Dance of the planets

Usually the picture illustrates the text, but i liked this image so much it's the other way 'round on this occasion :) The title reflects the little known idea that our solar system has a rather large number of unique characteristics facilitating the development of intelligent life, not least the presence of the large giants in the outer reaches of the solar system. (FFI on this, see the book called Rare Earth).

These Gas Giants - in particular Jupiter - are responsible for mopping up the vast numbers of cometry & rocky planetessimals that have bombarded the solar system since its dawn. If you haven't realised how violent a place the solar system is, just look up at the Moon, or check out any of the public domain images of Mercury, Mars or Jupiter's moons: they're covered with multitudes of craters - a permanent record of intense bombardment over the eons of time that have elaspsed since Sol went nuclear.

So Jupiter is in a very real sense our Neighbourhood Watch Vacuum Cleaner - this was demonstrated very spectacularly in 1994, when Comet Shoemaker-Levy 9 collided dramatically with Jupiter, scarring its gaseous surface with Earth-sized stains for hours, or even days in some cases. (Prior to its collision with Jupiter. the comet had been gravitationally broken up on a previous close-orbit encounter with the planet). Without it, there simply wouldn't be enough time for life to evolve, develop and progress beyond slimy soup.

Jupiter doesn't always get them all though - think of Chicxulub or the Barringer Crater for a minute & you'll see what I mean.

So next time you look upwards at night and see a bright yellow blob hanging in the sky, think about how it's possibly been instrumental in us being here. Rare Earth will tell you of a whole heap of other unique things about our Solar System too, though, including some facts that I didn't previously know about our Sun - such as its unusual high metallurgic content, which limits or regulates its variability.

Which is why I like the photo above - the biggest ball (from In the Night Garden), representing Sol (aka the Sun) is nicely varied in its colours, representing the pizza-base mix of elements it gained at its creation (presumably from a previously violent explosion of a more giant predecessor or two).

• For an overview of the conditions needed for complex life to develop, see here.