Do the Pieces

By combining the nebular and condensation theories, we have arrived at an explanation that appears to address the major constraints that we listed at the beginning of the chapter. Does that mean that this theory is "right"?

Perhaps.

But like any model, it is subject to future observations that might cause us to reject or revise it. Let's revisit some of the constraints that we outlined.

> A rotating cloud of gas, collapsing gravitationally, can account for the "counterclockwise" (as seen from the North Pole) orbit of the planets, rotation of the sun, and rotational orientation of their moons. What we are seeing in all of these is the direction of rotation of that original solar nebula.

> The rocky nature of the inner solar system, and the gaseous nature of the outer solar system follow directly from the temperature of these regions as the dust grains were formed. Only the heaviest materials (metals) survived intact close to the sun, whereas more fragile molecules (like water) survived in the outer reaches.

V The planets are all found close to the ecliptic because, as the solar nebula contracted gravitationally, it naturally flattened. This flattened disk was where the planets most likely formed.

V The existence and location of asteroids, comets, and other debris is a natural byproduct of the accretion and early gravitational interaction process.

Yet, as expressed here, the condensation theory does not account for absolutely everything we observe in the solar system. Various apparent anomalies and irregularities, which we shall consider when we discuss the planets in detail in Chapters 13, 14, and 15, exist. Do they threaten to topple the condensation theory?

Probably not.

For the theory allows for an element of randomness, primarily in the form of close encounters and collisions among the planetesimals and protoplanets, which probably influenced certain variations we see in the orbital motions and orientation of some of the planets. As we saw in the last chapter, it is very likely that our own moon is the remnant of a catastrophic collision between the earth and a planetesimal that was the size of Mars. That collision also likely explains the anomalous tip of the earth's rotational axis, and thus the seasons that grace our planet.

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