Oort cloud
Oort proposed that the bodies in this cloud replenish and keep constant the number of long-period comets entering the inner Solar System—where they are eventually consumed and destroyed during close approaches to the Sun.
[4][6] The innermost portion of the Oort cloud is more than a thousand times as distant from the Sun as the Kuiper belt, the scattered disc and the detached objects—three nearer reservoirs of trans-Neptunian objects.
[7] The outer Oort cloud is only loosely bound to the Solar System and its constituents are easily affected by the gravitational pulls of both passing stars and the Milky Way itself.
These forces served to moderate and render more circular the highly eccentric orbits of material ejected from the inner Solar System during its early phases of development.
[8] By the same token, galactic interference in the motion of Oort bodies occasionally dislodges comets from their orbits within the cloud, sending them into the inner Solar System.
[4][9] Astronomers hypothesize that the material presently in the Oort cloud formed much closer to the Sun, in the protoplanetary disc, and was then scattered far into space through the gravitational influence of the giant planets.
[10] Nevertheless, the cloud is thought to be the source that replenishes most long-period and Halley-type comets, which are eventually consumed by their close approaches to the Sun after entering the inner Solar System.
[12] In 1932, the Estonian astronomer Ernst Öpik proposed a reservoir of long-period comets in the form of an orbiting cloud at the outermost edge of the Solar System.
[19] Because it lies at the interface between the dominion of Solar and galactic gravitation, the objects comprising the outer Oort cloud are only weakly bound to the Sun.
This in turn allows small perturbations from nearby stars or the Milky Way itself to inject long-period (and possibly Halley-type) comets inside the orbit of Neptune.
[4][22] Formerly the outer cloud was thought to be more massive by two orders of magnitude, containing up to 380 Earth masses,[23] but improved knowledge of the size distribution of long-period comets has led to lower estimates.
If analyses of comets are representative of the whole, the vast majority of Oort-cloud objects consist of ices such as water, methane, ethane, carbon monoxide and hydrogen cyanide.
[27] 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.
[30] The Oort cloud is thought to have developed after the formation of planets from the primordial protoplanetary disc approximately 4.6 billion years ago.
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.
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.
[36] 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."
[47] 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.
For example, it is hypothesized that 70,000 years ago Scholz's Star passed through the outer Oort cloud (although its low mass and high relative velocity limited its effect).
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.
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.
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.
[66] 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.
