The Earth has an unusually large moon. Most planet-moon ratios are small: Phobos and Deimos are tiny next to Mars; Jupiter's moons are dwarfed by Jupiter. The Earth-Moon system is closer to a double planet, with the Moon roughly a quarter of Earth's diameter.

Before the Apollo missions, scientists had three competing theories for the Moon's origin. Either it formed alongside Earth from the same cloud of dust. Or Earth's gravity captured a passing rogue body. Or — strangest of all — a chunk of early Earth somehow flung itself away. None of the three matched the data well.

Then Apollo astronauts brought back 380 kilograms of Moon rocks. The chemistry was strange. The Moon's rocks have the same isotopic composition as Earth's mantle. Identical, to a fraction of a percent. Captured rogue moons should have alien chemistry. Co-formed moons should differ in iron content. Self-flung chunks should match Earth's crust, not its mantle.

In the 1970s, two American scientists proposed the Giant Impact hypothesis. About 4.5 billion years ago — soon after Earth itself coalesced — a Mars-sized planet, called Theia, slammed into the proto-Earth at an oblique angle. The impact vaporized most of Theia and a chunk of Earth's mantle. The vapor cooled in orbit and condensed into a ring. The ring assembled into the Moon.

Theia's heavy core sank into Earth's molten interior. Earth absorbed it. There is, today, a sister planet living inside our planet's mantle.

The Moon stabilizes Earth's axial tilt, keeps our seasons regular, and may have made complex life possible. We owe its existence to a planetary murder.