Over our Earth’s 4.5 billion-year history, we have been hit by things and possibly experienced small changes in orbit. The earliest evidence for life we have found so far on our world is 3.8 billion years old, and from then to the present day living things have been around in profusion.
Since even a small change in the Earth's orbit could sterilize our planet, things must have remained stable at least as long as there has been life on Earth.
Understandably, our ideas as to what other planetary systems might be like were moulded by what we saw in our own. For years it was the only one we could study; what we saw made sense.
Things start off with the collapse of a big cloud of gas and dust. It forms a gradually shrinking rotating disc, which condenses into a number of lumps. The lump in the middle, the biggest, becomes a star.
The lumps forming far from the star are not strongly affected by it, so the resulting planets hang onto lots of the gases present in the birth cloud.
In our solar system, these are the planets Jupiter, Saturn, Uranus and Neptune, known as the “gas giants.”
Further in, the heat, radiation and wind from the young star remove a lot of the gases, but not all, leaving rocky planets with relatively thin atmospheres, like Venus, Earth and Mars.
Planets closer in get all their gases blasted away, as with our planet Mercury. It all fits our theories and is consistent with a system that is stable over the long term, and ideal for life.
This explains the shock we had when we first started getting a good look at other planetary systems. Most of them were nothing like ours at all.
Some of them had Jupiter-sized gas giant planets orbiting close to their stars. They could not have formed there because the gas would have been blasted away; they must have moved somehow.
In all, very few of those other planetary systems look like ours. The picture we get is of planets changing their orbits quite drastically over time, going from the outer reaches to their system to close to their stars, and then somehow moving off to somewhere else.
Since even a tiny change in the Earth's orbit would freeze or fry us, we know this shuffling the planets tendency cannot have happened here, or if it did, it must have been a long time ago.
So the picture we get of our solar system is one of exceptional stability, and if life can only thrive in exceptionally stable planetary systems, it might be rarer than we thought.
This change in our thinking is interesting because years ago, at a conference, a scientist described work he had done on computer simulations of planetary systems, and the results suggested instability.
Books often describe planetary systems as a bunch of planets orbiting a star, held in their orbits by that star's gravity. If that were entirely the case. the systems would probably be extremely stable over the long term.
However, there is another factor. Each planet, as it moves in its orbit, is tugging at all the other planets. These tugs are tiny, but they are constantly changing as the planets move, and can gradually pump up strange instabilities, just as properly timed pushes on a swing can build up really dramatic movements.
That scientist said that the results of his simulations indicated these interactions can make planetary systems inherently unstable. This raises interesting questions.
Firstly, will our solar system remain stable?
Secondly, what percentage of planetary systems is inherently stable?
In this case, how many life-bearing planets can there be?
It is intriguing that as we puzzle about dark matter, dark energy, black holes and wormholes, we still have basic questions about our solar system.
It is also intriguing that the answers to these questions lie not in today's cutting-edge physics, but in the ideas developed by Isaac Newton, written down at around the beginning of the 18th century.
After sunset, Venus is very low in the southwest and Jupiter and Saturn low in the south.
The moon will be reach first quarter on Tuesday.
Ken Tapping is an astronomer with the National Research Council's Dominion Radio Astrophysical Observatory, near Penticton.