奥尔特云Oort cloud

The Oort cloud (/ɔːrt, ʊərt/),[1] named after the Dutch astronomer Jan Oort, sometimes called the Öpik–Oort cloud,[2] is a theoretical cloud of predominantly icy planetesimals proposed to surround the Sun at distances ranging from 2,000 to 200,000 au (0.03 to 3.2 light-years).[note 1][3] It is divided into two regions: a disc-shaped inner Oort cloud (or Hills cloud) and a spherical outer Oort cloud. Both regions lie beyond the heliosphere and in interstellar space.[3][4] The Kuiper belt and the scattered disc, the other two reservoirs of trans-Neptunian objects, are less than one thousandth as far from the Sun as the Oort cloud.

奥特云(/ Janrt,ʊərt/)[1]以荷兰天文学家扬·奥特(Jan Oort)的名字命名,有时也称为Öpik–Oort云,[2]是理论上主要由冰冷的小行星组成的云,提出围绕太阳的距离为 2,000到200,000 au(0.03到3.2光年)。[注1] [3]它分为两个区域:盘状内部奥尔特云(或丘陵云)和球形外部奥尔特云。 这两个区域都位于日球层以外,并且位于星际空间中。[3] [4] 跨海王星天体的另外两个储层-柯伊伯带和 scattered disc ,离太阳到奥尔特云只有不到千分之一的距离。

The outer limit of the Oort cloud defines the cosmographical boundary of the Solar System and the extent of the Sun’s Hill sphere.[5] The outer Oort cloud is only loosely bound to the Solar System, and thus is easily affected by the gravitational pull both of passing stars and of the Milky Way itself. These forces occasionally dislodge comets from their orbits within the cloud and send them toward the inner Solar System.[3] Based on their orbits, most of the short-period comets may come from the scattered disc, but some may still have originated from the Oort cloud.[3][6]

奥尔特云的外边界定义了太阳系的宇宙学边界和太阳山球的范围。[5] 外部的奥尔特云仅与太阳系松散地结合在一起,因此容易受到恒星和银河系自身引力的影响。 这些力量有时会将彗星从其在云中的轨道上移开,然后将它们送入内部太阳系。[3] 根据它们的轨道,大多数短周期彗星可能来自 scattered disc ,但有些可能仍然起源于奥尔特云。[3] [6]

Astronomers conjecture that the matter composing the Oort cloud formed closer to the Sun and was scattered far into space by the gravitational effects of the giant planets early in the Solar System’s evolution.[3] Although no confirmed direct observations of the Oort cloud have been made, it may be the source of all long-period and Halley-type comets entering the inner Solar System, and many of the centaurs and Jupiter-family comets as well.[6]

天文学家猜测,构成奥尔特云的物质形成得更靠近太阳,并在太阳系演化初期受到巨大行星的引力作用而分散到太空中。[3] 尽管尚未确认对奥尔特云的直接观测,但它可能是进入太阳系内部的所有长周期和哈雷型彗星的来源,也可能是许多半人马和木星族彗星的来源。[6]

The existence of the Oort cloud was first postulated by Estonian astronomer Ernst Öpik in 1932. Oort independently proposed it in 1950.

爱沙尼亚天文学家恩斯特·厄皮克(ErnstÖpik)于1932年首先提出了奥尔特云的存在。奥尔特(Oort)在1950年独立提出。

推测Hypothesis

There are two main classes of comet: short-period comets (also called ecliptic comets) and long-period comets (also called nearly isotropic comets). Ecliptic comets have relatively small orbits, below 10 au, and follow the ecliptic plane, the same plane in which the planets lie. All long-period comets have very large orbits, on the order of thousands of au, and appear from every direction in the sky.[7]

彗星主要分为两类:短周期彗星(也称为黄道彗星)和长周期彗星(也称为近乎各向同性的彗星)。 黄道彗星的轨道相对较小,在10 au以下,并遵循黄道平面,即行星所在的平面。 所有长周期彗星都有非常大的轨道,大约数千个au,并且从天空的各个方向出现。[7]

A. O. Leuschner in 1907 suggested that many comets believed to have parabolic orbits, and thus making single visits to the solar system, actually had elliptical orbits and would return after very long periods.[8] In 1932 Estonian astronomer Ernst Öpik postulated that long-period comets originated in an orbiting cloud at the outermost edge of the Solar System.[9] Dutch astronomer Jan Oort independently revived the idea in 1950 as a means to resolve a paradox:[10]

A. O. Leuschner在1907年提出,许多被认为具有抛物线轨道的彗星,因此单次造访太阳系,实际上是椭圆形的轨道,并且在很长一段时间后会返回。[8] 1932年,爱沙尼亚天文学家恩斯特·奥皮克(ErnstÖpik)推测,长周期彗星起源于太阳系最外层边缘的轨道云。[9] 荷兰天文学家扬·奥尔特(Jan Oort)在1950年独立地重新提出了这一想法,以解决以下的悖论:[10]

  • Over the course of the Solar System’s existence the orbits of comets are unstable, and eventually dynamics dictate that a comet must either collide with the Sun or a planet or else be ejected from the Solar System by planetary perturbations.
  • Moreover, their volatile composition means that as they repeatedly approach the Sun, radiation gradually boils the volatiles off until the comet splits or develops an insulating crust that prevents further outgassing.
  • 在太阳系的存在过程中,彗星的轨道是不稳定的,最终动力学表明,彗星必须与太阳或行星发生碰撞,或者必须通过行星扰动将其从太阳系中弹出。
  • 此外,它们的挥发性成分意味着,当它们反复接近太阳时,辐射逐渐使挥发物沸腾,直到彗星分裂或形成绝缘皮,防止进一步挥发。

Thus, Oort reasoned, a comet could not have formed while in its current orbit and must have been held in an outer reservoir for almost all of its existence.[10][11][7] He noted that there was a peak in numbers of long-period comets with aphelia (their farthest distance from the Sun) of roughly 20,000 au, which suggested a reservoir at that distance with a spherical, isotropic distribution. Those relatively rare comets with orbits of about 10,000 au have probably gone through one or more orbits through the Solar System and have had their orbits drawn inward by the gravity of the planets.[7]

因此,奥尔特(Oort)推断,彗星不可能在其当前轨道上形成,而必须几乎全部存在于外部库中。[10] [11] [7] 他指出,长周期彗星的顶峰(距太阳最远)约为20,000 au,这是一个高峰,这表明该距离处的储层呈球形,各向同性分布。 那些相对稀有的约10,000 au轨道的彗星可能已经通过了太阳系的一个或多个轨道,并且由于行星的引力而向内拉动它们的轨道。[7]

结构和组成Structure and composition

The presumed distance of the Oort cloud compared to the rest of the Solar System与太阳系其余部分相比,奥尔特云的假定距离

The Oort cloud is thought to occupy a vast space from somewhere between 2,000 and 5,000 au (0.03 and 0.08 ly)[7] to as far as 50,000 au (0.79 ly)[3] from the Sun. Some estimates place the outer edge at between 100,000 and 200,000 au (1.58 and 3.16 ly).[7] The region can be subdivided into a spherical outer Oort cloud of 20,000–50,000 au (0.32–0.79 ly), and a torus-shaped inner Oort cloud of 2,000–20,000 au (0.0–0.3 ly).

奥尔特云被认为占据了广阔的空间,从太阳到2000到5,000 au(0.03到0.08 ly)[7]到50,000 au(0.79 ly)[3]。一些估计将外边缘放置在100,000至200,000 au(1.58至3.16 ly)之间。[7]该区域可细分为20,000–50,000 au(0.32–0.79 ly)的球形外部奥尔特云和2,000–20,000 au(0.0–0.3 ly)的圆环状内部奥尔特云。

The outer cloud is only weakly bound to the Sun and supplies the long-period (and possibly Halley-type) comets to inside the orbit of Neptune.[3] The inner Oort cloud is also known as the Hills cloud, named after Jack G. Hills, who proposed its existence in 1981.[12] Models predict that the inner cloud should have tens or hundreds of times as many cometary nuclei as the outer halo;[12][13][14] it is seen as a possible source of new comets to resupply the tenuous outer cloud as the latter’s numbers are gradually depleted. The Hills cloud explains the continued existence of the Oort cloud after billions of years.[15]

外层云只与太阳弱结合,并向海王星的轨道内部提供长周期(可能是哈雷型)的彗星。[3]内部的奥尔特云也被称为希尔斯云,以杰克·希尔斯(Jack G. Hills)的名字命名,他于1981年提出存在该云。[12]模型预测,内部云的彗星核数应是外部光晕的数十倍或数百倍; [12] [13] [14]人们认为,新的彗星可能会为脆弱的外部云重新供应脆弱的外部云。数字逐渐枯竭。希尔斯云解释了数十亿年后奥尔特云的持续存在。[15]

The outer Oort cloud may have trillions of objects larger than 1 km (0.62 mi),[3] and billions with absolute magnitudes[16] brighter than 11 (corresponding to approximately 20-kilometre (12 mi) diameter), with neighboring objects tens of millions of kilometres apart.[6][17] Its total mass is not known, but, assuming that Halley’s Comet is a suitable prototype for comets within the outer Oort cloud, roughly the combined mass is 3×1025 kilograms (6.6×1025 lb), or five times that of Earth.[3][18] Earlier it was thought to be more massive (up to 380 Earth masses),[19] but improved knowledge of the size distribution of long-period comets led to lower estimates. The mass of the inner Oort cloud has not been estimated.

外部的奥尔特云可能具有数万亿个大于1公里(0.62英里)的物体[3],以及数十亿个绝对幅度[16]大于11毫米的物体(对应于大约20公里(12英里)的直径),而邻近的物体则数十个 相隔数百万公里。[6] [17] 它的总质量未知,但假设哈雷彗星是外部奥尔特云中彗星的合适原型,其总质量约为3×1025千克(6.6×1025磅),是地球的五倍。[3 ] [18] 早些时候人们认为它更大(最多380个地球质量),[19]但是对长周期彗星大小分布的了解增加导致估计值降低。 尚未估计内部奥尔特云的质量。

If analyses of comets are representative of the whole, the vast majority of Oort-cloud objects consist of ices such as watermethaneethanecarbon monoxide and hydrogen cyanide.[20] However, the discovery of the object 1996 PW, an object whose appearance was consistent with a D-type asteroid[21][22] in an orbit typical of a long-period comet, prompted theoretical research that suggests that the Oort cloud population consists of roughly one to two percent asteroids.[23] 

如果对彗星的分析能代表整体,那么奥尔特云的绝大多数物体都由冰组成,例如水,甲烷,乙烷,一氧化碳和氰化氢。[20] 但是,发现了1996 PW天体,该天体的外观与长周期彗星典型轨道中的D型小行星[21] [22]一致,这促使理论研究表明,奥尔特云群由 大约百分之一到百分之二的小行星。

Analysis of the carbon and nitrogen isotope ratios in both the long-period and Jupiter-family comets shows little difference between the two, despite their presumably vastly separate regions of origin. This suggests that both originated from the original protosolar cloud,[24] a conclusion also supported by studies of granular size in Oort-cloud comets[25] and by the recent impact study of Jupiter-family comet Tempel 1.[26]

对长周期和木星族彗星中碳氮同位素比的分析显示,尽管两者的起源区域可能相距甚远,但两者之间的差异很小。 这表明这两者都起源于原始的原生太阳云,[24]这个结论也得到了对奥尔特云彗星粒度的研究[25]和最近对木星家族彗星坦普尔1的影响研究的支持[26]。

Origin

The Oort cloud is thought to have developed after the formation of planets from the primordial protoplanetary disc approximately 4.6 billion years ago.[3] The most widely accepted hypothesis is that the Oort cloud’s objects initially coalesced much closer to the Sun as part of the same process that formed the planets and minor planets. After formation, strong gravitational interactions with young gas giants, such as Jupiter, scattered the objects into extremely wide elliptical or parabolic orbits that were subsequently modified by perturbations from passing stars and giant molecular clouds into long-lived orbits detached from the gas giant region.[3][27]

奥尔特云被认为是在大约46亿年前从原始原行星盘形成行星之后形成的。[3] 最广泛接受的假设是,与形成行星和次行星的过程相同,奥尔特云的物体最初聚结得更靠近太阳。 形成后,与年轻的天然气巨人如木星的强烈引力相互作用将这些物体散布到极宽的椭圆形或抛物线形轨道中,随后通过扰动将恒星和巨大的分子云通过扰动而改变为与天然气巨人区域分离的长寿命轨道。 [3] [27]

Recent research has been cited by NASA hypothesizing that a large number of Oort cloud objects are the product of an exchange of materials between the Sun and its sibling stars as they formed and drifted apart and it is suggested that many—possibly the majority—of Oort cloud objects did not form in close proximity to the Sun.[28] Simulations of the evolution of the Oort cloud from the beginnings of the Solar System to the present suggest that the cloud’s mass peaked around 800 million years after formation, as the pace of accretion and collision slowed and depletion began to overtake supply.[3]

美国宇航局引用了最近的研究假设,认为大量的奥尔特云物体是太阳与其同伴恒星形成并漂移时发生物质交换的产物,因此建议许多(可能是大多数)奥尔特 没有在太阳附近形成云物体。[28] 从太阳系开始到现在的奥尔特云演化的模拟表明,随着吸积和碰撞的速度减慢并且耗尽开始超过供应,云的质量在形成后约8亿年达到顶峰。[3]

Models by Julio Ángel Fernández suggest that the scattered disc, which is the main source for periodic comets in the Solar System, might also be the primary source for Oort cloud objects. According to the models, about half of the objects scattered travel outward toward the Oort cloud, whereas a quarter are shifted inward to Jupiter’s orbit, and a quarter are ejected on hyperbolic orbits. The scattered disc might still be supplying the Oort cloud with material.[29] A third of the scattered disc’s population is likely to end up in the Oort cloud after 2.5 billion years.[30]

JulioÁngelFernández的模型表明,  scattered disc 是太阳系周期性彗星的主要来源,也可能是奥尔特云物体的主要来源。 根据这些模型,大约一半的散布物体朝着奥尔特云向外传播,而四分之一则向内移到木星轨道,而四分之一则被抛到双曲线轨道上。  scattered disc 可能仍在为奥尔特云提供物质。[29] 25亿年后,三分之一的  scattered disc 可能最终会出现在奥尔特云中。[30]

Computer models suggest that collisions of cometary debris during the formation period play a far greater role than was previously thought. According to these models, the number of collisions early in the Solar System’s history was so great that most comets were destroyed before they reached the Oort cloud. Therefore, the current cumulative mass of the Oort cloud is far less than was once suspected.[31] The estimated mass of the cloud is only a small part of the 50–100 Earth masses of ejected material.[3]

计算机模型表明,在形成期间,彗星碎片的碰撞所起的作用远比以前想象的要大。 根据这些模型,太阳系历史早期的碰撞次数如此之多,以至于大多数彗星在到达奥尔特云之前就被摧毁了。 因此,当前奥尔特云的累积质量远小于曾经的怀疑。[31] 估计的云团质量仅是喷射物质在50-100个地球质量中的很小一部分。[3]

Gravitational interaction with nearby stars and galactic tides modified cometary orbits to make them more circular. This explains the nearly spherical shape of the outer Oort cloud.[3] On the other hand, the Hills cloud, which is bound more strongly to the Sun, has not acquired a spherical shape. Recent studies have shown that the formation of the Oort cloud is broadly compatible with the hypothesis that the Solar System formed as part of an embedded cluster of 200–400 stars. These early stars likely played a role in the cloud’s formation, since the number of close stellar passages within the cluster was much higher than today, leading to far more frequent perturbations.[32]

与附近恒星和银河潮的引力相互作用修改了彗星轨道,使它们更圆。 这解释了外部奥尔特云的近似球形。[3] 另一方面,更牢固地绑定到太阳的希尔斯云尚未获得球形。 最近的研究表明,奥尔特云的形成与太阳系形成为200-400颗恒星嵌入星团的一部分的假设基本兼容。 这些早期恒星可能在云层的形成中起了作用,因为星团中近距离恒星通道的数量比今天高得多,从而导致扰动更加频繁。[32]

In June 2010 Harold F. Levison and others suggested on the basis of enhanced computer simulations that the Sun “captured comets from other stars while it was in its birth cluster.” Their results imply that “a substantial fraction of the Oort cloud comets, perhaps exceeding 90%, are from the protoplanetary discs of other stars.”[33][34]

在2010年6月,Harold F. Levison等人在增强的计算机模拟的基础上建议说,“太阳在其诞生簇中捕获了其他恒星的彗星”。 他们的结果暗示“奥尔特云彗星的相当一部分,也许超过90%,是来自其他恒星的原行星盘。” [33] [34]

彗星Comets

Comet Hale–Bopp, an archetypical Oort-cloud comet

Comets are thought to have two separate points of origin in the Solar System. Short-period comets (those with orbits of up to 200 years) are generally accepted to have emerged from either the Kuiper belt or the scattered disc, which are two linked flat discs of icy debris beyond Neptune’s orbit at 30 au and jointly extending out beyond 100 au from the Sun. Long-period comets, such as comet Hale–Bopp, whose orbits last for thousands of years, are thought to originate in the Oort cloud. The orbits within the Kuiper belt are relatively stable, and so very few comets are thought to originate there. The scattered disc, however, is dynamically active, and is far more likely to be the place of origin for comets.[7] Comets pass from the scattered disc into the realm of the outer planets, becoming what are known as centaurs.[35] These centaurs are then sent farther inward to become the short-period comets.[36]

人们认为彗星在太阳系中有两个不同的起源点。 人们普遍认为短周期彗星(轨道长达200年的彗星)是从柯伊伯带或分散盘中出现的,它们是海王星在30 au轨道以外的两个相连的冰碎片的平盘,并共同向外延伸超过 太阳100 au。 人们认为,长周期彗星(如黑尔-波普彗星)的轨道持续了数千年,它们起源于奥尔特云。 柯伊伯带内的轨道相对稳定,因此认为很少有彗星起源于此。 但是,分散的盘是动态活动的,并且很可能是彗星的起源地。[7] 彗星从分散的圆盘进入外行星的领域,成为所谓的半人马。[35] 这些半人马然后被送入更远的地方成为短周期彗星。[36]

There are two main varieties of short-period comet: Jupiter-family comets (those with semi-major axes of less than 5 AU) and Halley-family comets. Halley-family comets, named for their prototype, Halley’s Comet, are unusual in that although they are short-period comets, it is hypothesized that their ultimate origin lies in the Oort cloud, not in the scattered disc. Based on their orbits, it is suggested they were long-period comets that were captured by the gravity of the giant planets and sent into the inner Solar System.[11] This process may have also created the present orbits of a significant fraction of the Jupiter-family comets, although the majority of such comets are thought to have originated in the scattered disc.[6]

短周期彗星有两种主要变体:木星家族彗星(半长轴小于5 AU的那些)和哈雷家族彗星。 以其原型哈雷彗星命名的哈雷家族彗星与众不同之处在于,尽管它们是短周期彗星,但据推测它们的最终起源是在奥尔特云中,而不是在分散的盘中。 根据它们的轨道,建议它们是被大行星的引力捕获并送入太阳系内部的长周期彗星。[11] 这个过程可能还产生了很大一部分木星家族彗星的当前轨道,尽管大多数这样的彗星被认为起源于散布的圆盘。[6]

Oort noted that the number of returning comets was far less than his model predicted, and this issue, known as “cometary fading”, has yet to be resolved. No dynamical process are known to explain the smaller number of observed comets than Oort estimated. Hypotheses for this discrepancy include the destruction of comets due to tidal stresses, impact or heating; the loss of all volatiles, rendering some comets invisible, or the formation of a non-volatile crust on the surface.[37] 

奥尔特指出,返回彗星的数量远远少于他的模型所预测的,这个被称为“彗星衰落”的问题尚未解决。 没有已知的动力学过程可以解释观测到的彗星数量少于Oort估计的数量。 这种差异的假说包括由于潮汐应力,撞击或加热而破坏了彗星; 所有挥发物的流失,使一些彗星不可见或表面上形成了不挥发的硬皮。[37]

Dynamical studies of hypothetical Oort cloud comets have estimated that their occurrence in the outer-planet region would be several times higher than in the inner-planet region. This discrepancy may be due to the gravitational attraction of Jupiter, which acts as a kind of barrier, trapping incoming comets and causing them to collide with it, just as it did with Comet Shoemaker–Levy 9 in 1994.[38] An example of typical Oort cloud comet could be C/2018 F4.[39]

假设的奥尔特云彗星的动力学研究估计,它们在外行星区域的发生率将是内行星区域的几倍。 这种差异可能是由于木星的引力所引起的,木星起着一种屏障的作用,捕获了进入的彗星并使它们碰撞,就像1994年对Shoemaker–Levy 9彗星所做的那样。[38] 一个典型的奥尔特云彗星的例子可能是C / 2018 F4。[39]

潮汐作用Tidal effects

Main article: Galactic tide

Most of the comets seen close to the Sun seem to have reached their current positions through gravitational perturbation of the Oort cloud by the tidal force exerted by the Milky Way. Just as the Moon‘s tidal force deforms Earth’s oceans, causing the tides to rise and fall, the galactic tide also distorts the orbits of bodies in the outer Solar System.

在银河系附近施加的潮汐力,通过对奥尔特云的引力扰动,大多数接近太阳的彗星似乎已经到达了它们当前的位置。就像月球的潮汐力使地球的海洋变形,导致潮汐上升和下降一样,银河潮汐也扭曲了外太阳系中物体的轨道。

In the charted regions of the Solar System, these effects are negligible compared to the gravity of the Sun, but in the outer reaches of the system, the Sun’s gravity is weaker and the gradient of the Milky Way’s gravitational field has substantial effects. Galactic tidal forces stretch the cloud along an axis directed toward the galactic centre and compress it along the other two axes; these small perturbations can shift orbits in the Oort cloud to bring objects close to the Sun.[40] The point at which the Sun’s gravity concedes its influence to the galactic tide is called the tidal truncation radius. It lies at a radius of 100,000 to 200,000 au, and marks the outer boundary of the Oort cloud.[7]

与太阳的引力相比,在太阳系的制图区域中,这些影响可以忽略不计,但是在系统的外围,太阳的引力较弱,银河系的引力场的梯度具有很大的影响。银河的潮汐力沿着指向银河中心的轴拉伸云,并沿其他两个轴压缩云。这些微小的扰动会改变奥尔特云中的轨道,使物体靠近太阳。[40]太阳的重力使它对银河系潮汐的影响减小的点称为潮汐截断半径。它的半径为100,000至200,000 au,标志着奥尔特云的外边界。

Some scholars theorise that the galactic tide may have contributed to the formation of the Oort cloud by increasing the perihelia (smallest distances to the Sun) of planetesimals with large aphelia (largest distances to the Sun).[41] The effects of the galactic tide are quite complex, and depend heavily on the behaviour of individual objects within a planetary system. Cumulatively, however, the effect can be quite significant: up to 90% of all comets originating from the Oort cloud may be the result of the galactic tide.[42] Statistical models of the observed orbits of long-period comets argue that the galactic tide is the principal means by which their orbits are perturbed toward the inner Solar System.[43]

一些学者的理论认为,银河潮可能通过增加具有大的视差(距太阳最大的距离)的小行星的近日点(距太阳最小的距离)来促成奥尔特云的形成。[41] 银河潮汐的影响非常复杂,并且在很大程度上取决于行星系统内单个物体的行为。 但是,累积的影响可能非常显着:起源于奥尔特云的所有彗星中多达90%可能是银河潮的结果。[42] 对观测到的长周期彗星轨道的统计模型认为,银河潮汐是扰动其轨道向内部太阳系运动的主要手段。[43]

恒星摄动和恒星伴随假设Stellar perturbations and stellar companion hypotheses

Besides the galactic tide, the main trigger for sending comets into the inner Solar System is thought to be interaction between the Sun’s Oort cloud and the gravitational fields of nearby stars[3] or giant molecular clouds.[38] The orbit of the Sun through the plane of the Milky Way sometimes brings it in relatively close proximity to other stellar systems. For example, it is hypothesized that 70 thousand years ago, perhaps Scholz’s star passed through the outer Oort cloud (although its low mass and high relative velocity limited its effect).[44] During the next 10 million years the known star with the greatest possibility of perturbing the Oort cloud is Gliese 710.[45] This process could also scatter Oort cloud objects out of the ecliptic plane, potentially also explaining its spherical distribution.[45][46]

除了银河潮,将彗星送入内部太阳系的主要诱因被认为是太阳的奥尔特云与附近恒星[3]或巨大分子云的引力场之间的相互作用。[38] 穿过银河系平面的太阳轨道有时使它相对接近其他恒星系统。 例如,假设7万年前,也许舒尔茨的恒星穿过了外部的奥尔特云(尽管其低质量和高相对速度限制了它的作用)。[44] 在接下来的1000万年中,最可能扰动奥尔特云的已知恒星是Gliese710。[45] 这个过程也可能使奥尔特云物体飞出黄道平面,可能也解释了其球形分布。[45] [46]

In 1984, Physicist Richard A. Muller postulated that the Sun has a heretofore undetected companion, either a brown dwarf or a red dwarf, in an elliptical orbit within the Oort cloud. This object, known as Nemesis, was hypothesized to pass through a portion of the Oort cloud approximately every 26 million years, bombarding the inner Solar System with comets. However, to date no evidence of Nemesis or the Oort cloud have been found, and many lines of evidence (such as crater counts), have thrown their existence into doubt.[47][48] Recent scientific analysis no longer supports the idea that extinctions on Earth happen at regular, repeating intervals.[49] Thus, the Nemesis hypothesis is no longer needed to explain current assumptions.[49]

1984年,物理学家理查德·A·穆勒(Richard A. Muller)推测,太阳在奥尔特云的椭圆轨道上有一个迄今未被发现的伴星,即棕矮星或红矮星。 假设这个被称为“复仇女神”的天体大约每2600万年就穿过一部分奥尔特云,并用彗星轰击太阳系内部。 但是,迄今为止,尚未找到克星或克星云的证据,许多证据(如陨石坑计数)使它们的存在受到怀疑。[47] [48] 最近的科学分析不再支持灭绝是有规律地重复发生的想法。[49] 因此,不再需要克星主义假设来解释当前的假设。[49]

A somewhat similar hypothesis was advanced by astronomer John J. Matese of the University of Louisiana at Lafayette in 2002. He contends that more comets are arriving in the inner Solar System from a particular region of the postulated Oort cloud than can be explained by the galactic tide or stellar perturbations alone, and that the most likely cause would be a Jupiter-mass object in a distant orbit.[50] This hypothetical gas giant was nicknamed Tyche. The WISE mission, an all-sky survey using parallax measurements in order to clarify local star distances, was capable of proving or disproving the Tyche hypothesis.[49] In 2014, NASA announced that the WISE survey had ruled out any object as they had defined it.[51]

路易斯安那大学拉法叶特大学的天文学家约翰·马塞斯(John J. Matese)在2002年提出了类似的假设。 仅是潮汐或恒星摄动,最可能的原因是遥远轨道上的木星质量物体。[50] 这个假想的天然气巨头被昵称为“ Tyche”。 WISE任务是一项使用视差测量的全天候调查,目的是弄清当地的恒星距离,它能够证明或反驳Tyche假设。[49] 2014年,NASA宣布WISE调查已排除了他们定义的任何物体。[51]

未来探索Future exploration

Artist’s impression of the TAU spacecraft

Space probes have yet to reach the area of the Oort cloud. Voyager 1, the fastest[52] and farthest[53][54] of the interplanetary space probes currently leaving the Solar System, will reach the Oort cloud in about 300 years[4][55] and would take about 30,000 years to pass through it.[56][57] However, around 2025, the radioisotope thermoelectric generators on Voyager 1 will no longer supply enough power to operate any of its scientific instruments, preventing any further exploration by Voyager 1. The other four probes currently escaping the Solar System either are already or are predicted to be non-functional when they reach the Oort cloud; however, it may be possible to find an object from the cloud that has been knocked into the inner Solar System.

太空探测器尚未到达奥尔特云地区。 目前离开太阳系的行星际空间探测器中最快的[52]和最远的[53] [54]旅行者1号将在大约300年[4] [55]到达奥尔特云,并且需要大约30,000年的时间才能通过 通过它。[56] [57] 但是,到2025年左右,旅行者1号上的放射性同位素热电发生器将不再提供足够的电力来运行其任何科学仪器,从而阻止旅行者1号进行任何进一步的探索。目前已经或预计,目前正在逃离太阳系的其他四个探测器将 当它们到达奥尔特云时将无法运行; 但是,有可能从被撞到内部太阳系的云中找到一个物体。

In the 1980s there was a concept for a probe to reach 1,000 au in 50 years called TAU; among its missions would be to look for the Oort cloud.[58]

在1980年代,有一个名为TAU的探针在50年内达到1,000 au的概念。 其任务之一是寻找奥尔特云。[58]

In the 2014 Announcement of Opportunity for the Discovery program, an observatory to detect the objects in the Oort cloud (and Kuiper belt) called the “Whipple Mission” was proposed.[59] It would monitor distant stars with a photometer, looking for transits up to 10,000 au away.[59] The observatory was proposed for halo orbiting around L2 with a suggested 5-year mission.[59] It was also suggested that the Kepler observatory could have been capable of detecting objects in the Oort cloud.[60]

在2014年的“发现机会的公告”计划中,提出了一个观测站,用于检测奥尔特云(和柯伊伯带)中的物体,称为“惠普尔任务”。[59] 它将用光度计监视远处的恒星,寻找距离高达10,000 au的过境。[59] 提议将天文台绕二号轨道进行晕圈运动,并建议进行5年的飞行。[59] 也有人认为开普勒天文台本来可以探测奥尔特云中的物体。[60]

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