At the edge of the solar system is a region that we think could be populated by trillions of different objects , miniature worlds made of rock and ice , orbiting the Sun every several thousand or even millions of years. This region is called the Oort cloud . We think that the long-period comets that very occasionally visit the interior of the solar system originate from it. However, we have never directly observed any of the objects that make up this cloud. So does it exist or not?
The name of this cloud is due to the Dutch astronomer Jan Oort , who in the 1950s proposed its existence. However, Oort’s was not the first such proposal, although it was the most convincing. Throughout the 19th and early 20th centuries, comets with parabolic orbits had been observed, orbits that brought them close to the Sun only once and would not repeat themselves over time. However Armin Otto Leuschner , an American astronomer proposed in 1907 that these orbits could actually be incredibly elliptical orbits (so much so that low precision measurements could pass them off as parabolic) and these comets come from a region of the solar system located thousands of miles away. times the distance that separates the Earth from the Sun, commonly known as an astronomical unit. In 1932 , following a similar reasoning, the Estonian astronomer Ernst Öpik proposed that long-period comets must come from a cloud located at the edge of the solar system .
Oort’s proposal sought to resolve an apparent paradox. We know that cometary orbits are unstable for sufficiently long times . Some comets can keep their orbits for a few thousand years and others for millions, but due to the presence of the planets, their orbits end up becoming destabilized over time , leading them to collide with the Sun or with one of these planets. In addition, as they approach the Sun they lose their material , which consists mainly of ice, so that after tens or hundreds of these approaches, the comet would end up reduced to a tiny part of its original size.
Despite this, we continue to observe long-period comets to this day , some 4.6 billion years after the solar system formed. These comets, having been orbiting for so long, should have been affected by the larger planets or shrunk to much smaller sizes. Therefore, there must be a “reservoir” in the outskirts of the solar system that contains a large number of these objects, of which only a few occasionally reach us.
This is what Jan Oort proposed should happen in that remote region. However, we believe that the bulk of the bodies that today form the Oort cloud were not created so far from the Sun. During its first million years the solar system had a different appearance than it does today. From what we have been able to observe, the giant planets formed closer to each other and closer to the Sun. These planets formed just beyond what is known as the snow line or frost line, the distance from the Sun from which water ice and other compounds can persist in solid form. In this region, too, the solar wind is not so intense as to rapidly disperse the gases that formed the interplanetary medium in those origins. All this meant that these planets were able to accumulate large amounts of mass rapidly , leaving the rest of the smaller bodies at their mercy.
During the first hundreds of millions of years the giant planets were interacting with the planetesimals that surrounded them , much more abundant then than now. Some ended up being absorbed, others fell towards the Sun, but the vast majority we think were thrown into wide orbits that took them to distances of thousands of astronomical units. These countless planetesimals expelled from the interior of the solar system would end up forming what we now think would be the Oort cloud. Objects in this cloud would still be subject to the Sun’s gravity and would orbit around it, but could be strongly affected by massive objects that may occasionally approach the edges of the solar system. We know that occasionally there are stars that pass close to our own (such as Gliese 710 which will pass at about 70,000 astronomical units in about a million and a half years). Also the gravitational forces exerted by the galaxy as a whole can perturb the orbits of these bodies , bringing them closer to the Sun and, very occasionally, to the Earth.
This process of cleaning up the space around the gas giants also affected their orbits. Specifically, we think that the result of all this was that Jupiter saw its orbit reduced by several million kilometers , while Saturn moved about 150 million kilometers from the Sun and Uranus and Neptune several hundred or even more than a billion kilometers, to their current orbits.
V. V. Emelyanenko et al, 2007, The fundamental role of the Oort Cloud in determining the flux of comets through the planetary system, Monthly Notices of the Royal Astronomical Society. 381 (2): 779–789, doi:10.1111/j.1365-2966.2007.12269.x