bonham@calgary.UUCP (Mike Bonham) (12/17/85)
> Recently, someone posted on the net that one of the possible > comet formation theories has the comets forming inside the orbit > of Neptune and then being thrown out to form the Oort cloud. > I was hoping a real astrophysician might post a more detailed description of this theory of cometary formation, but here goes: 4 1/2 billion years or so ago the solar system was condensing from a cloud of gas, dust and small rocks. Blobs of nebular material would form, gravitationally attract, coalesce, and break apart, but the general tendency was to contract toward the centre of the nebula, where the biggest blob was forming (eventually, the sun). Because of the cloud's initial rotational momentum, as it contracted it spun out into a disk shape. Blobs in the disk accreted other blobs of gas & dust and grew into planets etc. It is thought that blobs not swept up by planets would resemble the comets of today. When the central blob (the sun) became massive enough, it collapsed and heated and thermonuclear-ly ignited. Radiation pressure and solar wind drove off the gaseous atmospheres of the inner planets, which up to that time resembled the hydrogen/methane/ammonia outer planets of today. The same thing must have happened to Jupiter, Saturn & co. but to a lesser extent. A curious phenomenon is that while the sun has 99% of the solar system's mass, the planets have most (95% ?) of the angular momentum. Somehow the cloud's momentum was transferred or retained in the outer disk of the proto- solar system. Various tidal and electromagnetic forces were at work in the early solar system, which, it is thought, may have been able to accelerate the proto-comets out past the orbits of the present-day planets. If such a mechanism were demonstrated conclusively, it would relieve astrophysicists' anxieties about "where the angular momentum went". If this theory is correct, comets are frozen blobs of the original pre- solar nebula, which makes the upcoming Halley's comet flyby by scientific probes so exciting to planetary physicists. Note that we're not talking about a purely gravitational system such as exists today -- if a comet were to form inside the orbit of Neptune today it would probably not be thrown out to the Oort cloud. In the proto-solar disk, there was more mass in the outlying parts of the cloud, and some think the disk had a rigid magnetic field embedded in it (until the sun condensed and started spinning too fast). I apologise if this contains gross inaccuracies. Maybe this will start a discussion on the net. -- __|__ __/___ Mike Bonham __|__ /___ Dept. of Computer Science ___|___ | | University of Calgary /___, |____| Calgary, Alberta, CANADA / \./ /| T2N 1N4 __/ \__ _/ `__| ..!{ubc-vision,ihnp4}!alberta!calgary!bonham