Having reviewed the components of the Solar System and the forces at work upon them, the course turns towards the events that brought these bodies into being. Starting with the Big Bang, we journey through the birth of atoms, early galaxy formation, and the ignition of our Sun. We then take a closer look at the formation of the Earth, our Moon, and the outer bodies of the Solar System.

This week focuses on the planetary bodies that orbit our Sun. Although all are composed of a mixture of ice, rock, and gas, we divide them into three categories according to their main composition (rocky, icy, and gaseous). This division conveniently divides the Solar System into regions. We take a virtual trip to the Earth, the Moon, Mars, and Venus to view their surfaces and atmospheres.

This week, starting with Earth, we explore atmospheres (the layers of vapor that surround some of the planets) and their effects on all the planetary bodies. After all, a planet’s atmospheres—or its absence—is as responsible as its crust or its interior for defining a planet and shaping the conditions on its surface. We journey to the far reaches of the Solar System to view the atmospheres of the outer planets and Saturn’s moon Titan and learn how dramatically these atmospheres differ from our own.

Is the controversy over Pluto's status semantic or scientific? We begin this week with a history of how early astronomers described the objects they discovered orbiting the Sun, and how the list of planets has changed over time. The scientists that led the New Horizon mission to Pluto explain what they learned about the dwarf planet and even more distant objects in the Kuiper Belt. Pondering Pluto gives us a framework to think about what makes a planet a planet.

Are we alone? Most astrophysicists accept the likelihood that Earth is not unique in harboring life. But there is great debate about where extraterrestrial life might be found and what characteristics it might possess. This week we explore the link between life and liquid water, a precondition for life on Earth and perhaps anywhere. The final journeys of the course take us to the moons of Jupiter and Saturn, and then further away to the extrasolar Trappist system in the search for life.