Oort Cloud
  • Flock of many millions of comets presumed to exist far beyond the planets and marking the outermost reaches of the solar system. Jan Oort (1900–1992) estimated in 1950 that the cloud must contain about 100 billion comets, of which a few a year are diverted by the gravitation of stars near the Sun into the inner solar system. The cloud probably lies some 40 000AU from the Sun.

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