<P> The outer Oort cloud may have trillions of objects larger than 1 km (0.62 mi), and billions with absolute magnitudes brighter than 11 (corresponding to approximately 20 - kilometre (12 mi) diameter), with neighboring objects tens of millions of kilometres apart . 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 × 10 kilograms (6.6 × 10 lb), or five times that of 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 led to lower estimates . The mass of the inner Oort cloud has not been characterized . </P> <P> 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 . However, the discovery of the object 1996 PW, an object whose appearance was consistent with a D - type asteroid 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 . 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, a conclusion also supported by studies of granular size in Oort - cloud comets and by the recent impact study of Jupiter - family comet Tempel 1 . </P> <P> The Oort cloud is thought to be a remnant of the original protoplanetary disc that formed around the Sun approximately 4.6 billion years ago . 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, but that gravitational interaction with young gas giants such as Jupiter ejected the objects into extremely long elliptic or parabolic orbits . 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 . 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 . </P> <P> 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 . A third of the scattered disc's population is likely to end up in the Oort cloud after 2.5 billion years . </P>

Where are the objects in the oort cloud thought to have been formed initially
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