What remnants of early solar system structure remain? All planets orbit the Sun in the same direction (CCW). Most planets' orbits lie in nearly the same plane. Most planetary moons orbit their planets in the same direction (CCW). The planets' orbits are nearly circular. There is a progression of planetary properties; the planets farther from the Sun tend to be less dense and richer in VOLATILE materials (e.g., ice, hydrogen gas). All solid-surfaced bodies have been CRATERED. BODE'S PROGRESSION probably arose from gravitational interactions among the planetary bodies. d (AU) ~ 0.4 AU + 0.3 AU x (0, 1, 2, 4, 8, 16,...) PLANET Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Predicted 0.4 0.7 1.0 1.6 2.8 5.2 10.0 19.6 38.8 Actual 0.387 0.723 1.000 1.524 5.203 9.53 19.2 30.0 Ceres 2.78

SOLAR NEBULA (Condensation) Theory

COLLAPSE of an interstellar cloud It is possible that collapse of an interstellar cloud was triggered by the supernova explosion of a nearby previous-generation massive star. The pressure waves from the explosion helped the collapse, and short-lived radioactive isotopes from the supernova decayed into stable isotopes of elements that we now find in very ancient meteorites.

Formation of a hot, spinning disk If the collapsing matter was not perfectly spherical, it would have started to spin. The shrinking generated heat, especially in the denser center where an infant Sun would eventually appear.

Astronomers do not know whether the disk ever got hot enough everywhere to vaporize the solid dust grains in it, but the inner part probably did. As the collapsing disk got denser, its collapse slowed. Heating by shrinkage and cooling by radiation were always competing. With collapse slowed, cooling started to become more important.

CONDENSATION of grains in cooling disk Solid particles (grains) started to condense from the cooling gas. Which ones condensed depended upon the local temperature. The more VOLATILE (low boiling point) materials probably could not form in large quantity near the infant Sun.

Iron could certainly form near the Sun, and elsewhere Iron could certainly form near the Sun, and elsewhere. Space would have been filled with floating iron grains.

Silicate materials (“sand grains”) would also condense over a wide region of the nebula ― but could not survive quite as near the Sun as iron could.

Farther from the Sun, it was cold enough for more volatile materials like ICE to freeze out. An inner planet might be expected to be small, but with a large share of iron. Farther out, planets might be larger, made of similar proportions of silicates and iron. Even more distant planets would be larger still because of the extra volatiles in their structure.

ACCRETION of planetesimals from grains, forming protoplanets ACCRETION differs from CONDENSATION not only because the particles get bigger, but also because larger particles tend to settle toward the central plane of the nebula. That is why most of the planets’ orbits lie in nearly the same plane.

Accretion builds larger bodies from smaller ones.

Accreted bodies may grow large enough to gravitate themselves into spherical shapes, but they are not yet planets ― just collections of debris.

COMPOSITIONAL SEPARATION of the larger bodies (protoplanets) Near the end of the accretion process, the larger objects had enough internal heat to melt, separating into a heavy core, a lighter mantle and a low-density crust. Finally, full-fledged planets came into existence.

CRATERING of planets and moons by leftover planetesimals Excess debris was CLEARED from the nebula, partly through cratering, partly through the tugs of other planets and partly through the young Sun’s activity.

CLEARING of the Solar Nebula by solar wind, planetary encounters and other processes

INTERNAL EVOLUTION of the planets, as separate closed systems

Some notable EXCEPTIONS to the nice, neat theory may "prove the rule:" Venus's rotation is retrograde (E to W). Uranus's equator is tilted 98° to its orbital plane. Neptune's largest moon Triton is in a nearly circular but retrograde (CW) orbit. Comets have very large and very elliptical (hairpin) orbits, both direct (CCW) and retrograde (CW). Many of Jupiter's and Saturn's outer moons are in retrograde (CW) orbits. Catastrophic events DO fit into a uniformitarian picture as long as they are part of the overall process!