亮成形碰撞产生的能量会熔化地球表面,将温度提高到2000K左右,并对地球表面进行消毒。在2017年发表的一篇论文中,研究人员如何用zircon晶体来计算月球的年代。这提供了极精确的4.5吉尔(不确定只有10米尔)的年代。由于地球的年代大约是4.56吉尔,这一发现支持了在地球形成后发生月亮成形碰撞60米尔的理论。这是一个令人振奋奋人心的结果,因为它使我们能够计算出地球的冷却时间,这是估计地球生命升生的最早时间所需要的。

    Current ideas about t he evolution of the moon are nicely explained in the     below. Watch this and then answer the questions below.
    ::下面很好地解释了当前关于月球进化的想法。看这个,然后回答下面的问题。

    What was the Late Heavy Bombardment?  What evidence does the moon present for this event? 
    ::最近的重轰炸是什么?

    When did volcanic activity on the moon end? 
    ::月球上的火山活动何时结束?

    Is the smoother surface of the moon geologically younger or older than the cratered surface?
    ::月球平滑的表面在地质上比弹坑的表面年轻还是更老?

     

    Astronomers speculate  that planet formation might  be hampered by the presence of a second star. If a binary star system has a short orbital period, then the protoplanetary disk will  be truncated, limiting planet formation around either or both stars.  However, one of the surprises in the past 20 years is that planet formation is incredibly robust. In the case of the shortest-period binary stars, astronomers have found planets circling  outside of the  binary star orbits.  For binary star systems  with  orbital periods of many years, planets have been found orbiting one or both stars individually.  Therefore, i t  seems a good bet  that planets exist in orbits around the three alpha Centauri stars, the likely first destination for space probes that venture out beyond our solar system, and indeed a planet has been discovered orbiting Proxima Centauri. 
    ::天文学家推测行星形成可能会受到第二颗恒星存在的影响。 如果二星系统运行时间短,那么原行星磁盘将会被截断,限制行星在其中一颗或两颗恒星周围的形成。 但是,过去20年的一个惊喜是行星的形成是令人难以置信的强大。对于最短时期的双星来说,天文学家发现行星在二星轨道之外环绕着行星。对于轨道周期多年的二星系统来说,行星已经发现在其中一颗或两颗恒星的轨道上运行。因此,似乎可以肯定的是,行星存在于三个阿尔法半人马星的轨道上,这是在太阳系之外探索空间探测器的首个可能目的地,事实上,一个行星已经被发现在半人马星轨道上运行。