Hi All

The stability studies of terrestrial formation are fascinating but they presume the correctness of the nebula-disk/Safronov-accretion model. This has yet to be demonstrated chiefly because no one has yet explained how our Sun became a slow rotator and yet so much angular momentum is tied up in the planets - what coupling transferred momentum from Sun to the disk? And why is the Sun's rotational axis not aligned perfectly to the plane of the planets if they were originally so strongly coupled?

The current best contender for the necessary process is some sort of magnetohydrodynamic coupling which tranferred momentum to the disk like a huge electric motor. This hasn't been as nearly successfully modelled as the subsequent accretion of planets, but there are encouraging efforts in that direction.

Alternatively the planets could have been captured through a Sun-protostar near-miss (~ 1000 AU) thus solving the mismatch between planes and momenta. The two would be largely unrelated except through the process of circularisation that reduced the original elliptical orbits of the planets via viscous braking in a captured disk. Also the slow infall of disk material into the Sun would bring the Sun and disk into close, but not perfect alignment, before the disk decayed away.

Of relevance to the opinion of planetary scientists that Earths will be common is the proposed Earth/Venus formation mechanism in the Capture Theory - a planetary collision. As planets round off from highly elliptical orbits, they precess around their orbital planes and significant interactions between them become likely. Earth and Venus could either be the remnant cores of two sub-Jovian planets, A & B, the split core of one, or the accreted debris of a total fragmentation. Mercury, Mars and Theia (the Moon-making collider) were once moons, abraded to varying degrees by the blast.

But the odds aren't as good as for a pure disk model. Planetary interactions can only occur during round off which takes ~ 3 myr before the disk is blown away. In the case of the proposed planet A and B round-off times are 2 and 6 myr respectively. Their average interaction time is ~ 33 myr, thus the odds are ~ 0.2. Interaction times are similar for the other big planets, with decent odds for collisions with Jupiter or total ejection for both planet A and B. Thus large terrestrials may not be as frequent as expected.

Planet data from the ESO's Eddington and NASA's Kepler will give bulk statistics for planet radii and types. If Earth-mass planets are rare then the collision mechanism will be the likely cause. In planetesimal accretion models big terrestrials appear in virtually all simulation runs and thus every system with a sufficient disk should have them. In Dormand and Woolfson's scenario for the Solar System planet B - the innermost - had only 5 Earth masses. Such a "small" sub-Jovian would have a radius ~ 5.5 Earth, so it would be obvious... if it's made of chiefly Jovian planet "stuff" with [rho] ~ 0.8. Different densities could lead to a false exaggeration of "terran" planet numbers. Spectroscopic follow-up studies - if possible - will be vital.

Adam

PS Capture theory predicts planets around ~ 8-25% of Sun-like stars depending on the density of the birth-nebula. Fewer red-dwarfs are expected to have planets chiefly because their collapse to high-density sufficient for successful capture is slower than for Sun-like stars, and the timing is critical. Nebula density declines rapidly as the generated stars disperse, thus the probability of Capture declines. Red dwarfs should have a lower overall planet formation rate.