1. Good Day Sunshine

How the solar system came into being

Age of the Solar System

Precisely when did the solar system originate? Its precise age is determined by examining primitive meteorites, ancient rocks returned from the Moon, and deep ocean sediments. These relics have remained unaffected by the erosion that removed the primordial record from most terrestrial rocks.

Their ages can be determined by measuring the relative amounts of radioactive materials and their non-radioactive products. When this ratio is combined with the known rates of radioactive decay, the time since the rock solidified and locked in the radioactive atoms is found.

Rounding off the numbers and allowing for possible systematic errors, we can say that the Earth, Moon and meteorites solidified at the same time some 4.6 billion years ago, with an uncertainty of no more than 0.1 billion years. If the solar system originated as one entity, then this should also be the approximate age of the Sun and the rest of the solar system.

Regular Planetary Orbits and the Nebular Hypothesis

Although Newton’s and Kepler’s laws describe the present behavior of the solar system, they cannot explain the remarkable arrangement. Some additional constraints are required, that describe the state of affairs before the planets were formed and set in motion. These initial conditions are provided by the nebular hypothesis, in which the Sun and planets formed out of a single collapsing, rotating cloud of interstellar gas and dust, called the solar nebula (Fig. 1.13).

Fig. .. 

Although Newton’s and Kepler’s laws describe the present behavior of the solar system, they cannot explain the remarkable arrangement. Some additional constraints are required, that describe the state of affairs before the planets were formed and set in motion. These initial conditions are provided by the nebular hypothesis, in which the Sun and planets formed out of a single collapsing, rotating cloud of interstellar gas and dust, called the solar nebula (Fig. 1.13).

Modern versions of the nebular hypothesis provide additional caveats, but the basic tenants of the original idea are still valid. Billions of years ago the spinning solar nebula, attracted by its own gravity, fell in on itself, getting denser and denser, until its middle became so packed, so tight and hot, that the Sun began to shine. The planets formed at the same time, within a flattened rotating disk centered on the contracting proto-Sun.

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Copyright 2010, Professor Kenneth R. Lang, Tufts University