内奥尔特云1981

根据推测，内奥尔特云区域呈圆盘状，内部的彗核是奥尔特星云的数十甚至数百倍，是长周期彗星很好的补充源。如果内奥尔特云区域确实存在的话，位于这一区域内的彗星数量将是奥尔特云的五倍。

历史

1950年，荷兰天文学家扬·奥尔特提出了奥尔特云的概念。他认为，在太阳系边缘存在一个能孕育长周期彗星的云团。在1981年，美国天文学家杰克·希尔斯提出在奥尔特云内侧存在一个能产生彗星的区域，这就是今天所说的“内奥尔特云”或“希尔斯云”区域。希尔斯认为，这一区域中产生的彗星比奥尔特云外侧更多。在希尔斯提出内奥尔特云的概念后，西德尼·范登贝赫和马克·贝利两名天文学家分别在1982年和1983年推测了内奥尔特云的结构。

概览

内奥尔特云区域受到太阳影响较少，因此很可能较好维持了45亿年前太阳系形成时的环境。因为内奥尔特云受到的太阳引力作用比奥尔特云外侧更强，内奥尔特云得以保持圆盘状，而不是像奥尔特云外侧那样呈球壳状。根据推测，内奥尔特云区域的彗核和彗星的数目都比奥尔特云外侧要多，这一区域甚至可能是太阳系中彗星最密集的区域。内奥尔特云可以产生大量进入太阳系中心的彗星雨，来自内奥尔特云区域的彗星甚至有可能曾撞击地球，造成过地球上的生物大灭绝。内奥尔特云内的物质会源源不断地进入奥尔特云外侧，补充奥尔特云的质量。这也很可能是奥尔特云内一直有天体在离开太阳系或靠近太阳中心，却能一直维持存在的原因。

内奥尔特云区域天体的总质量目前尚无精确值，一项研究认为内奥尔特云天体的总质量大约是地球质量的13.8倍，也有人估算这一数字应该是500-1000倍地球质量。

In astronomy, the Hills cloud (also called the inner Oort cloud and inner cloud) is a theoretical vast circumstellar disc, interior to the Oort cloud and roughly aligned with the solar ecliptic plane, whose outer border would be located at around 20,000 to 30,000 astronomical units (AU) from the Sun, and whose inner border, less well defined, is hypothetically located at 250–1500 AU, well beyond planetary and Kuiper Belt object orbits—but distances might be much greater. If it exists, the Hills cloud likely contains roughly five times as many comets as the Oort cloud.

Overview

The need for the Hills cloud hypothesis is intimately connected with the dynamics of the Oort cloud: Oort cloud comets are continually perturbed in their environment. A non-negligible fraction leaves the Solar System, or tumbles into the inner system where they evaporate, fall into the Sun, or collide with or are ejected by the giant planets. Hence, the Oort cloud should have been depleted long ago, but it is still well supplied with comets.

The Hills cloud hypothesis addresses the persistence of the Oort cloud by postulating a densely populated, inner-Oort region—the "Hills cloud". Objects ejected from the Hills cloud are likely to end up in the classical Oort cloud region, maintaining the Oort cloud.

History

Original Oort cloud model

Between 1932 and 1981, astronomers believed that the Oort cloud proposed by Ernst Öpik and Jan Oort, and the Kuiper belt were the only reserves of comets in the Solar System.

In 1932, Estonian astronomer Ernst Öpik hypothesized that comets were rooted in a cloud orbiting the outer boundary of the Solar System. In 1950, this idea was revived independently by Dutch astronomer Jan Oort to explain an apparent contradiction: Comets are destroyed after several passes through the inner Solar System, so if any had existed for several billion years (since the beginning of the Solar System), no more could be observed now.

Oort selected 46 comets for his study that were best observed between 1850 and 1952.

New model

In the 1980s, astronomers realized that the main cloud could have an internal section that would start at about 3,000 AU from the Sun and continue up to the classic cloud at 20,000 AU. Most estimates place the population of the Hills cloud at about 20 trillion (about five to ten times that of the outer cloud), although the number could be ten times greater than that.

The main model of an "inner cloud" was proposed in 1981 by the astronomer Jack G. Hills, from the Los Alamos Laboratory, who gave the region its name. He calculated that the passage of a star near the Solar System could have triggered a "comet rain," thereby causing extinctions on Earth.

Characteristics

Structure and composition

Oort cloud comets are constantly disturbed by their surroundings and distant objects. A significant number either leave the Solar System or go much closer to the Sun. The Oort cloud should therefore have broken apart long ago, but it still remains intact. The Hills cloud proposal could provide an explanation; J. G. Hills and other scientists suggest that it could replenish the comets in the outer Oort cloud.

It is also likely that the Hills cloud is the largest concentration of comets across the Solar System. The Hills cloud should be much denser than the outer Oort cloud: If it exists, it is somewhere between 5,000 and 20,000 AU in size. In contrast, the Oort cloud is between 20,000 and 50,000 AU (0.3 and 0.8 ly) in size.

Possible Hills cloud objects

Bodies in the Hills cloud are made mostly of water ice, methane and ammonia. Astronomers suspect many long-period comets originate from the Hills cloud, such as Comet Hyakutake.

In their article announcing the discovery of Sedna, Mike Brown and his colleagues asserted that they observed the first Oort cloud object. They observed that, unlike scattered disc objects like Eris, Sedna's perihelion (76 AU) was too remote for the gravitational influence of Neptune to have played a role in its evolution. The authors regarded Sedna as an "inner Oort cloud object", located along the ecliptic and placed between the Kuiper belt and the more spherical part of the Oort cloud.