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? Ŷ