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太阳系的形成与演化 - Wikipedia

太阳系的形成与演化

维基百科，自由的百科全书

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主条目：太阳系和恒星演化論

太阳系的形成和演化开始于46亿年前一个巨大分子云的一小部分的重力塌陷。大多塌陷的质量集中在中心，形成了太阳，其余部分平摊成了一个原始星盘，继而形成了行星，卫星，陨星和其他小的太阳系天体系统。

这一广泛被接受的模型，叫星云假想，最先由18世纪的Emanuel Swedenborg, Immanuel Kant 和Pierre-Simon Laplace提出。其随后的发展交织以天文学，物理学，地质学和行星学等多种科学领域。自从1950年代太空时代的到来和1990年代太阳系外行星的发现，这一模型在解释新发现的过程中即受到挑战又被完善化。 从最初形成，太阳系经历了了深刻的演化。有很多卫星由环绕母体行星的气体和尘埃组成的星盘中形成，其他一些卫星据信是俘获而来，或来自于巨大的碰撞（地球的卫星月球属此情况）。天体间的碰撞一直到今天都持续发生，对太阳系的演化起到核心的作用。行星的位置经常内外变化，行星们经常交换位置。这种行星迁移现在被认为对太阳系早期演化起到绝大部分的作用。 就如同太阳和行星的出生一样，它们最终将灭亡。大约50亿年后，太阳会冷却，向外膨胀超过现在的直径很多倍（成为一个红巨星），抛去它的外层成为行星云，留下的行星的尸骸叫白矮星。行星会跟随太阳的历程：在遥远的未来,经过的恒星的重力会卷走太阳的环绕行星。 有些会被毁掉，其它一些会被抛向星际间的太空，但最终，千万亿年之后，太阳终将会独自一个，不再有其它天体在轨道上。


目录

[编辑] 历史

主条目：太阳系形成和演化史假说
Pierre-Simon Laplace, one of the originators of the nebular hypothesis
Pierre-Simon Laplace, one of the originators of the nebular hypothesis

涉及世界起源和命运的主张可以追述到已知的最早的文字记载：但那时几乎所有的都没有试图把这样的理论根“太阳系”的存在联系起来，原因不过是我们现在理解意义上的太阳系的存在总的来说并不为人所知。 迈向太阳系形成和演化理论的第一步是对heliocentrism的广泛认同,这一模型把太阳放在系统的中心，把地球放在环绕其的轨道上. 这一理论孕育了数千年，但直到17世纪末才广泛被接受。第一次有记载的“太阳系”术语的使用是在1704年。[1]

现今太阳系形成的标准理论，星云假说,从其在18世纪被Emanuel Swedenborg, Immanuel Kant, 和 Pierre-Simon Laplace提出之日起就屡经采纳和摒弃。对该假说最有意义的批评是其明显的无法解释太阳相对行星缺少角动量.[2] 但，自从1980年代早期对新恒星的研究显示，正如星云假想预测的那样，它们被冷的气体和灰尘的盘环绕着，才导致这一假想的重新被接受。[3]

对太阳将如何继续演化的了解需要对它的能量之源有所了解. Arthur Stanley Eddington对爱因斯坦的 相对论的确认导致他认识到太阳的能量来自于它核心的核聚变.[4] 1935年, Eddington 进一步提议元素也有可能是在恒星中形成。[5] Fred Hoyle 进一步详尽阐释这一假设，认为演化成为的红巨星会在核心产生很多比氢和氦重的元素。当红巨星最终抛掉它的外层，这些元素将被回收用以形成其它星系.[5]

[编辑] Formation

参见：Nebular hypothesis

[编辑] Pre-solar nebula

星云假说主张太阳系从一巨大的有几光年跨度的分子云的碎片重力塌陷的过程中形成[6]直到几十年前，传统的观点认为太阳在相对孤立中形成，但对古陨石的研究发现短暂的同位素如铁-60的踪迹,其只能在爆炸的寿命较短恒星中形成。 这显示在太阳形成的过程中附近发生了若干次超新星爆发.这些超新星之一的冲击波可能在分子云中造成了超密度区域，导致了这个区域塌陷，从而触发了太阳的形成。因为只有大质量、短寿恒星才会产生超新星爆发，太阳一定是在一个产生了大质量恒星的一个大恒星诞生区域里（可能类似于猎户座星云)形成。[7][8]

Hubble image of protoplanetary discs in the Orion nebula, a light-years-wide "stellar nursery" likely very similar to the primordial nebula from which our Sun formed
Hubble image of protoplanetary discs in the Orion nebula, a light-years-wide "stellar nursery" likely very similar to the primordial nebula from which our Sun formed

这些塌陷气体区域中的一部分(叫"前太阳星云")[9] 将形成太阳系.这一区域直径在7000到20,000 天文单位s (AU)[6][10][11] 和刚好超过太阳的质量.它的组成跟今天的太阳差不多. 由大爆炸核合成产生的氢,和氦和少量的锂 组成,占塌陷星云质量的98%. 其余2%的质量由在前代恒星中的核合成中产生的金属重元素组成.[12] 在这些恒星的晚年它们把这些重元素抛射成为星际物质.[13]

因为角动量守恒, 星云塌陷时转动加快. 随着星云密度增高，其中的原子相互碰撞频率增高,把它们的动能转化成热能. 其质量集中的中心越来越比周边环绕的盘热.[6] 大约经过100,000年,[14] 竞争的重力、气体压力、磁场和转动导致收缩的星云扁平化成一个直径约200 AU[6]的原始星盘,在中心形成一个又热又致密的原恒星(内部氢聚变还没有开始的恒星) .[15]

在这太阳的这个星系演化时刻, 其据信曾是一颗金牛T星. 对金牛T星的研究表明它们常伴以0.001–0.1 solar masses.[16]的前行星物质组成的盘。这些盘伸展可达几百 AU—哈勃太空望远镜已经观察过在恒星诞生区域(如猎户座星云[17])直径达1000 AU的原星盘—且相当冷，最热只能达到一千开尔文.[18] 在五千万年内, 太阳核心的温度和压力变得如此巨大，它的氢开始聚变，产生内部能源抗拒重力收缩的力直到达至静力平衡.[19]这标志太阳进入了它生命中的一个主要阶段，叫主序星. 主序星从它们的核心的氢聚变为氦的过程中产生能量.太阳至今还是一颗主序星.[20]

[编辑] Formation of planets

参见：Protoplanetary disc
Artist's conception of the solar nebula
Artist's conception of the solar nebula

不同的行星被认为是从“太阳星云”中形成的，太阳星云是太阳形成中剩下的气体和尘埃的形成的圆盘状的云。[21] 目前被接受的行星形成的方法叫吸积, 行星以绕原恒星的轨道上的尘埃颗粒开始形成. 通过直接收缩，这些颗粒形成一到十公里直径的块状物, 然后它们互相碰撞形成更大的尺寸约~5 km 的(微行星). 进一步相撞逐渐加大它们的尺寸, 在接下来的几百万年中大约一年增加几厘米.[22]

太阳系内4AU的内太阳系温度过高以至于易挥发的如水和甲烷分子难以聚集,所以那里形成的微行星只能由高熔点的物质形成，如铁，镍和铝和石状硅酸盐. 这些石质天体会成为类地行星(水星，金星和火星). 这些物质在宇宙中很稀少，大约只占星云质量的0.6%,所以类地行星不会长得很大.[6] 类地行星胚胎在太阳形成100，000年后长到0.05地球质量，然后就停止聚集质量; 随后的这些行星大小的天体间的相互撞击与合并使它们这些类地行星长到它们今天的大小(见下面的 [[#类地行星).[23]

类木行星 (木星, 土星, 天王星, 和海王星)形成于更远的冻结线之外, 在火星和木星之间物质足以冷到使易挥发的冰状化合物保持固态.类木行星上的冰比类地行星上的金属和硅酸盐更丰富，使得类木行星的质量长得足够大到可以俘获氢和氦这些最轻和最丰富的元素.[6] 冻结线以外的微行星在3百万年间聚集了4倍地球的质量.[23] 今天，四个类木行星囊括了刚好少于99%的所有环绕太阳的质量.[24]理论学者认为木星处于刚好在冻结线之外的地方并不是偶然的. 因为冻结线聚集了大量由向内降落的冰状物质蒸发而来的水，其生成了一个低压区，加速了轨道上环绕的尘埃颗粒的速度阻止了它们向太阳落去的运动。在效果上，冻结线起到了一个壁垒的作用，导致物质在距离太阳~5 AU处迅速聚集。这些过多的物质聚集成一个大约有10个地球质量的胚胎, 然后其开始通过吞噬周围星盘的氢而迅速迅速增长，只用了1000年就达到150 地球质量并最终达到318 地球质量. 土星质量显著地小可能只是因为它比木星晚了几百万年形成，当时所能使用的气体少了。[23]

年轻的太阳这样的金牛T星星风远比稳定的老的恒星强烈.天王星和海王星据信是在木星和土星之后当太阳风把吧星盘物质大部分吹走之后形成。结果导致这两个行星上聚集的氢和氦很少，各自仅达一个地球质量。天王星和海王星有时被成为失败的核.[25] 形成理论在这些行星上的主要问题是它们的形成时间。在它们目前的位置，它们的核需要数亿年的时间聚集。这意味着天王星和海王星可能是在更靠近太阳的地方形成的，接近，甚至是在木星和土星之间，后来才向外迁移。(见下面的Planetary migration below).[26][25] 在微行星的时代运动并不全是向内朝向太阳; 从彗星Wild 2上取回的Stardust 样本表明太阳系早期形成的物质从温暖的太阳系内部向Kuiper带区域迁移。[27]

过了三百万到一千万年，[23]年轻的太阳的太阳风会清净原星盘内所有的气体和尘埃，把它们吹向星际空间，从而结束行星的生长。[28][29]

[编辑] Subsequent evolution

行星原先被认为是在我们今天看到的它们的轨道内或附近形成的。但这一观点在20世纪晚期和21世纪初期发生了巨变。现在认为太阳系在最初形成之后看上去跟现在很不一样：有几个至少跟金星一样大的天体在内太阳系，外太阳系也比现在紧密，Kuiper belt离太阳要近得多。[30]

[编辑] Terrestrial planets

Artist's conception of the giant impact event that may have created the Moon, a collision typical of the later stages of the inner Solar System's formation
Artist's conception of the giant impact event that may have created the Moon, a collision typical of the later stages of the inner Solar System's formation

行星形成时代结束后内太阳系有50–100个月球到火星大小的行星胚胎.[31][32] 进一步的生长可能只是由于这些天体的相互碰撞和合并, 这一过程持续了大约1亿年.这些天体互相产生重力作用，互相拖动对方的轨道直到它们相撞，长得更大，直到最后我们今天所知的4个类地行星初具雏形。[23] 其中的一个这样的巨大碰撞据信导致了月球的形成(see Moons below), 另外一次剥去了年轻的水星的外壳.[33]

这个模型的一个未决问题是它不能解释这些原类地行星的初始轨道-需要是相当的偏心园形才能相撞-是如何形成今天这样相当稳定且接近圆形的轨道的。[31] 此“偏圆去除”的一个假说认为在气体盘中形成的类地行星尚未被太阳驱离.这些残余气体的 "重力拖拉" 终将降低行星的能量，平滑化它们的轨道.[32] 但是，如果存在这样的气体，一开始它就会防止类地行星的轨道变得如此偏圆.[23]另一个假说认为重力拖拉不是发生在行星和气体之间，而是发生在行星和余留的小天体之间. 当大的天体行经小天体群时，小天体手受到大天体的重力吸引，在大天体的路径形成了一个高密度区，一个“重力唤醒”，由此降低了大天体使其进入一个更正规的轨道.[34]

[编辑] Asteroid belt

类地区外围边缘，离太阳2到4个AU，叫小行星带.小行星带开始有多于足够的物质可以形成2到3个地球一样的行星 ,并且确实，有很多微行星在那里形成. 如同类地行星，这一区域的微行星后来合并形成20到30个月亮到火星大小的行星胚胎;[35] 但因为在木星附近，意味着太阳形成3百万年后这一区域的历史发生了巨大变化.[31] 在小行星带木星和土星的轨道共振特别强烈，并且与更多的大质量的行星胚胎的的引力交互作用使更多的微行星散步到到这些共振中。木星的重力增加了这些在共振中的天体的速度，使它们与其它天体相撞后破碎而不是聚集。[36] 随着木星的形成后的向内迁移（见下文行星迁移），共振将横扫小行星带，动态地激发这一区域的个体数目，加大它们之间的相对速度[37]。共振和行星胚胎的累计作用要么使微行星脱离小行星带，要么激发它们的轨道倾角和偏心率变化。[35][38] 一些大质量的行星胚胎也被木星抛出，其它一些可能迁移到内太阳系，在类地行星的最终聚集中发挥了作用。[39][35][40] 在这个初始消竭时期，大行星和行星胚胎的作用下在小行星带剩下的主要由微行星组成的总质量不到地球的1%。 [ 38 ]这仍是目前在主带的质量的10到20倍，约1/2000地球质量。 [38]第二消竭阶段据信是当木星和土星进入临时2:1轨道共振时发生，使小行星带的质量下降接近至目前规模（见下文） 。

内太阳系的巨大撞击期可能对地球从小行星带获取其目前的水成分（ 〜 6 × 1021公斤）起到了一定的作用。水太易挥发，不会在地球的形成时期就存在，一定是其后从太阳系外部较冷的地方送来的。 [41] 水可能是由被木星甩离小行星带的行星胚胎和小的微行星带过来的。[39]2006年发现的一些主带彗星也被认为可能是地球的水的来源之一。 [41][42] 在相比之下，从Kuiper带或更远的区域的彗星带来的不过约6%地球的水。[43][44] panspermia假说认为，生命本身可能是通过这种方式播撒到地球上，虽然这种想法不被广泛接受。 [45]

[编辑] Planetary migration

主条目：Planetary migration

根据星云假说，外层的两个行星处于“错误位置”。天王星和海王星(以"巨冰行星"著称)所处的区域的太阳星云的低密度和它们的更长的轨道周期时间使它们的形成看似非常不合理. 这两个行星被认为形成于有更多物质的木星和土星的轨道附近,但后来历经几亿年迁移到了它们今天所处的位置.[25]

Simulation showing outer planets and Kuiper belt: a) Before Jupiter/Saturn 2:1 resonance b) Scattering of Kuiper belt objects into the Solar System after the orbital shift of Neptune c) After ejection of Kuiper belt bodies by Jupiter
Simulation showing outer planets and Kuiper belt: a) Before Jupiter/Saturn 2:1 resonance b) Scattering of Kuiper belt objects into the Solar System after the orbital shift of Neptune c) After ejection of Kuiper belt bodies by Jupiter[44]

外层行星的迁移对于解释太阳系最外围区域的存在和特性的解释也是必要的..[26] 海王星之外,太阳系延伸到柯伊伯带,黄道离散天体, 和奥尔特云, 这三个稀疏的小冰状天体群落被认为是绝大多数被观测到的彗星的起源地.以它们离太阳的距离，在太阳星云散离前聚集的速度太慢以至于不能足以形成行星，所以最开始的星盘缺乏足够的物质密度来形成行星.柯伊伯带处于距离太阳30到55AU的地方，更远的黄道离散天体延展到100AU,[26]，而遥远的奥尔特云起始于大约50,000AU的地方。[46] 但起初，柯伊伯带离太阳近得多也致密得多，外围边缘离太阳大约30AU. 它的内部边缘刚好在天王星和海王星的轨道外，天王星和海王星的轨道在形成的时候离太阳要近得多（可能15-20AU), 并且位置相反，天王星离太阳要比海王星更远。[26][44]

太阳系形成之后,巨大行星的轨道持续缓慢变化，主要是受到它们与剩下的大量的微行星之间的相互作用的影响 .过了5亿到6亿年（大约40亿年前)木星和土星进入2：1共振;土星每当木星环绕太阳两周环绕太阳一周.[26]这一共振对外围行星造成重力推力,造成海王星超过天王星，耕入古柯伊伯带.这些行星把大部分小冰状天体向内部散播，它们自己却向外移动. 这些微行星继而以类似的方式驱散它们遇到的下一颗行星，把行星的轨道向外移动，它们自己向内移动。[47] 这一过程持续到微行星与木星相互作用，木星的强大引力使它们轨道变得高度椭圆，甚至把它们径直抛出太阳系。这使得木星略微向内移动。这些被木星驱散进入高度椭圆轨道的天体形成了奥尔特云;[26] 那些被迁移中的海王星驱散程度较轻的天体形成了现在的柯伊伯带和黄道离散天体。[26] 此情形可解释柯伊伯带和黄道离散天体现今的低密度。 这些被驱散的天体，包括冥王星，开始被海王星重力束缚, 被拉入轨道共振.[48] 最终，在微行星盘里的摩擦力使得天王星和海王星的轨道又变圆了。[26][49]

与外围行星比，内部行星在太阳系的历史中并未发生显著的迁移，因为它们的轨道在大撞击期保持了稳定。[23]

[编辑] Late Heavy Bombardment and after

主条目：Late Heavy Bombardment

外围行星的迁移带来的重力干扰会把大量小行星送到内太阳系，严重地耗竭原地带，直到它降到今天的特别低的质量水平。[38] 该事件可能触发了大约40亿年前、太阳系形成5到6亿年后的后期重击。[44][50] 这一时期的重击持续了几亿年，在地质死亡了的内太阳系天体如水星和月球上的有陨坑为证。[44][51] 地球生命最早的证据可以早到38亿年前，几乎是紧接着后期重创的结束。[52]

Meteor Crater in Arizona. Created 50,000 years ago by an impactor only 50m across, it is a stark reminder that the accretion of the Solar System is not over.
Meteor Crater in Arizona. Created 50,000 years ago by an impactor only 50m across, it is a stark reminder that the accretion of the Solar System is not over.

（陨石）撞击据信是太阳系演化的常规部分（如果说现在不是很频繁的话）。其在继续发生有1994年的苏梅克-列维9号彗星撞击木星, 和亚利桑那陨石坑为证，因此还没有结束，可能还会对地球上的生命造成威胁。[53][54]

外太阳系的演化好像曾经受到过附近的超新星和途径的星际云的影响。太阳系外围天体的表面可能经历过由太阳风，微陨星和星际物质的中性成分带来的太空风化。[55]

后期重创后的小行星带的演化主要是由碰撞主宰。[56] 大质量的天体有足够的重力留住任何强烈撞击溅出的物质.在小行星带通常不是这样。结果，很多大的天体分裂,有时候不太激烈的碰撞的残余物会合并成新的天体。[56]有些小行星现在周围的卫星只能以聚敛从母天体飞出的没有足够能量完全逃脱它的重力来解释。[57]

[编辑] Moons

参见：Giant impact hypothesis

卫星存在于多数行星和其他太阳系天体周围。这些天然卫星有三个可能的来源机制:

  • co-formation from a circum-planetary disc (only in the cases of the gas giants);
  • formation from impact debris (given a large enough impact at a shallow angle); and
  • capture of a passing object.

Jupiter and Saturn have a number of large moons, such as Io, Europa, Ganymede and Titan, which may have originated from discs around each giant planet in much the same way that the planets formed from the disc around the Sun.[58] This origin is indicated by the large sizes of the moons and their proximity to the planet. These attributes are impossible to achieve via capture, while the gaseous nature of the primaries make formation from collision debris another impossibility. The outer moons of the gas giants tend to be small and have eccentric orbits with arbitrary inclinations. These are the characteristics expected of captured bodies.[59][60] Most such moons orbit in the direction opposite the rotation of their primary. The largest irregular moon is Neptune's moon Triton, which is believed to be a captured Kuiper belt object.[54]

Moons of solid Solar System bodies have been created by both collisions and capture. Mars's two small moons, Deimos and Phobos, are believed to be captured asteroids.[61] The Earth's Moon is believed to have formed as a result of a single, large oblique collision.[62][63] The impacting object likely had a mass comparable to that of Mars, and the impact probably occurred near the end of the period of giant impacts. The collision kicked into orbit some of the impactor's mantle, which then coalesced into the Moon.[62] The impact was probably the last in series of mergers that formed Earth. It has been further hypothesized that the Mars-sized object may have formed at one of the stable Earth-Sun Lagrangian points (either L4 or L5) and drifted from its position.[64] Pluto's moon Charon may also have formed by means of a large collision; the Pluto-Charon and Earth-Moon systems are the only two in the Solar System in which the satellite's mass is at least 1% that of the larger body.[65]

[编辑] Future

Astronomers estimate that the Solar System as we know it today will not change drastically until the Sun has fused all the hydrogen fuel in its core into helium, beginning its evolution off of the main sequence of the Hertzsprung-Russell diagram and into its red giant phase. Even so, the Solar System will continue to evolve until then.

[编辑] Long-term stability

The Solar System is chaotic,[66] with the orbits of the planets open to long-term variations. One notable example of this chaos is the Neptune-Pluto system, which lies in a 3:2 orbital resonance. Although the resonance itself will remain stable, it becomes impossible to predict the position of Pluto with any degree of accuracy more than 10–20 million years (the Lyapunov time) into the future.[67] Another example is Earth's axial tilt which, thanks to friction raised within Earth's mantle by tidal interactions with the Moon (see below) will be rendered chaotic at some point between 1.5 and 4.5 billion years from now.[68]

The planets' orbits are chaotic over longer timescales, such that the whole Solar System possesses a Lyapunov time in the range of 2–230 million years.[69] In all cases this means that the position of a planet along its orbit ultimately becomes impossible to predict with any certainty (so, for example, the timing of winter and summer become uncertain), but in some cases the orbits themselves may change dramatically. Such chaos manifests most strongly as changes in eccentricity, with some planets' orbits becoming significantly more—or less—elliptical.[70]

Ultimately, the Solar System is stable in that none of the planets will collide with each other or be ejected from the system in the next few billion years.[69] Beyond this, within five billion years or so Mars's eccentricity may grow to around 0.2, such that it lies on an Earth-crossing orbit, leading to a potential collision. In the same timescale, Mercury's eccentricity may grow even further, and a close encounter with Venus could theoretically eject it from the Solar System altogether[66] or send it on a collision course with Venus or Earth.[71]

[编辑] Moon-ring systems

The evolution of moon systems is driven by tidal forces. A moon will raise a tidal bulge in the object it orbits (the primary) due to the differential gravitational force across diameter of the primary. If a moon is revolving in the same direction as the planet's rotation and the planet is rotating faster than the orbital period of the moon, the bulge will constantly be pulled ahead of the moon. In this situation, angular momentum is transferred from the rotation of the primary to the revolution of the satellite. The moon gains energy and gradually spirals outward, while the primary rotates more slowly over time.

The Earth and its Moon are one example of this configuration. Today, the Moon is tidally locked to the Earth; one of its revolutions around the Earth is equal to one of its rotations about its axis, which means that it always shows one face to the Earth. However, as the Moon recedes from Earth, Earth's spin will gradually slow, until, in about 50 billion years, the two worlds will become tidally locked to each other. Each will only be visible from one hemisphere of the other.[72] Other examples are the Galilean moons of Jupiter (as well as many of Jupiter's smaller moons)[73] and most of the larger moons of Saturn.[74]

Neptune and its moon Triton, taken by Voyager 2. Triton's orbit will eventually take it within Neptune's Roche limit, tearing it apart and possibly forming a new ring system.
Neptune and its moon Triton, taken by Voyager 2. Triton's orbit will eventually take it within Neptune's Roche limit, tearing it apart and possibly forming a new ring system.

A different scenario occurs when the moon is either revolving around the primary faster than the primary rotates, or is revolving in the direction opposite the planet's rotation. In these cases, the tidal bulge lags behind the moon in its orbit. In the former case, the direction of angular momentum transfer is reversed, so the rotation of the primary speeds up while the satellite's orbit shrinks. In the latter case, the angular momentum of the rotation and revolution have opposite signs, so transfer leads to decreases in the magnitude of each (that cancel each other out).[75] In both cases, tidal deceleration causes the moon to spiral in towards the primary until it either is torn apart by tidal stresses, potentially creating a planetary ring system, or crashes into the planet's surface or atmosphere. Such a fate awaits the moons Phobos of Mars (within 30 to 50 million years),[76] Triton of Neptune (in 3.6 billion years),[77] Metis and Adrastea of Jupiter,[78] and at least 16 small satellites of Uranus and Neptune. Uranus' Desdemona may even collide with one of its neighboring moons.[79]

A third possibility is where the primary and moon are tidally locked to each other. In that case, the tidal bulge stays directly under the moon, there is no transfer of angular momentum, and the orbital period will not change. Pluto and Charon are an example of this type of configuration.[80]

Prior to the 2004 arrival of the Cassini–Huygens spacecraft, the rings of Saturn were widely thought to be much younger than the Solar System and were not expected to survive beyond another 300 million years. Gravitational interactions with Saturn's moons were expected to gradually sweep the rings' outer edge toward the planet, with abrasion by meteorites and Saturn's gravity eventually taking the rest, leaving Saturn unadorned.[81] However, data from the Cassini mission led scientists to revise that early view. Observations revealed 10 km-wide icy clumps of material that repeatedly break apart and reform, keeping the rings fresh. Saturn's rings are far more massive than the rings of the other gas giants. This large mass is believed to have preserved Saturn's rings since the planet first formed 4.5 billion years ago, and is likely to preserve them for billions of years to come.[82]

[编辑] The Sun and planetary environments

参见：Stellar evolution

长远地，太阳系最大的改变将来自于太阳自身因衰老而带来的改变。随着太阳烧掉它的氢供给，它会变得更热且更快地烧掉余下的燃料。结果，太阳每11亿年就会更亮10%.[83] 在10亿年的时间，随着太阳的辐射输出增强,它的适宜居住区就会外移，地球的表面会热到液态的水无法在地球表面继续存在。此时地面上所有的生命都将绝迹。[84] 从海平面而来的水蒸气，一种强温室气体,可以加速温度升高，可以潜在地更早地结束地球上的所有生命。[85] 这时候可能火星的表面温度逐渐升高,现在冻结在表面土壤下的水和二氧化碳会被释放到大气里，产生温室效应暖化这颗行星直到它达到今天地球一样的条件，提供一个未来的生命的居住场所。[86] 从现在开始35亿年，地球的表面条件就会变得跟今天的金星类似。[83]

距今54亿年之后，太阳核心的所有的氢都会聚变成氦。核心将不再支撑得住重力塌陷，将会开始收缩，加热核周围的一个外壳直到里面的氢开始聚变。[84] 这将使其外层急剧扩张, 这颗恒星将进入它生命中的红巨星阶段.[87][88] 在75亿年内，太阳会膨胀到半径为1.2 AU—256倍于它现在的大小. 在它红巨枝星的顶峰, 因为巨量增大的表面积、,太阳的表面会比现在冷却很多(大约2600 K)， 它的光度会高很多，会达到现在太阳光度的2700倍. 它红巨星的某阶段，太阳会有很强的星风，其将带走它自身33%的质量.[84][89][90] 在這時候，土星的衛星泰坦，有可能可以達到維持生命的表面溫度。[91][92]

當太陽膨胀，它很可能會吞掉水星和金星。地球的命運不是很清楚。尽管太阳会吞噬地球的现在的轨道，这颗恒星的质量损失（既而更弱的重力）会导致行星的轨道向外移动。如果只是如此，地球可能会逃离火海，但2008年的研究提议地球还是会因为与太阳附着不紧密的外层的潮汐作用而被吞噬掉。 [89] [84]

逐渐，太阳核心周围的壳里的燃烧的氢将增大核的质量直到达到现今太阳质量的45%. 此时密度和温度如此高以至于氦开始聚变成碳,导致氦闪; 太阳的半径将从250倍于现在（主序）的半径缩至11倍于现在的半径.作为结果，它的光度会从3000倍于今天的水平跌至54倍于今天的水平,它的表面温度会升至约4770 K.t The Sun will become a horizontal branch star, burning helium in its core in a stable fashion much like it burns hydrogen today. The helium-fusing stage will last only 100 million years. Eventually, it will have to again resort to the reserves of hydrogen and helium in its outer layers and will expand a second time, turning into what is known as an asymptotic giant branch star. Here the luminosity of the Sun will increase again, reaching about 2090 present luminosities, and it will cool to about 3500 K.[84] This phase lasts about 30 million years, after which, over the course of a further 100,000 years, the Sun's remaining outer layers will fall away, ejecting a vast stream of matter into space and forming a halo known (misleadingly) as a planetary nebula. The ejected material will contain the helium and carbon produced by the Sun's nuclear reactions, continuing the enrichment of the interstellar medium with heavy elements for future generations of stars.[93]

The Ring nebula, a planetary nebula similar to what the Sun will become
The Ring nebula, a planetary nebula similar to what the Sun will become

This is a relatively peaceful event, nothing akin to a supernova, which our Sun is too small to undergo as part of its evolution. Any observer present to witness this occurrence would see a massive increase in the speed of the solar wind, but not enough to destroy a planet completely. However, the star's loss of mass could send the orbits of the surviving planets into chaos, causing some to collide, others to be ejected from the Solar System, and still others to be torn apart by tidal interactions.[94] Afterwards, all that will remain of the Sun is a white dwarf, an extraordinarily dense object, 54% its original mass but only the size of the Earth. Initially, this white dwarf may be 100 times as luminous as the Sun is now. It will consist entirely of degenerate carbon and oxygen, but will never reach temperatures hot enough to fuse these elements. Thus the white dwarf Sun will gradually cool, growing dimmer and dimmer.[95]

随着太阳的死亡，它作用于如行星、彗星和小行星这些天体的重力引力会随着它的质量的丢失而减弱.[89] [96] 如果地球在這時候還生存，它的軌道會大約1.85 AU; 火星的軌道將大約在2.8 AU。它們和其它剩餘的行星將成為昏暗，寒冷外壳，完全没有任何形式的生命。 它們將繼續圍繞他們的恒星，他們速度因为距离太阳的距离增大和太阳的降低的重力而减慢。 二十億年后，当太阳冷却到6000到8000K的范围,太阳核心的碳和氧将冷却，它所剩的90%的质量将形成结晶结构。 最終，再过数十亿年，太阳将完全停止闪耀，成為黑矮星。[97]

[编辑] Galactic interaction

Location of the Solar System within our galaxy
Location of the Solar System within our galaxy

The Solar System travels alone through the Milky Way galaxy in a circular orbit approximately 30,000 light years from the galactic centre. Its speed is about 220 km/s.[98] The period required for the Solar System to complete one revolution around the galactic centre, the galactic year, is in the range of 220-250 million years.[99] Since its formation, the Solar System has completed at least 18 such revolutions.

A number of scientists have speculated that the Solar System's path through the galaxy is a factor in the periodicity of mass extinctions observed in the Earth's fossil record. One hypothesis supposes that vertical oscillations made by the Sun as it orbits the galactic centre cause it to regularly pass through the galactic plane. When the Sun's orbit takes it outside the galactic disc, the influence of the galactic tide is weaker; as it re-enters the galactic disc, as it does every 20–25 million years, it comes under the influence of the far stronger "disc tides", which, according to mathematical models, increase the flux of Oort cloud comets into the Solar System by a factor of 4, leading to a massive increase in the likelihood of a devastating impact.[100]

However, others argue that the Sun is currently close to the galactic plane, and yet the last great extinction event was 15 million years ago. Therefore the Sun's vertical position cannot alone explain such periodic extinctions, and that extinctions instead occur when the Sun passes through the galaxy's spiral arms. Spiral arms are home not only to larger numbers of molecular clouds, whose gravity may distort the Oort cloud, but also to higher concentrations of bright blue giant stars, which live for relatively short periods and then explode violently as supernovae.[101]

[编辑] Galactic collision and planetary disruption

主条目：Andromeda-Milky Way collision
An artist's rendition of the collision of the Milky Way and Andromeda galaxies, as it might be seen from Earth
An artist's rendition of the collision of the Milky Way and Andromeda galaxies, as it might be seen from Earth

Although the vast majority of galaxies in the Universe are moving away from the Milky Way, the Andromeda Galaxy, the largest member of our Local Group of galaxies, is heading towards it at about 120 km/s.[102] In 2 billion years, Andromeda and the Milky Way will collide, causing both to deform as tidal forces distort their outer arms into vast tidal tails. When this initial disruption occurs, astronomers calculate a 12% chance that the Solar System will be pulled outward into the Milky Way's tidal tail and a 3% chance that it will become gravitationally bound to Andromeda and thus a part of that galaxy.[102] After a further series of glancing blows, during which the likelihood of the Solar System's ejection rises to 30%, the galaxies' supermassive black holes will merge. Eventually, in roughly 7 billion years, the Milky Way and Andromeda will complete their merger into a giant elliptical galaxy. During the merger, if there is enough gas, the increased gravity will force the gas to the centre of the forming elliptical galaxy. This may lead to a short period of intensive star formation called a starburst.[102] In addition the infalling gas will feed the newly formed black hole transforming it into an active galactic nucleus. The force of these interactions will likely push the Solar System into the new galaxy's outer halo, leaving it relatively unscathed by the radiation from these collisions.[102][103]

It is a common misconception that this collision will disrupt the orbits of the planets in the Solar System. While it is true that the gravity of passing stars can detach planets into interstellar space, distances between stars are so great that the likelihood of the Milky Way-Andromeda collision causing such disruption to any individual star system is negligible. While the Solar System as a whole could be affected by these events, the Sun and planets are not expected to be disturbed.[104]

However, over time, the cumulative probability of a chance encounter with a star increases, and disruption of the planets becomes all but inevitable. Assuming that the Big Crunch or Big Rip scenarios for the end of the universe do not occur, calculations suggest that the gravity of passing stars will have completely stripped the dead Sun of its remaining planets within 1 quadrillion (1015) years. This point marks the end of the Solar System. While the Sun and planets may survive, the Solar System, in any meaningful sense, will cease to exist.[105]

[编辑] Chronology

The time frame of the Solar System's formation has been determined using radiometric dating. Scientists estimate that the Solar System is 4.6 billion years old. The oldest known mineral grains on Earth are approximately 4.4 billion years old.[106] Rocks this old are rare, as Earth's surface is constantly being reshaped by erosion, volcanism, and plate tectonics. To estimate the age of the Solar System, scientists use meteorites, which were formed during the early condensation of the solar nebula. Almost all meteorites (see the Canyon Diablo meteorite) are found to have an age of 4.6 billion years, suggesting that the Solar System must be at least this old.[107]

Studies of discs around other stars have also done much to establish a time frame for Solar System formation. Stars between one and three million years old possess discs rich in gas, whereas discs around stars more than 10 million years old have little to no gas, suggesting that gas giant planets within them have ceased forming.[23]

[编辑] Timeline of Solar System evolution

Note: All dates and times in this chronology are approximate and should be taken as an order of magnitude indicator only.

Phase Time since formation of the Sun Event
Pre-Solar System Billions of years before the formation of the Solar System Previous generations of stars live and die, injecting heavy elements into the interstellar medium out of which the Solar System formed.[13]
~5×107 years before formation of the Solar System If the Solar System formed in an Orion nebula-like star-forming region, the most massive stars are formed, live their lives, die, and explode in supernovae. One supernova possibly triggers the formation of the Solar System.[7][8]
Formation of Sun 0–1×105 years Pre-solar nebula forms and begins to collapse. Sun begins to form.[23]
1×105–5×107 years Sun is a T Tauri protostar.[14]
1×105–7 years Outer planets form. By 107 years, gas in the protoplanetary disc has been blown away, and outer planet formation is likely complete.[23]
1×107–8 years Terrestrial planets and the Moon form. Giant impacts occur. Water delivered to Earth.[44]
Main sequence 5×107 years Sun becomes a main sequence star.[19]
2×108 years Oldest known rocks on the Earth formed.[106]
5–6×108 years Resonance in Jupiter and Saturn's orbits moves Neptune out into the Kuiper belt. Late Heavy Bombardment occurs in the inner Solar System.[44]
8×108 years Oldest known life on Earth.[52]
4.6×109 years Today. Sun remains a main sequence star, continually growing warmer and brighter by ~10% every 109 years.[83]
6×109 years Sun's habitable zone moves outside of the Earth's orbit, possibly shifting onto Mars' orbit.[86]
7×109 years The Milky Way and Andromeda Galaxy begin to collide. Slight chance the Solar System could be captured by Andromeda before the two galaxies fuse completely.[102]
Post-main sequence 10–12×109 years Sun exhausts the hydrogen in its core, ending its main sequence life. Sun begins to ascend the red giant branch of the Hertzsprung-Russell diagram, growing dramatically more luminous (by a factor of up to 2700), larger (by a factor of up to 250 in radius), and cooler (down to 2600 K): Sun is now a red giant. Mercury, Venus, and possibly Earth are swallowed.[84]
~12×109 years Sun passes through helium-burning horizontal branch and asymptotic giant branch phases, losing a total of ~30% of its mass in all post-main sequence phases. Asymptotic giant branch phase ends with the ejection of a planetary nebula, leaving the core of the Sun behind as a white dwarf.[84][93]
Remnant Sun >12×109 years The white dwarf Sun, no longer producing energy, begins to cool and dim continuously, eventually reaching a black dwarf state.[95][97]
1015 years Sun cools to 5 K.[108] Gravity of passing stars detaches planets from orbits. Solar System ceases to exist.[105]

[编辑] See also

Solar System主題 Solar System主題首頁

  • Age of the Earth
  • History of Earth
  • Tidal locking

[编辑] Notes

  1. ^ "Solar system"．Merriam Webster Online Dictionary（2008年）．於2008年4月15日查閱．
  2. ^ M. M. Woolfson（1984年）．“Rotation in the Solar System”．Philosophical Transactions of the Royal Society of London．313：5．DOI:10.1098/rsta.1984.0078．
  3. ^ Nigel Henbest（1991年）．Birth of the planets: sThe Earth and its fellow planets may be survivors from a time when planets ricocheted around the Sun like ball bearings on a pinball table．New Scientist．於2008年4月18日查閱．
  4. ^ David Whitehouse（2005）．The Sun: A Biography．John Wiley and Sons．ISBN 978-0470092972． 
  5. ^ 5.0 5.1 Simon Mitton（2005）．“Origin of the Chemical Elements”，Fred Hoyle: A Life in Science．Aurum，197–222．ISBN 978-1854109613． 
  6. ^ 6.0 6.1 6.2 6.3 6.4 6.5 Ann Zabludoff (University of Arizona)（Spring 2003）．Lecture 13: The Nebular Theory of the origin of the Solar System．於2006年12月27日查閱．
  7. ^ 7.0 7.1 J. Jeff Hester, Steven J. Desch, Kevin R. Healy, Laurie A. Leshin（21 May 2004）． “The Cradle of the Solar System”．Science．304：1116–1117．DOI:10.1126/science.1096808．于2007年1月11日访问．
  8. ^ 8.0 8.1 Martin Bizzarro, David Ulfbeck, Anne Trinquier, Kristine Thrane, James N. Connelly, Bradley S. Meyer（2007年）．“Evidence for a Late Supernova Injection of 60Fe into the Protoplanetary Disk”（abstract page）．Science．316（5828）：1178–1181．DOI:10.1126/science.1141040．
  9. ^ W. M. Irvine (1983). "The chemical composition of the pre-solar nebula". T. I. Gombosi (ed.) Cometary Exploration 1: 3–12. 
  10. ^ 一个天文单位AU, 是地球到太阳之间的平均距离, 或大约1.5亿公里. 其为测量星际距离的标准单位.
  11. ^ J. J. Rawal（1986年）．“Further Considerations on Contracting Solar Nebula”（PDF）．Earth, Moon, and Planets．34：93–100．DOI:10.1007/BF00054038．于2006年12月27日访问．
  12. ^ Template:Harvtxt
  13. ^ 13.0 13.1 Charles H. Lineweaver（2001年）． “An Estimate of the Age Distribution of Terrestrial Planets in the Universe: Quantifying Metallicity as a Selection Effect”．Icarus．151：307．DOI:10.1006/icar.2001.6607．arXiv:astro-ph/0012399．
  14. ^ 14.0 14.1 Thierry Montmerle, Jean-Charles Augereau, Marc Chaussidon（2006年）．“Solar System Formation and Early Evolution: the First 100 Million Years”．Earth, Moon, and Planets．98：39–95．DOI:10.1007/s11038-006-9087-5．
  15. ^ Jane S. Greaves（2005年）． “Disks Around Stars and the Growth of Planetary Systems”．Science．307：68．DOI:10.1126/science.1101979．
  16. ^ M. Momose, Y. Kitamura, S. Yokogawa, R. Kawabe, M. Tamura, S. Ida (2003). "Investigation of the Physical Properties of Protoplanetary Disks around T Tauri Stars by a High-resolution Imaging Survey at lambda = 2 mm" (PDF). Ikeuchi, S., Hearnshaw, J. and Hanawa, T. (eds.) The Proceedings of the IAU 8th Asian-Pacific Regional Meeting, Volume I 289: 85, Astronomical Society of the Pacific Conference Series. 
  17. ^ Deborah L. Padgett, Wolfgang Brandner, Karl R. Stapelfeldt et al.（1999年3月）．“Hubble Space Telescope/NICMOS Imaging of Disks and Envelopes around Very Young Stars”．The Astronomical Journal．117：1490–1504．DOI:10.1086/300781．
  18. ^ M. Küker, T. Henning, G. Rüdiger（2003年）．“Magnetic Star-Disk Coupling in Classical T Tauri Systems”．Astrophysical Journal．589：397．DOI:10.1086/374408．
  19. ^ 19.0 19.1 Sukyoung Yi, Pierre Demarque, Yong-Cheol Kim, Young-Wook Lee, Chang H. Ree, Thibault Lejeune, Sydney Barnes（2001年）．“Toward Better Age Estimates for Stellar Populations: The Y2 Isochrones for Solar Mixture”．Astrophysical Journal Supplement．136：417．DOI:10.1086/321795．arXiv:astro-ph/0104292．
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[编辑] References

  • Michael A. Zeilik, Stephen A. Gregory（1998）．Introductory Astronomy & Astrophysics，4th ed.，Saunders College Publishing．ISBN 0030062284． 

[编辑] External links


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