The planet that hit Earth is called Theia, a roughly Mars-sized body that collided with our young planet about 4.5 billion years ago. That collision, known as the Giant Impact, flung enough molten and vaporized rock into orbit to form the Moon. Theia wasn’t a rogue object from deep space. Recent research published in Science suggests it originated in the inner solar system, making it a neighbor of the early Earth before the two worlds collided.
What Was Theia?
Theia was a protoplanet, one of many small, partially formed worlds jostling for space in the early solar system. It’s estimated to have been about the size of Mars, which means it was roughly half the diameter of Earth and perhaps a tenth of its mass. The name comes from Greek mythology: Theia was the mother of Selene, the goddess of the Moon.
At the time of the collision, Earth itself was still forming. The solar system was only about 60 million years old, and the inner planets were still sweeping up leftover material. Theia’s orbit gradually became unstable, eventually putting it on a direct collision course with the proto-Earth.
How the Impact Happened
The collision wasn’t a head-on crash. Computer simulations consistently show it was an oblique, glancing blow at roughly the speed two bodies would reach falling toward each other under their own gravity (called the mutual escape velocity). This angle and speed were critical. A direct hit at higher speed would have shattered both bodies in a way that couldn’t produce the Earth-Moon system we see today.
The impact was extraordinarily violent. It liquefied and partially vaporized both worlds, launching a vast arc of superheated debris into orbit around what remained of Earth. Within about a day, material with orbits too low had crashed back into the planet’s surface, while the rest settled into a ring-shaped disk of molten and vaporized rock. Temperatures in this disk reached roughly 3,000 to 4,000 degrees Kelvin, hot enough to turn silicate rock into a mix of vapor and liquid. From that disk, the Moon gradually accumulated just beyond the point where Earth’s gravity would have torn it apart.
The process was remarkably efficient. Simulations show that less than 5% of the disk material escaped into space. Nearly everything that didn’t become the Moon fell back onto Earth.
How Scientists Know This Happened
The strongest evidence comes from the chemistry of Moon rocks brought back by the Apollo missions. Every object in the solar system has a slightly different chemical fingerprint, particularly in the ratios of different forms of oxygen. Planets that formed in different parts of the solar system have measurably different oxygen signatures. Mars is different from Earth, Earth is different from meteorites from the asteroid belt, and so on.
The Moon, however, is a near-perfect match. High-precision measurements published in the Proceedings of the National Academy of Sciences show that Earth and the Moon are isotopically identical to within 0.2 parts per million. That’s an astonishing level of similarity, and it tells scientists that the Moon formed from the same pool of material as Earth’s outer layers. A capture scenario (where the Moon formed elsewhere and was grabbed by Earth’s gravity) can’t explain this match.
Tungsten isotopes tell a complementary story. A specific form of tungsten that results from radioactive decay shows that before a late phase of asteroid bombardment, the Earth’s mantle and the Moon were indistinguishable in their tungsten signatures as well. This points to thorough mixing of both bodies’ material during or shortly after the impact.
Pieces of Theia May Still Be Inside Earth
One of the most striking recent findings is that parts of Theia may still exist deep inside our planet. In the 1980s, geophysicists discovered two continent-sized structures sitting near the base of Earth’s mantle, one beneath Africa and one beneath the Pacific Ocean. These formations, called large low-velocity provinces, slow down seismic waves passing through them, suggesting they’re denser and chemically different from the surrounding mantle rock. They appear to be unusually rich in iron.
A 2023 study from Arizona State University made the case that these blobs are actual remnants of Theia. Using simulations of mantle behavior over billions of years, the researchers showed that dense, iron-rich material from Theia could have sunk to the bottom of Earth’s mantle after the impact and stayed there for the entire 4.5-billion-year history of the planet. As ASU professor Mingming Li put it, the Moon and these deep-Earth blobs share the same origin: both are pieces of the world that crashed into ours.
The Synestia Model
The classic version of the Giant Impact hypothesis has one nagging problem. If the Moon formed mostly from Theia’s material (as older simulations suggested), it should have a different chemical composition from Earth. But it doesn’t. This puzzle has led to an updated model called the synestia hypothesis.
In this version, the impact was even more energetic than traditionally assumed. Instead of simply knocking debris into orbit, it transformed the entire Earth into a massive, donut-shaped cloud of vaporized rock extending tens of thousands of kilometers into space. This structure, called a synestia, is neither a planet nor a disk but something in between. As the synestia cooled, droplets of rock condensed out of the vapor, and those droplets gradually built up into the Moon while still surrounded by bulk silicate vapor at tens of bars of pressure.
The key advantage of this model is that it naturally explains why the Moon’s chemistry matches Earth so closely. The Moon didn’t form from a separate chunk of Theia. It condensed from a well-mixed vapor that blended material from both Theia and proto-Earth, effectively erasing any chemical differences between the two. The Moon inherited Earth’s isotopic fingerprint because it literally crystallized out of the same cloud of rock vapor.
What Theia Left Behind
The collision reshaped Earth in ways that persist to this day. It likely gave Earth its current spin rate, tilted its axis, and delivered enough energy to melt the entire surface into a global magma ocean. The Moon that formed from the impact has been slowly receding from Earth ever since, and its gravitational pull stabilizes Earth’s axial tilt, contributing to the relatively stable climate cycles that allowed complex life to evolve.
The Moon’s first solid crust, formed as lighter minerals floated to the top of its own magma ocean, dates to about 4.4 billion years ago. That early crust is what Apollo astronauts sampled in the bright highland regions of the lunar surface, providing the rock samples that ultimately confirmed the giant impact origin story.