Ken Tapping, June 16 2010

The conventional story for the formation of planetary systems is that a huge cloud of gas and dust collapses under its own gravity, and settles into a flat, slowly-rotating disc. The biggest concentration forms in the middle of the disc, and becomes a star. The rest of the disc forms smaller lumps – planets. The planets nearest the new star tend to be smaller and then get most of their atmospheres blown away by radiation from the young star. Planets further away are larger and tend to be able to hang onto more of their atmospheres.  This idea sounds reasonable and accounts for Mercury, Venus, Earth and Mars being small, rocky planets, and the more distant planets: Jupiter, Saturn, Uranus and Neptune being large planets with very deep atmospheres. This theory is intuitively satisfying. However, there is so far one very big problem with it. We have now detected a few hundred planets orbiting other stars, and as far as we can see at the moment, none of them are members of planetary systems like our Solar System. In many cases there are gas giant planets, bigger than Jupiter, lying far closer to their stars than Mercury is to our Sun. We have only one Solar System, ours, which seems to fit our theory. Obviously Mother Nature does not agree with us, so it is up to us to change our ideas. On the whole it seems our theories as to how stars and planetary systems form are roughly correct, but we do not have a good understanding of what can happen after the planets have mostly formed. 

It not very difficult to set up a computer simulation of several planets orbiting their star and then see what happens over time. You can do this on a PC. What happens is surprising. Two bodies orbiting one another turn out to be well-behaved. A star with a single planet is stable, with that lone planet moving around its star in a simple orbit that does not change much with time. Having a stable orbit is essential for a planet to develop life that has a chance to evolve to more complex forms. 

Add more planets and things get complicated. The orbits become unstable and change dramatically with time. Gas giant planets may end up close to their stars, and other planets might be thrown out of the system altogether. In short, we get systems resembling those we see orbiting other stars. 

Our Solar System was born some 4.5 billion years ago. Life appeared on the Earth very soon after our planet had cooled enough for liquid water to accumulate on its surface, between 2.5 and 3 billion years ago. We are here today thinking about these issues because the Earth’s orbit around the Sun has not changed much since then. There are theories that ice ages are due to tiny changes in the Earth’s orbit, but that is about it. We have much to learn about the stability of planetary systems. 

Since at the moment we know for sure of only one planet with life on it, trying to get a reliable answer to whether we are alone is impossible. That life appeared here on Earth pretty well as soon as conditions allowed suggests life appears wherever there’s an opportunity.  Having just us in an immense universe would be a dreadful waste.