Astronomers and geologists have several techniques for dating Earth, and, therefore, the age of the solar system. From the radiometric dating of rocks, which measures the known decay rates of radioactive elements, we know Earth and the solar system are approximately 4.6 billion years old. The knowledge does not come from Earth rocks, however, the oldest of which are about 3.9 billion years old. (Earth rocks are constantly involved in vigorous erosion — by plate tectonics and volcanism — making the oldest rocks on Earth extremely hard to find.)
Instead, meteorites — chunks of asteroids, the Moon, and Mars — make dating the solar system more accurate. These bodies were left in more pristine form. The oldest radiometrically dated thus far are 4.6 billion years old, and so the solar system itself must have formed near this time.
While many ideas in astronomy have changed radically over time, the notion of how the solar system formed has changed little in the last 250 years. In 1755, German philosopher Immanuel Kant first proposed the nebular hypothesis, in which a great cloud of material, the solar nebula, preceded the Sun and planets.
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In 1796, French astronomer Pierre Simon Laplace put forth a similar theory. Although he was unable to draw on supporting evidence from observations of deep space, Kant proposed the solar nebula was part of a much larger cloud of gas and dust that fell in by the weight of its own gravity and began to rotate. This gravitational contraction led to the formation of planets, both gaseous and rocky. Although the scope of knowledge about how this happened has grown considerably since Kant’s time, the basic idea is the same, and it has been borne out by repeated bits of evidence.
Astronomers now know when the solar system’s molecular cloud began to collapse, it measured 100 astronomical units across (1 astronomical unit is the average distance between the Sun and Earth) and had about two or three times the Sun’s mass. The cloud’s gravitational collapse may have commenced by the flash of a nearby supernova and the resulting pressure wave.