Were Mercury and Mars separated at birth? – space – 19 January 2009 – New Scientist.
Brad Hansen discovers that modelling a ring of debris makes two large planets and interrupts the growth of two others, thus reproducing the planets as we know them. In the process a Mars-like object would be needed to collide with Earth to make the moon and something akin is needed to strip Mercury’s mantle bare, but those are secondary details.
Question is: How did the debris ring arise?
Michael Woolfson’s Capture Theory has a natural answer – the solar system had two more gas giants than its present configuration and they collided, their silicate/metallic remnants forming a debris ring from which planets could form. But why only gas giants? That’s the trick – as Woolfson points out, gas giants form naturally via gravitational instability from gas filaments produced in a tidally distorted protostar. Terrestrial planets don’t, and have to be explained another way.
The little publicised gap in planet formation modelling is that going from the observed circumstellar dust to the planetesimals assumed by standard planet-forming theories is very, very hard to do. The dust if too small gets blown away, and if too large (but not large enough) falls into the star due to gas drag. Finding a process to counter-attack these two loss mechanisms is a damned hard problem, and one Woolfson-style theories tries to avoid entirely.
In science telling such origin theories apart needs better observational data at higher resolutions than we can presently attain. But bigger and better telescopic devices are coming soon…
Maybe. The collision in Woolfson’s model isn’t all that improbable given the number of planets he was proposing and their highly eccentric orbits. According to his modelling of planet formation in the embedded phase of star formation +20% of stars end up with at least single planet. Multi-planet systems are harder to form via the Capture Theory process, which might be reflected in the current number distribution data of exoplanets.
But if the dust-to-planetesimal problem can be solved then the formation of terrestrial planets is pretty straight forward. There’s indications that 30% of sun-like stars have a hot Neptune or two – what that means is anyone’s guess.