Asked by William Cartwright
In short, we don’t currently know exactly why Charon is more than half the size of the dwarf planet Pluto, even though it is technically the latter’s moon.
Of all the planet-moon systems in our Solar System the one that most resembles that of Charon and Pluto is actually our own – the Earth and the Moon. The Moon is roughly a quarter of the size of the Earth. It’s therefore possible that they may well both have had a similar formation process: a collision with another large celestial body. The collision may have resulted in a large portion of the parent body to have separated off, but then captured as a moon.
Answered by Zoe Baily from the National Space Centre
Image Credit: NASA