Jan Hendrik Oort [1900-1992] was a pretty smart Dutch astronomer who [amongst many other things] empirically derived the Oort Constants that characterize the local rotational properties of our galaxy [the Milky Way].
However, Oort is probably best remembered for inventing the Oort Cloud in 1950.
This was no trivial invention. Oort invented a cloud of 100,000,000,000 comets orbiting the Sun at a distance of 50,000 to 150,000 [“and possibly still further”] AU.
Jan Oort did not rely upon Fred Whipple’s “visualized” technique. Instead, he simply invented 100,000,000,000 comets by unilaterally declaring that they “must” exist.
The structure of the cloud of comets surrounding the Solar System
and a hypothesis concerning its origin
J. H. Oort
Bulletin of the Astronomical Institutes of the Netherlands, vol. 11, p. 91-110 (1950).
SAO/NASA ADS Astronomy Abstract Service
Although there are no observations that confirm the existence of the Oort Cloud it has become deeply embedded into the mythology of modern astronomy.
Although no confirmed direct observations of the Oort cloud have been made, astronomers believe that it is the source of all long-period and Halley-type comets entering the inner Solar System and many of the centaurs and Jupiter-family comets as well.
In fact, during the last 60 years the Oort Cloud has become an industry which has provided astronomers with an endless supply of projects which have refined and extended Oort’s original invention:
Structure and composition
The Oort cloud is thought to occupy a vast space from somewhere between 2,000 and 5,000 AU [0.03 and 0.08 light years] to as far as 50,000 AU [0.79 light years) from the Sun. Some estimates place the outer edge at between 100,000 and 200,000 AU [1.58 and 3.16 light years].
The region can be subdivided into a spherical outer Oort cloud of 20,000–50,000 AU [0.32–0.79 light years], and a doughnut-shaped inner Oort cloud of 2,000–20,000 AU [0.03–0.32 light years].
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 years.
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 3×1025 kg (7×1025 lb or roughly five times the mass of the Earth)
The invisible Oort Cloud has many problematic issues regarding its genesis, location and retention [and re-supply] of orbiting objects over billions of years.
However, the greatest problems arise when the Oort Cloud is deemed to be the source of the long period comets that enter the inner solar system.
Firstly, it is difficult to explain how passing stars [and the galactic tide] manage to perturb the orbits of only a very select few [out of the trillions of objects in the Oort Cloud] so that they become long period comets in the inner solar system.
Secondly, to generate the observed long period comets the Oort Cloud must include billions of comets in both prograde and retrograde elliptical orbits [with ecliptic inclinations ranging from 0 to 360 degrees] that are between 2,000 and 200,000 AU from the Sun. Therefore, perturbations of the Oort Cloud [by passing stars or the galactic tide over billions of years] would result in the depopulation of the Oort Cloud as the trillions of orbiting objects either a) collided, b) were expelled from the cloud, or c) were propelled towards the Sun and a fiery demise.
Overall, astronomers have been working very hard and very creatively [for over sixty years] to maintain the myth of the Oort Cloud.
However, the Voyager 1 spacecraft appears to be providing astronomy with a reality check because at a distance of only 122 AU [from the Sun] the spacecraft appears to be leaving the solar system.
The Voyager 1 spacecraft is a 722 kilogram (1,592 lb) space probe launched by NASA on September 5, 1977 to study the outer Solar System and interstellar medium. Operating for 35 years, 1 month and 23 days as of 28 October 2012, the spacecraft receives routine commands and transmits data back to the Deep Space Network.
At a distance of about 122 AU (1.83×1010 km) as of September 2012, it is the furthest manmade object from Earth. Voyager 1 is now in the heliosheath, which is the outermost layer of the heliosphere. On June 15, 2012, NASA scientists reported that Voyager 1 may be very close to entering interstellar space and becoming the first manmade object to leave the Solar System.
Voyager 1 May Have Left the Solar System
by Nancy Atkinson on October 8, 2012
While there’s no official word from NASA on this, the buzz around the blogosphere is that Voyager 1 has left the Solar System. The evidence comes from this graph, above, which shows the number of particles, mainly protons, from the Sun hitting Voyager 1 across time.