Every other planet (eight now, right?) has several moons. And they are small compared to their planet. Why do we only have one, and why is it, comparatively, so small? Why hasn't Earth attracted in an asteroid or something to make a second moon?
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Earth actually DOES temporarily capture an asteroid as a second "moon", but it really doesn't orbit Earth. It's called Cruithne.
Earth's Moon was created from the Earth when an object with the mass of Mars impacted Earth at a glancing angle . Earth had already started to differentiate chemically so that it had am iron/nickel core, and a Fe/Mg rich mantle with titanium and very likely a basaltic crust. The Earth had considerable more mass before the impact, so it had a larger radius, and Earth's surface was at its gravitational Roche limit, like it is today. This massive body impacts the Earth at a glancing angle, then reflects off Earth. This celestial body even has a name, Theia. Theia's trajectory after the impact carries it out of the Solar System. Most of the mass of the ejecta from the impact gets thrown into orbit around the Earth INSIDE the Earth's FORMER Roche limit, where it coalesces to form the Moon, which has enough mass to chemically differentiate into a core, mantle and crust. The titanium that WAS in the Earth's mantle became part of the Moon's crust and near surface rocks. How do we know this? The Moon rocks that were bought back to Earth by the Apollo astronauts have a higher percentage of titanium than rocks from the crust and near surface of the Earth has. The Moon still has a liquid outer iron/nickel core that is 55 kilometers thick that still has not crystallized.
The Jovian planets ALL have large, spherical moons that are not captured asteroids. It is assumed, but not confirmed, that these moons formed by accretion processes similar to the processes that created the Solar System and the planets. Two planets and one dwarf planet were either impacted by multiple massive objects or one or two very massive objects that had enough momentum to flip the rotational of axes of Venus, Uranus AND Pluto so that they now rotate (NOT revolve around the Sun) retrograde. ALL of the planets and dwarf planets revolve around the Sun prograde, no matter what their orbital inclination to the ecliptic might be, counterclockwise when seen from "above" the Sun's north rotational pole.
Earth's Moon was created from the Earth when an object with the mass of Mars impacted Earth at a glancing angle . Earth had already started to differentiate chemically so that it had am iron/nickel core, and a Fe/Mg rich mantle with titanium and very likely a basaltic crust. The Earth had considerable more mass before the impact, so it had a larger radius, and Earth's surface was at its gravitational Roche limit, like it is today. This massive body impacts the Earth at a glancing angle, then reflects off Earth. This celestial body even has a name, Theia. Theia's trajectory after the impact carries it out of the Solar System. Most of the mass of the ejecta from the impact gets thrown into orbit around the Earth INSIDE the Earth's FORMER Roche limit, where it coalesces to form the Moon, which has enough mass to chemically differentiate into a core, mantle and crust. The titanium that WAS in the Earth's mantle became part of the Moon's crust and near surface rocks. How do we know this? The Moon rocks that were bought back to Earth by the Apollo astronauts have a higher percentage of titanium than rocks from the crust and near surface of the Earth has. The Moon still has a liquid outer iron/nickel core that is 55 kilometers thick that still has not crystallized.
The Jovian planets ALL have large, spherical moons that are not captured asteroids. It is assumed, but not confirmed, that these moons formed by accretion processes similar to the processes that created the Solar System and the planets. Two planets and one dwarf planet were either impacted by multiple massive objects or one or two very massive objects that had enough momentum to flip the rotational of axes of Venus, Uranus AND Pluto so that they now rotate (NOT revolve around the Sun) retrograde. ALL of the planets and dwarf planets revolve around the Sun prograde, no matter what their orbital inclination to the ecliptic might be, counterclockwise when seen from "above" the Sun's north rotational pole.