The Oort Cloud The Oort cloud is a vast swarm of some 2 trillion comets orbiting our star in the most distant reaches of our solar system, extending from beyond the orbits of Neptune and Pluto out to 100,000 times the Earth-Sun distance. Almost one-third the distance to the nearest star. While the planets are confined to a flattened disk in the solar system, the Oort cloud forms a spherical shell centered on the Sun, which gradually flattens down to an extended disk in the inner region, called the Kuiper belt.
Bright comets observed through telescopes or with the naked eye get thrown out of the Oort cloud or Kuiper belt, and become visible when they get close to enough so that the Sun’s energy can transform the surface ices into gases. These gases drag off the embedded dust, and we see the light reflected from the dust as a tail. Comets are the leftover icy building blocks from the time of planet formation, which formed in the region of the outer planets. Essentially thesecomets are dirty snowballs, composed primarily of water ice, with some carbon monoxide and other ices, in addition to interstellar dust.
When their orbits passed close enough to the giant planets to be affected, some were thrown toward the Sun and some were tossed outward toward the distant reaches of the solar system, the spherical swarm we now call the Oort cloud. Some of the comets sent inward hit the inner rocky planets, and probably contributed a significant amount of ocean water and organic material, the building blocks of life, to Earth. Comets that live in the Oort cloud are especially important scientifically because they have been kept in a perpetual deep freeze since the formation of our solar system 4. 6 billion years ago.
This means that they preserve, nearly intact, a record of the chemical conditions during the first few million years of the solar system’s history, and can be used to unravel our solar system’s origins much like an archaeologist uses artifacts to decipher an ancient civilization. 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) to as far as 50,000 AU (0. 79 ly) from the Sun. Some estimates place the outer edge at between 100,000 and 200,000 AU (1. 58 and 3. 16 ly).  The region can be subdivided into a spherical outer Oort cloud of 20,000–50,000 AU (0. 2–0. 79 ly), and a doughnut-shaped inner Oort cloud of 2,000–20,000 AU (0. 03–0. 32 ly). 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.  The inner Oort cloud is also known as the Hills cloud, named after J. G. Hills, who proposed its existence in 1981.  Models predict that the inner cloud should have tens or hundreds of times as many cometary nuclei as the outer halo; it is seen as a possible source of new comets to resupply the relatively 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 year The outer Oort cloud is believed to contain several trillion individual objects larger than approximately 1 km (0. 62 mi) (with many billions with absolute magnitudes brighter than 11—corresponding to approximately 20 km (12 mi) diameter), with neighboring objects typically tens of millions of kilometres apart. Its total mass is not known with certainty, but, assuming that Halley’s comet is a suitable prototype for all comets within the outer Oort cloud, the estimated combined mass is 3? 025 kg (7? 1025 lb or roughly five times the mass of the Earth). Earlier it was thought to be more massive (up to 380 Earth masses), but improved knowledge of the size distribution of long-period comets has led to much lower estimates. The mass of the inner Oort Cloud is not currently known. If analyses of comets are representative of the whole, the vast majority of Oort-cloud objects consist of various ices such as water, methane, ethane, carbon monoxide and hydrogen cyanide.
However, the discovery of the object 1996 PW, an asteroid in an orbit more typical of a long-period comet, suggests that the cloud may also contain rocky objects. Analysis of the carbon and nitrogen isotope ratios in both the Oort cloud and Jupiter-family comets shows little difference between the two, despite their vastly separate regions of origin. This suggests that both originated from the original protosolar cloud,a conclusion also supported by studies of granular size in Oort-cloud comets by the recent impact study of Jupiter-family comet.
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