MOON • THEIA • PLANETARY FORMATION
Could the Moon Have Formed in Five Hours?
New simulations add a neglected ingredient to the giant-impact problem: the temperature-dependent strength of planetary rock.
SCIENCE UPDATE • OCTOBER 2026
The headline needs a qualifier
A 2026 study does not establish that Earth's Moon actually formed in five hours. It demonstrates that, under some modeled conditions, a large coherent satellite can emerge within roughly five hours of the giant impact between the proto-Earth and a Mars-sized impactor commonly called Theia. The distinction matters: this is a physically plausible pathway revealed by simulations, not a measured stopwatch for lunar history.
The classic giant-impact picture
For decades, a leading family of models has pictured Theia striking the young Earth roughly 4.5 billion years ago. The collision places hot rock into orbit, creating a protolunar debris disk. Material in that disk subsequently accretes into the Moon. Giant-impact calculations often treated the colliding planets largely as strengthless fluids—a defensible approximation when shock pressures, gravity, melting and vaporization dominate the physics.
What the 2026 simulations changed
Researchers led by C. Adeene Denton at Southwest Research Institute, working with University of Arizona scientists, used smoothed-particle hydrodynamics simulations that include temperature-dependent material strength. Rock's ability to resist deformation changes with temperature, so otherwise similar collisions can evolve differently depending on the thermal state of Earth and Theia.
In some simulations, Theia is disrupted and a protolunar disk forms. In another canonical case that included material strength, the rebounding impactor behaved differently and a large intact satellite emerged within about five hours. The work therefore adds strength and temperature to impact angle, velocity and composition as important variables in models of the Moon's birth.
Five hours does not mean a finished modern Moon
The result concerns the rapid dynamical emergence of a coherent satellite. Cooling, differentiation, crust formation, tidal evolution and the Moon's outward orbital migration are much longer processes. The simulated body also need not reproduce every property of the present Moon.
Why temperature may reveal timing
Young protoplanets cool as they age. Because temperature influences material strength, the physical state of Earth and Theia at impact could potentially connect the mode of Moon formation to when the collision occurred. That makes present-day lunar composition and volatile content possible clues to the thermal state of the colliding worlds.
The isotope problem remains
Any successful origin model must confront the remarkable compositional similarity between Earth and Moon material. The new strength-based simulations change the mechanics of the collision, but they do not by themselves settle why Earth and Moon are so isotopically alike. Possibilities explored in lunar-origin research include extensive mixing and Earth and Theia having formed from compositionally similar material in the same broad region of the early Solar System.
Does this replace the giant-impact hypothesis?
No. It changes a possible outcome within giant-impact modeling. The central collision remains; the open question is whether that collision primarily produced a disk that later assembled into the Moon, rapidly captured a large coherent remnant, or followed another pathway consistent with all of the geochemical and dynamical evidence.
MKP takeaway
The important discovery is not simply “the Moon formed in five hours.” It is that planetary geology can alter the outcome of a giant collision. A physical property once considered secondary—rock strength—may determine whether the aftermath is a debris disk or a large satellite almost immediately. That is a reminder that better physics can change the story even when the underlying giant-impact idea survives.
Sources & further reading
Southwest Research Institute — New scenarios for Moon formation →
The Astrophysical Journal Letters — Collisional Capture of an Intact Moon Depends on Strength →
NASA — Earlier high-resolution simulations of rapid Moon formation →