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# 9.16 Gradual Evolution and a Few Catastrophies

Gas and dust condensed quickly into small particles throughout the collapsing solar nebula. Most planetesimals accreted onto the planets, scattered into the Oort cloud, or were left stranded in the asteroid belt within 50 to 100 million years, or the first 1% to 2% of the solar system’s age. The leftover planetesimals then crashed into the planets and their satellites, forming craters. Lunar rocks taught us that this process continued throughout the Solar System for first 500 million years, or about 10% of the Solar System’s age. At this point, the mayhem stopped, for the most part. Most planetesimals had disappeared from interplanetary space. The remaining gas and dust in the solar nebula had been blown away by the outward pressure of radiation and gases streaming out from the Sun. The comets and asteroids that remain today hit planets at a lower (and roughly constant) rate. They are basically leftover planetesimals.

TheÂ orbitsÂ of the bodies in the Solar System to scale (clockwise from top left). Click here for original source URL

Georges-Louis Leclerc, comte de Buffon. Click here for original source URL.

The growth of the planets according to this scenario produced certain regularities in the Solar System: planetary orbits that lie in the plane of the Sun's equator, regular spacing of planet orbits, orbits that move in the same direction as the Sun’s rotation, planets that spin in the same sense as they move around the Sun, and small axial tilts of planets. These regularities require smooth evolutionary growth from a system of many small planetesimals, not a catastrophic creation in a chaotic system. Early theories of Solar System formation tended to invoke rare catastrophes in order to explain the existence of planets. For example, French naturalist Georges Buffon suggested in the mid-1700s that the Sun had crashed into a passing star, and the resulting debris formed the planets. More recent calculations show that such collisions would be extremely rare. If planets required such an accident, less than one in a million stars would have planets. However, we have detected enough extrasolar planets to suggest that formation of planets may be a fairly "normal" process — a byproduct of how stars themselves form.

One of the advantages of the modern picture of planet formation is that it can explain the few irregularities of the Solar System by a few catastrophic events. These catastrophic events involved collisions or near-misses of the young planets with the remaining planetesimals. Most planetesimals were very small compared to planets, but the largest one or two near any given planet would have been an appreciable fraction of the size of that planet. As each planet grew, it experienced collisions of different magnitudes. If the largest collision was strong enough, it could have had lasting effects on the planet.

Image of Uranus. Uranus' rotation axis lies nearly in the plane of the solar system, instead of nearly perpendicular like the other planets. Click here for original source URL.

Artist's depiction of the giant impact that is hypothesized to have formed the Moon. Click here for original source URL.

There are several examples of this in the Solar System. A Mars-sized planetesimal may have hit the Earth late in its growth. The material that was blown off coalesced into the Moon. Uranus was probably also hit by a relatively large planetesimal, tipping its rotation axis to lie nearly in the plane of the Solar System, instead of perpendicular to it like most other planets. Other properties of the Solar System — such as the geological differences between the hemispheres of Mars, Venus’s retrograde rotation, and Mercury’s large core — may all trace their origins back to large impacts. The largest impacts may not have been big enough to completely randomize the properties of planets and their orbits, but they were large enough to give planets their individuality!

The “detective work” that led to a theory for the formation of the Solar System was very successful. Astronomers can account for all the general features. However, this raises some general questions about the scientific method. Do we know the theory is unique, or could there be another explanation for these facts? How many "peculiarities" do we have to explain away before the formation scenario is compromised? Remember that we are engaged in a form of archaeology; all the relevant events took place several billion years ago. The legend of the Cyclops started long ago when hunters found the skull of an elephant and misinterpreted the hole where the trunk connects for a large, central eye socket. Could we be misinterpreting evidence, too? At present, most scientists don’t think so.

The theory of formation may not get every detail of the Solar System right, but it accounts for the main features and it has great explanatory power. The physical principles of gravitational collapse and accretion are well understood. The limitations of the theory reflect a fundamental property of complex physical systems: they are not deterministic. Determinism means that the entire history of the Solar System can be predicted, once you know the starting conditions. We know this is not true. Orbits are chaotic so small perturbations lead to large changes in the eventual outcome. This is not a deterministic process. We might have enough confidence to predict that planets are a natural consequence of star formation, but we can’t say how many planets might form around a star, or what their characteristics might be, because the system is chaotic. There’s only one way to find out — we have to look.