Hi Guys

Michael Woolfson, at the University of York, first proposed the Capture Theory of planet formation in 1964. With John Dormand and others he has elaborated, modelled and refined the theory, with his latest results presented in "The Origin and Evolution of the Solar System" (2000.)

The basic idea is that the planets formed via Jeans collapse from a filament of material drawn off a protostar that made a near miss (~ 100 AU) on the Sun. The Sun was very likely born in a dense nebula so this isn't a wildly unlikely event, especially since possible "free" planets have been seen in similar nebulas.

At first there were six planets, all Jovians, which were captured into highly eccentric orbits around the early Sun. As they made their first orbits they bulged tidally and so formed their moons.

Their orbits were rounded off by tidal interactions and a resisting medium [dust and gas] around the Sun. But during round off the planets would have interacted with each other to produce the erratic range of axial tilts as observed today.

The Moon, Mars and Mercury are explained as heavily abraded moons of the inner two planets. Abraded by what?
The explosive collision of the inner two planets that eventually ejected the remnants of one, and split the other into debris that became Earth and Venus.

[Zechariah Sitchin would be intrigued... but Nibiru's orbit is still ridiculous]

Sounds wild, but the collision wasn't as wildly improbable as it sounds in such a crowded system. Dynamical modelling of multi-Jovian systems frequently show ejections, collisions and/or inspiralling of planets. Also other researchers have independently modelled the proposed Sun/protostar interaction with similar results. Woolfson is looking at modifying his model based on such insights, so he's not a "knows-everthing" crank.

Woolfson's book compares and contrasts the various leading models of planet formation and (naturally) backs his own model, and it's hard to argue with him.

I think Jeans collapse is the preferred mechanism of big planet formation, but the collisional origin of Earth/Venus is hard to swallow. Until you see the math.

The book is online. I initially found the Prelude, References and Chapter 8...

http://bookmarkphysics.iop.org/fullbooks/075030457x/woolfsonch08.pdf

...via Google. Then I downloaded the lot. Chapter 8 covers the collision of the two inner planets.

As for SETI well it means Earth-like planets may be rarer, but Earth-like moons may not.

Adam