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.

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.

Two of the brightest stars in the evening sky lie at opposite corners of the rectangle: bright orange-red Betelgeuse at the northeastern corner and even brighter Rigel at the southwest.