Although no direct observations of the Oort Nebula have been made, the elliptical orbits of comets have been observed, and it is thought that many comets enter the inner solar system from the Oort Nebula, and that some of the shorter-period comets may come from the Kuiper Belt.
In 1932, the Estonian astronomer Ernst Pik proposed that comets are clouds from the outer edge of the solar system. But in 1950, the Dutch astronomer Jan Hendrick Oort pointed out that there was a contradiction in Pik's hypothesis, in that a comet traveling back and forth between the inner and outer solar system would eventually be destroyed by a variety of factors, and its life cycle would never be as long as the age of the solar system. The cloud is very stable with weak solar radiation, and there are millions of comet nuclei that can keep producing new comets to replace the destroyed ones.
The Oort Cloud is the remnant of the nebula that formed the Solar System 5 billion years ago and surrounds it.
It was thought that other stars outside the Sun would have their own Oort nebulae, and that if two stars were close together, their Oort clouds would overlap, leading to comets traveling into the inner solar system of the other star.
Until now, only the asteroid 90377 has been recognized as a possible Oort Nebula object, with an orbit between 76 and 850 astronomical units, which is closer to the Sun than expected, and possibly from the inner Oort Nebula. If this theory is correct, then the Oort Nebula must be closer to the Sun than estimated, and would be denser. It has also been suggested that the Sun was originally part of a cluster of stars when it formed.