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 composite of enhanced colour images of ... Read full article.