PALEODYNAMICS - SOLAR-SYSTEM FORMATION AND THE EARLY ENVIRONMENT OF THE SUN

Our present paradigm of the formation of stars and planetary systems s uggests that the early Solar System may have existed in much denser en vironments than the galactic disk, e.g., the cores of giant molecular clouds and open stellar clusters. Some constraints on a hypothetical p rimordial environment can be inferred from the present dynamical state of the Sun and planets. Tremaine (1991, Icarus 89, 85-92) has propose d that the large obliquities of the outer planets may have arisen from twisting of the ecliptic plane during the initial collapse of the pro tosolar cloud. I find that the current solar obliquity could only have been produced in the very dense environment of the initial collapse o f the protoplanetary cloud: Mean post collapse densities would have a neglible effect on Solar System dynamics. The inclinations of the orbi ts of Uranus and Neptune, sensitive to perturbations on time scales of less than 1 myr, place a constraint on the number density of stars in the solar neighborhood. I place a 3 sigma limit on the product of the stellar density and residence time of 3 x 10(4) M.pc(-3) Myr, still c onsistent with the average densities and lifetimes of observed open cl usters. I examine the possible effect of the tidal field on planet for mation. Tidal torquing can suppress runaway accretion of a massive bod y by maintaining a high velocity dispersion. I derive a limiting semim ajor axis of similar to 50 AU, but find that the sensitivity to parame ters does not permit the observed extent of the planetary system to be used to infer limits on the tidal field during the epoch of planet fo rmation. Stronger tides and a higher stellar density on a time scale o f 10(8) year will affect comet cloud formation. I conclude that a tran sient comet cloud of a few M(+) could have formed at a few thousand at omic units but this would have been rapidly destroyed by stellar encou nters over 10(8) year. Uranus- and Neptune-crossing planetesimals coul d have been scattered into a belt about 200 AU from the Sun. Although the dynamical lifetimes of these objects are comparable to the Solar S ystem, a time-scale comparison suggests that little mass could have re ached such a cloud before the end of the cluster epoch. (C) 1995 Acade mic Press, Inc.