How the planets formed

I confess I'm a fan of Masterchef. This years' contestants have been great and I've enjoyed the episodes. Except… I channel-surf when the ads are on, quite often flicking to old episodes of Spicks and Specs (a music trivia show) or the food network.

Then, while I was channel-surfing three weeks ago on a Sunday night, I came across The Planets, hosted by Britain's rock star astrophysicist, Professor Brian Cox. From then on, Sunday nights became watching The Planets, with a quick flick now and then to see how Masterchef was getting along.

If you haven't seen this series – do. It's chock full of the all the new things scientists have learned from unmanned missions to all the planets in the last decade or so. Water on Mercury and on asteroids, why Mars lost its atmosphere, what turned Venus into a death trap. And last Sunday, it was Jupiter's turn. What was particularly interesting about this episode is that Prof Cox ventured into cosmology and discussed the formation of the solar system.

When I was at school the conventional wisdom (at least for kids) was that the planets formed from a cloud of material surrounding the new-born Sun. The four rocky planets (Mercury, Venus, Earth, and Mars) were formed first from the heavier material closest to the sun. The lighter gases escaped further out and formed the gas giants (Jupiter, Saturn, Uranus, and Neptune). Back then Pluto was still a planet but even then it was something of an anomaly.

Public interest in space exploration waned quickly after the first Moon landing. Not too many people tuned into the pictures from the last manned mission in 1972. Funding or space exploration collapsed. For many years the best information we had came from the Voyager flights, launched in 1977. But more recently we have data from planetary explorers like Cassini-Huygens, Galileo, Juno, Dawn, and others. So we know more about the outer planets and – most importantly – because of Kepler's search for exo-planets, we know a lot more about what solar systems look like.

And they don't look much like ours.

Kepler has found a whole bunch of 'super earths', rocky planets much larger than our home. Scientists have also found lots of gas giants as big as Jupiter whipping around their stars in close orbits, nothing like the configuration of our system, with rocky planets innermost.

So why does our solar system look the way it does?

Professor Cox explained that Jupiter formed at about the same time as the Sun. In some other universe it probably went on to become another star, forming a binary system. There are heaps of them out there. But while Jupiter has two-and-a-half times more mass than every other object in the solar system combined – planets, moons, asteroids - [1] it's not enough to start nuclear fusion. However, Jupiter's presence dictated how the rest of the planets formed the way they did. At one time in its history Jupiter moved toward the sun, coming closer than the asteroid belt and into Mars's orbit. As it did so, it sort of vacuumed up matter in its orbit, leaving less material to be amalgamated into a planet. That's why Mars is so much smaller than Earth and Venus. What dragged Jupiter out again to its present orbit was the development of Saturn, which set up a resonance with Jupiter. The larger planet orbits the sun twice in the time it takes Saturn to orbit once.

These days Jupiter's vast gravitational field protects the inner planets from debris coming in from the Oort cloud or outside the solar system, but millions of years ago, the planet's close encounter with a large asteroid deflected its path, sending the giant rock hurtling through the inner solar system where it collided with a young planet Earth. In fact, that probably happened many times with smaller asteroids – but the big one wiped out the dinosaurs.

I love this stuff. It proves that science is never static. As they learn more about the universe scientists need to revise their hypotheses so they fit the new facts. So much has changed, even in my lifetime.

Maybe sometime, somewhere, we'll find that elusive hint of life on another planet. Won't that be wonderful?

Here's a bit more about the wonderful BBC series The Planets. Catch it if you can.