Ken Tapping, June 6, 2018
Last year an asteroid from interstellar space flew through our Solar System and is now on its way back out into the space between the stars. Now, astronomers have identified an asteroid likely to be another interstellar visitor that became trapped in our Solar System. The thing that attracted the attention of the astronomers is that it is orbiting the Sun in the opposite direction to the planets and most other objects in the Solar System.
Our Solar System formed around 4.5 billion years ago from the collapse of a cloud of cosmic gas and dust. This formed into a rotating disc. The Sun formed from the central part and the rest formed all the planets and other objects making up the Solar System. Since all these bodies formed from the same rotating disc, we would expect them all to orbit around the Sun in the same direction. So far we have found almost 800,000 asteroids. Of these, 95 of them go around the Sun the wrong way. We call such orbits "retrograde", as opposed to "prograde", which is how most Solar System objects move. How did the retrograde bodies get to be like that? One possibility is that one or more of them came from outside. Isolated incidents don't really tell us much. However, if a second one happens, we can deduce a lot more. That is why the possibility of two or more encounters is getting the astronomical community excited.
The nearest star to us after the Sun lies 4.3 light years away, where a light year is just under 1e13 (one followed by thirteen zeroes) kilometres. If that stellar system were to throw off an asteroid in a random direction, there would be a roughly 1.5 billion to one chance it would pass through our Solar System. For more distant stars the odds get far worse, very rapidly. These odds would apply to the asteroid that went through the Solar System and kept going. What would be the chance of an asteroid having an encounter on its way through that would lead to it being trapped in orbit around the Sun, albeit moving against the traffic?
In its current mature state, our Solar System is probably not throwing things off into space. However, when it was forming, this was much more likely. As the birth disc collapsed, some objects had close encounters, where one object fell deeper into the collapsing disc and one got thrown out. So maybe the birth of a new planetary system results in a few asteroids and other things being thrown off into space. Considering the sheer size of space, the fact that since our Solar System formed 4.5 billion years ago, we have collected at least two, suggests that there must be a lot of these unattached asteroids wandering around.
In general driving the wrong way on a highway is a short-term experience. The same applies to bodies going against the flow in the Solar System. Their orbits get changed by the gravitational attractions of the planets, or their careers end in a more spectacular fashion. Our retrograde asteroid is protected because its orbit is being stabilized by the planet Jupiter. This means it may have been in the Solar System for a very long time, which could explain how it became trapped. When our Solar System was forming, there was a lot of gas, dust and small objects in the process of coagulating into larger objects. Being forced to plough through the gas, dust and other objects could have slowed the intruder until it became trapped. At this point however, the only object we are sure came from outside is the one that flew past last year. Before we can be sure about our retrograde asteroid we need to collect more evidence. Whether we have had one interstellar visitor or two, getting here took millions or even billions of years in the cold and dark of interstellar space, a sobering thought.
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