LECTURE

Differences in the materials in different orbits explain the differences between the planets. The young Sun’s heat drove more volatile elements away from the center, so they collected further out. As a result, the inner planets contained less matter and were dominated by rocky materials while the outer planets were larger and consisted mainly of gassy materials. The asteroids, at the border between these two regions (between Mars and Jupiter), may represent a failed planet that was broken up by the powerful gravitational force of Jupiter, the largest planet in our solar system. The Earth’s Moon was probably created by a collision between the young Earth and a Mars-sized protoplanet. This would have gouged material from the Earth’s outer layers, forming a small, dusty nebula from which the Moon formed by accretion. Material not swept up inside planets formed the other bodies of the solar system—its comets, asteroids, meteorites, and many of its moons—some of them orbiting in strange patterns because of erratic collisions.

What’s the evidence for this story? The idea that the Sun and solar system had all condensed from a single solar nebula that collapsed under the pressure of gravity was rst proposed by German philosopher Immanuel Kant in 1755. It ts what we know of the workings of gravity and the shape and structure of the solar system. It also ts recent observations of other regions where planetary systems appear to be forming.

We cannot determine the age of the solar system by dating Earth rocks because the Earth is so geologically active that its original surface is now unrecognizable. However, the surface of the Moon, like that of most meteorites and asteroids, has changed little since the time of accretion. The fact that all these objects yield similar dates increases the con dence of astronomers that the entire solar system was formed more or less at the same time, about 4.5 billion years ago.

This lecture has described how planetary systems form, including our own, the solar system. Now we focus on our home planet, the Earth. What was the early Earth like? How different was it from the Earth we know today?