Where do comets and moonlets come from?

A couple of debates have recently come to the fore over the origin of many of our smaller planetary bodies.

First up we have comets. Comets are often divided into short period and long period comets (there are also comets on parabolic orbits that pass by the Sun never to return and those that hit the thing). Short period ones tend to pass by us with, you guessed it, a shorter orbital time. They also tend to be in the same plane as the planets. They are believed to be a mixture of long period comets captured by Jupiter and Trans Neptunian Objects cast out of the Kuiper-Edgeworth Belt. Long period comets on the other hand come from farther afield. It takes them a lot longer to make their round trip and they can come at us from any direction. Analysis of their orbits can provide an indication of where they’re coming from and the resultant ‘basin’ of comet populations is referred to as the Oort cloud. The numbers of comets coming in tell us about how many objects exist in the Oort cloud ready to be ejected by gravitational interactions with each other or passers by. The trouble is, the answers suggest a few more than are expected according to our present understanding of how the Sun and the planetary disc that gave rise to them formed – about ninety percent more. However, there is a get-out clause. The Sun is believed to have formed in a cluster that was then dispersed by the radiation pressure of the young stars it contained, and it is surprisingly easy for one star in a cluster to grab a few comets off another. So as you look at Comet C/2009 R1 McNaught or any other, so long as it is a long period comet, you have a nine to one chance of that comet having been born outside of our solar system and pushed away like a solar sail in the dark night between the stars.

A second set of objects has also left astronomers scratching their heads. This one is a set of younger than expected objects – the smaller moons of Saturn. Such tiny objects should long ago have been battered to pieces by comets and asteroids, should they be the age of everything else hanging around that area. However, they display a key difference to their larger siblings lying further out. These moons, which do the job of shepherding ring particles to keep them in place, have turned out to be less dense than water. Whereas the other moons are more compacted, these ones are nothing more than a loose conglomeration. In fact it looks as though the very moons that keep the ring particles in place are themselves composed of ring particles that have added themselves together over the years. The reason the moonlets have managed to live until now is that they were born around ten million years ago, not the four billion or so previously assumed. The moonlets are features of the rings themselves.

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