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Remnants from the Beginning

Yet many remained, and are still with us today: a small planet (and maybe more than one), icy planetesimals that orbit in the frozen realm beyond the planets, and giant boulders closer to the Sun, including some that cross Earth's orbit, presenting a potential threat to our planet.

All of these objects are important to astronomers because they help tell the full story of how the solar system formed and how it evolved. They also help scientists interpret their observations of other star systems, where big disks of material may be giving birth to planets.

Realm of the Giants

The solar system's outer realm contains four giant planets. The largest is Jupiter, at about 11 times Earth's diameter. Despite their great size, though, only two are easily visible to the unaided eye; the others are so remote that they were not discovered until after the invention of the telescope.

Exploring Our Own Back Yard

The structures of the terrestrial planets are similar. Each planet has a layered structure that developed through a process called differentiation. After the small bodies (called planetesimals) clumped together and melted, the gravity of the newborn planets pulled heavier elements to their centers, while lighter elements floated toward the surface.

Gemini, the Twins

Many cultures have seen two humans in this star pattern, which consists of two roughly parallel lines of stars capped by two of the brightest stars in the night sky. But the legend that endures is that of Castor and Pollux. Gemini's two brightest stars bear the names of the twins.

Canis Major, the Great Dog

The brightest star in Canis Major also is the brightest in the entire night sky — brilliant Sirius, which is just 8.6 light-years away. That's only twice as far as our closest stellar neighbor. Because it is the brightest star of the Great Dog, Sirius is known as the Dog Star. Its first appearance in the dawn sky in August heralds the "dog days" of summer.