Kissing moons may explain why Earth's moon is so large

Dwarf planet Pluto and our Earth are the only two worlds in our solar system with very large moons. These may have come about by a "kiss and capture" process, which preserves a moon's large size.

One of the mysteries in our solar system is why our moon is so large compared to our planet. At 3,476 kilometres in diameter, our moon is about one quarter the width of the Earth. 

Most of the other moons in our solar system are a tiny fraction the size of their parent planets. Most of the these relatively small moons were likely asteroids gravitationally captured by the planets they now orbit around. Whereas our solar system's oldest moons, such as the inner moons of Jupiter and Saturn, likely formed during the emergence of our solar system from the disk of gas and dust that rotated around the early sun.

The only other exception is dwarf planet Pluto, which itself is small at 2,250 kilometres across, but has a moon, Charon, that is half its size.

This large planet to moon ratio is sometimes referred to as a double-planet system rather than planet and moon because both objects are orbiting each other rather than a small moon going around a big planet.

So how did Earth and Pluto get such large moons?

Normally, planets like Mars, Jupiter and Saturn acquire moons by gravitationally capturing them, a process that wouldn't happen with larger moons. 

Scientists at the Southwest Research Institute in Texas who study the Pluto-Charon system have come up with a model that could solve the mystery. Their research was published this month in the journal Nature Geoscience.

Both Pluto and Charon began as independent objects in the Kuiper Belt, a band of icy objects that surrounds our solar system out beyond the orbit of Neptune.

The new research suggests that, at some point, they approached each other at relatively low speeds, made contact and stuck together in a cosmic kiss, becoming a double-lobed body resembling a dumbbell. 

Over time, they separated again but remained in orbit around each other, captured by their mutual gravity.

This relatively gentle kiss and capture process allows both objects to retain their original size. 

The Earth's moon is believed to have formed from a similar process, the researchers explained in a written statement, but it was more like a slap in the face rather than a kiss.

Billions of years ago, a Mars-sized object named Theia slammed into Earth in a violent collision that destroyed most of Theia and sheared off part of the Earth's surface.