How did the Moon form?

The Moon is a celestial object that orbits the earth and is our only natural satellite. But how did it form? Astronomers have yet to pinpoint the origins of our Moon, but several leading hypotheses have been proposed over the past few decades.

However, the leading theory is referred to as the "Giant-impact hypothesis". 

The Giant-impact hypothesis

This theory postulates that a Mars-sized protoplanet, (a planet that is in its developing stages) named 'Theia' after the female Titan Theia from Greek mythology, collided with the earth approximately 4.5 billion years ago to form the Moon. This hypothetical event took place roughly 20 - 100 million years after the formation of the Solar System. It is currently the preeminent hypothesis in terms of explaining the formation of the moon as aforementioned. 

Artist's rendition of Theia colliding with a primitive earth

There are several reasons why that is so, some of which are described below : 

- Lunar rock samples indicate that the Moon's surface was once molten

- Evidence of similar events have been observed in other star systems

- There's a common origin since isotope ratios of terrestrial and lunar rock are identical

- Giant collisions are congruous with the predominant theories that explain the formation of the Solar System

Other theories

Accretion

This hypothesis suggests that the earth and the Moon attracted each other in the pristine accretion disk of the Solar System.  An accretion disk is made up of various rocks, gases, etc. which orbit a central massive body.

An accretion disk around a suspected black hole in galaxy NGC 4261, an elliptical galaxy about 98 mega light years away.

Capture

This hypothesis implies that the earth captured the Moon. Evidence of this hypothesis includes tidal locking and the Moon's size as well as orbit. However, the conflict with this hypothesis is that close encounters with with earth of this proportion usually result in a clash of orbital trajectories. Earth's atmosphere must've had to be significantly larger for this happen, so it could slow down the Moon before it collided or escaped.