The Moon most likely formed about 4.5 billion years ago, when a planet about the size of Mars, called Theia, struck the young Earth. The collision threw hot rock and vapour into orbit, and that debris gathered into the Moon. This is called the giant impact hypothesis, and it is the leading explanation among scientists, though some details are still debated.
This guide explains the evidence for it, the puzzle that remains, where Theia may have gone, and how it compares with older ideas.
The Moon’s origin at a glance
| Fact | Detail |
|---|---|
| Leading theory | Giant impact (Theia) |
| Age | About 4.5 billion years |
| Size of Theia | About the size of Mars |
| Moon’s width | 3,474 km, about a quarter of Earth’s |
| Moon’s mass | About 1/81 of Earth’s |
| Density | About 3.3 g/cm³, lower than Earth’s 5.5 g/cm³ |
| Rock samples returned | About 382 kg from the Apollo missions alone |
For the Moon’s size and distance, see our guide to measuring the Moon’s distance with lasers.
Explore the theory
The widget below has two parts. First, step through the giant impact from Theia’s approach to the Moon’s slow drift outward. Second, compare the idea against its rivals and see where each one succeeds or struggles.
How did the Moon form?
Step through the leading explanation, then test it against the rivals.
1. The giant impact, step by step
2. Rival ideas, tested against the evidence
Scenes are simplified and not to scale. This is the leading scientific explanation, but details such as how Theia’s material mixed with Earth’s are still debated. Learn how the Moon’s distance is measured in our guide to lunar laser ranging. Sources: NASA and the Apollo sample studies.
What is the giant impact hypothesis?
The hypothesis says that early in the solar system's history, the young Earth was hit by another planet-sized body. That body is called Theia, after the Greek titan who was the mother of the Moon goddess Selene.
In this scenario:
- Theia struck Earth at an angle, with a glancing blow.
- The collision melted large parts of Earth and vaporised rock.
- Most of Theia's material merged with Earth, while the rest, together with a lot of Earth's outer rock, was thrown into orbit.
- The debris formed a hot disk around Earth.
- Clumps in the disk merged into the Moon. Depending on the computer model, this took anywhere from months to about a century.
The idea was proposed in the 1970s and gained wide support in the 1980s. Computer simulations have refined it ever since.
What is the evidence for the giant impact?
Several lines of evidence point to it.
The Moon has a small iron core. Earth's iron core makes up about half of its width. The Moon's core is much smaller, only about a fifth of its width, and the Moon's density is lower than Earth's. That fits a Moon built mostly from the rocky outer layers of the colliding bodies.
The Moon was once molten. Apollo samples from the bright highlands are rich in a pale rock called anorthosite. It is thought to have floated to the top of a global ocean of molten rock, which suggests the young Moon was extremely hot, as you would expect after a violent birth.
The Earth–Moon system spins fast. The combined spin and orbital motion of Earth and the Moon is large. A glancing collision can explain it, as simulations show.
The Moon is dry in some elements. Moon rocks have less of certain easily evaporated elements than Earth rocks. This fits material that was heated to very high temperatures.
The Moon is old. Dating of Apollo rocks shows the Moon formed around 4.4 to 4.5 billion years ago, soon after the solar system itself.
The isotope puzzle
There is one big puzzle. Moon rocks and Earth rocks have almost exactly the same mix of oxygen isotopes, which are slightly different forms of the same element. Different bodies in the solar system usually have different mixes. If the Moon were built partly from Theia, you would expect a noticeable difference.
Scientists have proposed several fixes:
- Theia was chemically similar to Earth. The two may have formed from material in the same part of the solar system.
- Mixing was thorough. A violent, high-energy impact could have blended the material from both bodies in the disk.
- A fast-spinning Earth. A more energetic collision with a rapidly spinning young Earth could have thrown out more of Earth's own rock.
- A synestia. In 2017, researchers suggested that the impact turned Earth and the debris into a huge, spinning, doughnut-shaped cloud of vapour called a synestia, which then condensed into Earth and the Moon.
- Multiple smaller impacts. Some models suggest the Moon came together from many smaller collisions.
No single version has won out yet, which is why the theory is still an active area of research.
Where did Theia go?
If Theia hit Earth, some of it should still be inside us. A 2023 study suggested that two huge, hot, dense blobs deep in Earth's mantle, one under Africa and one under the Pacific, could be remnants of Theia. This is an intriguing hypothesis, but it is not proven, and other explanations for those blobs exist.
Theia itself no longer exists as a separate planet. It was absorbed into Earth and the Moon.
What other ideas have been proposed?
Before the giant impact hypothesis, scientists suggested three other explanations. Try them in the widget above.
- Fission: George Darwin proposed in the 1800s that a fast-spinning Earth flung off a piece of itself. It explains the matching chemistry but fails on the Earth–Moon system's spin.
- Capture: Earth captured a passing body. It would struggle to explain the match with Earth's chemistry, and capturing something so large is difficult.
- Co-formation: Earth and the Moon formed side by side. It would give the Moon far more iron than it actually has.
Why does the Moon matter for Earth?
The Moon is not just a bystander. It affects Earth in several ways:
- Tides. The Moon's gravity raises the ocean tides, and tidal pull is slowly pushing the Moon away.
- A steadier tilt. Calculations suggest the Moon helps keep Earth's tilt stable, which makes climate more predictable over long periods of time. Learn more about what makes a planet suitable for life in our habitable zone guide.
- Longer days. Right after the impact, Earth spun once every few hours. Tidal forces have slowed it to 24 hours.
How do we keep testing the theory?
The best tests come from rocks. Apollo astronauts brought back about 382 kg of lunar samples between 1969 and 1972, and Soviet Luna missions added small amounts. China's Chang'e-5 mission returned samples in 2020, and Chang'e-6 brought back the first samples from the far side of the Moon in 2024.
New missions, including NASA's Artemis programme, aim to collect samples from new regions, such as the south pole. See Artemis pictures from space for more, and read about the earlier missions in our space exploration history section.
Common myths
- "Theia has been found." No. Theia is a hypothesis about a body that no longer exists. Scientists look for traces of it.
- "The Moon is a captured asteroid." This idea does not fit the evidence, because the Moon's chemistry is too close to Earth's.
- "The giant impact theory is settled." The overall picture is widely accepted, but details such as the origin of the matching isotopes are still debated.
Frequently asked questions
How was the Moon formed?
Most scientists think a Mars-sized planet called Theia hit the young Earth about 4.5 billion years ago, and the debris formed the Moon.
How old is the Moon?
About 4.5 billion years old, based on dating of Apollo lunar samples.
What was Theia?
A hypothetical planet about the size of Mars that collided with the early Earth.
Is the giant impact theory proven?
It is the leading explanation, supported by lunar samples and computer models, but some details are still unresolved.
How quickly did the Moon form?
Models suggest anywhere from months to about a century, which is very fast on a cosmic timescale.
Why is the Moon so similar to Earth in composition?
That is the main puzzle. Possible answers include a very thorough mixing during the impact, or a Theia that was already similar to Earth.
Was the Moon always the same distance from Earth?
No. It formed much closer and has been drifting away ever since, at about 3.8 cm a year today.
How many moons does Earth have?
One permanent moon. See the full list in our guide to how many moons each planet has.
Key takeaways
- The Moon most likely formed about 4.5 billion years ago from debris of a giant impact between Earth and a Mars-sized planet called Theia.
- Evidence includes the Moon's small iron core, its molten past and its old age.
- The big puzzle is why the Moon's isotopes match Earth's so closely.
- Older ideas, such as fission and capture, fit the evidence less well.
- New samples from the Moon will keep testing the theory.
Continue exploring in our Solar System guide.