How to build a solar system: full instructions included
Accepted wisdom is that the planets formed from the gradual accretion of material orbiting the Sun – rocks randomly banged into each other and stuck together, then more rocks banged into those and so on. Eventually the four outer planets grew large enough to attract an enormous, gaseous atmosphere around their rocky core. This is called the Core Accretion scenario and it has two principal problems.
First, if we consider the very first stage of this process, how would random collisions between small lumps of rock cause those rocks to start sticking together? And second, one would expect the rotation of planets formed in this random way to be, well, random – so the odds against all eight planets in one system spinning in the same direction are, not to put too fine a point on it, astronomical.
Dr Sergei Nayakshin in our Theoretical Astrophysics Research Group, who is a specialist in the physics of accretion discs, has proposed an elegant new theory which turns conventional wisdom on its head. In essence, instead of the gas giants being rocky planets which have gained a gas atmosphere, what if the rocky planets are gas giants which have lost their atmosphere?
Dr Nayakshin’s suggestion is that gas orbiting a new star – rather than rocks – comes together towards the outer edges of the accretion disc and gradually moves in towards the star itself. Because this process is caused by the gravitational pull of the star the direction of spin would be consistent across all of these ‘giant planet embryos’. So that solves problem two.
As these giant planet embryos (described by Sergei as “initially fluffy and cool”) grow in size, rocks in the accretion disc would be pulled towards them – again, it’s gravity at work, not random collisions – creating rocky cores. Solved problem one.
However as these gas planets spiral slowly inwards, they would pass a certain point beyond which tidal forces rip apart their gaseous outer layer and any loose lumps of rock still floating around therein. The gas dissipates and the rocks remain in situ. Where would this point be? Sergei’s calculations suggest it would be at about the orbital distance of the Asteroid Belt.
Dr Nayakshin’s ideas and calculations, proposing this ‘Tidal Downsizing’ scenario, were recently published in three papers in the Monthly Notices of the Royal Astronomical Society (MNRAS) and presented at several national and international astronomical meetings.
Of course, formation of our Solar System, and that of currently 500 observed exo-planets is not an easy problem to crack, and even with the two further papers submitted to MNRAS, there is much work to do to test this new model, according to Dr Nayakshin. Nor should we forget that the Solar System is much more than eight planets* and one asteroid belt. For any theory of planetary formation to survive, it also has to explain the Kuiper Belt, the Oort Cloud, comets and all the other miscellanea orbiting our Sun.
But as exciting new theories go, Tidal Downsizing is definitely one to watch.
- Formation of planets by tidal downsizing of giant planet embryos (DOI: 10.1111/j.1745-3933.2010.00923.x)
- Department of Physics and Astronomy
*Pluto, long considered a planet, is now regarded as a ‘minor planet’ ie. just a particularly big rock in the Kuiper Belt. Please adjust your orrery as appropriate.