How Far Can a Comet Come before Reaching Our Solar System? Water on Earth Is Older Than the Sun.

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    Sep 27, 2014 1:25 AM GMT
    From how far can a comet come before reaching our solar system?

    News Item: Up to 50% of the water on Earth is older than the Sun.

    http://www.weather.com/news/science/earth-water-older-sun-20140926
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    Sep 27, 2014 6:00 AM GMT
    Although no direct observations have been made of such a cloud, it is believed to be the source of most or all comets entering the inner solar system (some short-period comets may come from the Kuiper belt), based on observations of the orbits of comets.

    The Oort cloud was proposed in 1950 by Dutch astronomer Jan Oort to explain an apparent contradiction: comets are destroyed by several passes through the inner solar system, yet if the comets we observe had existed since the origin of the solar system, all would have been destroyed by now.

    According to the theory, the Oort cloud contains millions of comet nuclei, which are stable because the sun’s radiation is weak at their distance. The cloud provides a continual supply of new comets, replacing those that are destroyed.

    The Oort cloud is a remnant of the original nebula that collapsed to form the sun and planets five billion years ago, and is loosely bound to the solar system.

    It is thought that other stars are likely to possess Oort clouds of their own, and that the outer edges of two nearby stars’ Oort clouds may sometimes overlap, causing the occasional intrusion of a comet into the inner solar system.

    See also the Kuiper belt, a belt of cometary material that lies between Neptune’s orbit and the Oort.

    http://www.redorbit.com/education/reference_library/space_1/solar_system/2574588/oort_cloud/
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    Sep 27, 2014 6:04 AM GMT
    oortcloud2.gif
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    Sep 27, 2014 6:10 AM GMT
    The Oort cloud (/ˈɔrt/ or /ˈʊərt/;[1]) or Öpik–Oort cloud,[2] named after Dutch astronomer Jan Oort and Estonian astronomer Ernst Öpik, is a spherical cloud of predominantly icy planetesimals believed to surround the Sun at up to 50,000 AU.[3] This places the cloud a quarter of the distance to Proxima Centauri, the nearest star to the Sun. 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. The outer limit of the Oort cloud defines the cosmographical boundary of the Solar System and the region of the Sun's gravitational dominance.[4]

    The Oort cloud is thought to comprise two regions: a spherical outer Oort cloud and a disc-shaped inner Oort cloud, or Hills cloud. Objects in the Oort cloud are largely composed of ices, such as water, ammonia, and methane.

    - Wikipedia