Hi RL

Fair point if it actually applied to the Capture theory that Woolfson describes - he's talking about tidal distortion of a proto-star that's still mostly extended to about 1000 AU - exactly how the Sun began. The tidal effects cause the gas/dust cloud to form higher density filaments that then pinch off thanks to their own self-gravity i.e. higher than the Jeans mass for that sub-volume of the filament. Woolfson and colleagues have done plenty of Smoothed-Particle Hydrodynamic simulations of just such events and their evolution computes out just fine.

What Woolfson is basically describing has a lot in common with Alan Boss' gravitational instability scenario for making gas planets - direct gravitational collapse of higher density patches within the original "nebula". Woolfson derives the nebula from a protostar rather than the Sun to explain the mystery of the angular momentum distribution in the solar system. The disk-accretion scenario hasn't solved that problem yet, except via extremely high early magnetic fields (as yet unobserved) or formation of massive tidal spiral arms in the disk itself - not healthy for making planets either.

What's more the disk accretion scenario suffers from huge amounts of planetary migration which eats all the gas planets in virtually every simulation of the model with sufficient resolution so far. Tell me how that's somehow superior to the Capture Model?

So which approach is more in line with the physics we know? We can keep waving a magic wand over the momentum problem, because we like accretion scenarios, or else we can bite the bullet and try to explain it. Woolfson's theory does that. And it's not just Woolfson. Other cosmogonists have simulated disk-star and disk-disk interactions which produce outcomes very similar to the Capture scenario.

Finally you might say "but I was talking about the terrestrial planets and we all know how well accretion explains them."

I agree. The Safronov model works brilliantly for the terrestrial planets. It might even be how it happened in the Capture Model too if the planetary collision totally blew the inner gas giants to bits. But to then get the current planets we have to posit a lot of independent late-stage collision events. The Capture theory suggests that instead of separate events, the apparent hemisperical asymmetries in Mercury, Mars and the Moon all derive from the planetary collision - most of Mercury's mantle was abraded away, some of Mars' and the Moon less so, perhaps. There are also a lot of isotopic anomalies that Safronov accretion says nothing about, but the Capture Scenario explains naturally via the collision.

So which theory is more rational?

Well maybe both (or all three if we include gravitational instability) explain the diversity we see in exoplanet systems? Currently a lot of exoplanet workers are saying that instability might explain the low metallicity planets, while disk accretion might account for the planet/metallicity correlation. The Capture theory would probably bolster the statistics for both low/high metallicity stars also - it would make planets even around the oldest stars, but metallicity might improve efficiency of the mechanism is newer stellar nurseries.

Woolfson's own papers on the different mechanisms are pretty even handed. He points out the flaws and the unexplained facts in all the different systems, with an obvious preference for his own idea in his older papers, but a more conciliatory attitude in his later work. Does that happen when a nebula-disk theorist encounters the Capture theory? No. The problems - and angular momentum is the biggest - get glossed over.

But only more data and more modelling will work it out.

Adam