It's relevant to questions so important that most adults don't think to ask them - why are there only eight planets?  Or nine, or however many there are now?  Why are they where they are?  Because of all the objects that formed during the birth of the solar system, the ones we see are the ones that survived.  It's likely that many small proto-planets formed as the stellar dusts condensed into larger bodies - some collided and merged, some may have been pinballed out of the system by the varying gravitational fields, and it seems that some got smashed to bits in the confusion.  The existence of the asteroid belt supports such planetary pile-ups - but we can look at these antarctic rocks much closer.

Mineral analysis of the fragments reveals a large concentration of feldspar - large enough to have needed a planet to create it.  You might not think of rocks as hot and runny (unless you're the Human Torch), but when you gather enough of anything together the pressure and heat provided by gravity will melt it.  Materials of different densities float to different heights (just like oil in water), and as the system cools these differentiated layers are frozen in.  If you hang around for a while you have a hard shell around a layered liquid core, like our own Earth, and eventually the entire system solidifies, like the Moon.  If you lose a fight with another extremely solid body you get blown to pieces - but each of those pieces still shows evidence of the layer-cake structure.

It'll take more than a royal cavalry regiment to put this proto-planetary Humpty Dumpty together again.  It's thought that many of the pieces could be in the asteroid belt between and Jupiter, while others may have impacted on other planets or even been burned up in the Sun.  Spectral analysis of asteroids may confirm this hypothesis, but it seems clear that the early solar system was a dangerous place - and we have one more "We were lucky there" coincidence to thank for life as we know it.

Posted by Luke McKinney.