Mercury, the smallest planet in our solar system and the one that orbits closest to the Sun, has long baffled scientists. Unlike its rocky siblings—Earth, Venus, and Mars—Mercury hides an unusual secret within: a massive metallic core that makes up nearly 70% of its total mass. Wrapped around it lies only a thin shell of silicate rock, a mantle and crust that are astonishingly small compared to the other terrestrial planets.
For decades, the leading explanation for this imbalance pointed to a violent, catastrophic event. Scientists believed that long ago Mercury may have suffered a colossal collision with a massive body. In this scenario, the impact blasted away much of the planet’s rocky mantle, leaving behind the dense metal-rich world we see today. But there has always been a problem with this idea: simulations show that such extreme, unequal collisions are vanishingly rare. The odds of Mercury’s unique structure being the result of such an improbable accident never sat comfortably with planetary scientists.
Now, a new study suggests that Mercury’s story might be more subtle—and perhaps more common—than once thought.
A New Hypothesis Emerges
In research recently published in Nature Astronomy, a team led by Patrick Franco, an astronomer with a Ph.D. from the National Observatory in Brazil and now a postdoctoral researcher at the Institut de Physique du Globe de Paris, offers a fresh perspective. Their findings suggest that Mercury did not require an extraordinary cosmic accident to become the planet it is today. Instead, its strange composition may have been shaped by a far more ordinary event in the early solar system: a grazing collision between two planetary embryos of similar size.
“Through simulation, we show that the formation of Mercury doesn’t require exceptional collisions,” Franco explains. “A grazing impact between two protoplanets of similar masses can explain its composition. This is a much more plausible scenario from a statistical and dynamic point of view.”
This shift in thinking represents an important moment in planetary science. Instead of treating Mercury as the product of a one-in-a-million event, researchers can now see its formation as part of a larger, natural process of planetary growth and competition in the crowded, chaotic nursery of the early solar system.
A Solar System in Chaos
To understand this new model, it helps to travel back in time—4.5 billion years, when the solar system was young. In those days, the space near the newborn Sun was filled with planetary embryos: rocky worlds in the making, ranging in size from the Moon to Mars. These bodies orbited chaotically, tugging on each other’s paths through gravity, colliding, merging, or shattering apart.
“They were evolving objects, within a nursery of planetary embryos, interacting gravitationally, disturbing each other’s orbits, and even colliding, until only the well-defined and stable orbital configurations we know today remained,” Franco explains.
In this turbulent environment, direct head-on collisions between large bodies of very different sizes were rare. But glancing blows—where two planets of roughly equal mass brushed past each other, exchanging energy and stripping material—were much more common. Franco and his colleagues wondered whether such an event could have sculpted Mercury’s odd metallic heart.
Simulating a Cosmic Collision
To test this idea, the researchers turned to advanced computational modeling, using a method called smoothed particle hydrodynamics (SPH). This numerical technique allows scientists to simulate gases, liquids, and solids as they move, collide, and deform. SPH is widely applied in astrophysics, cosmology, and even engineering and computer graphics, because it can realistically capture the dynamics of large, violent events.
The method is rooted in the Lagrangian approach to mechanics, which traces how individual points—or “particles”—move through time and space. Unlike the Eulerian method, which focuses on fixed points, the Lagrangian perspective lets scientists follow the motion of each fragment of matter during a collision, giving a far more detailed view of what happens when worlds collide.
Using SPH, Franco’s team simulated near-collisions between protoplanets of similar size. What they found was remarkable: such an event could indeed strip away much of a planet’s mantle while preserving its metallic core. In fact, their simulations reproduced Mercury’s current mass and its unusual ratio of metal to silicate with less than 5% error.
“We assumed that Mercury would initially have a composition similar to that of the other terrestrial planets,” Franco explains. “The collision would have stripped away up to 60% of its original mantle, which would explain its heightened metallicity.”
Where Did the Missing Mantle Go?
One lingering question in Mercury’s mystery has always been: if a collision tore away much of the planet’s rocky mantle, where did all that material go?
Previous models struggled with this. They suggested that most of the ejected matter would eventually fall back onto the planet, negating the effect and restoring its mantle. But the new simulations propose a different outcome. Depending on the initial conditions of the impact, some of the stripped material may have been ejected into space and never returned.
This opens fascinating possibilities. If Mercury lost its mantle material in a glancing collision, perhaps that debris didn’t just disappear. Franco suggests that some of it could have been captured by neighboring planets—possibly even Venus. Such an exchange of planetary material would mean that the inner planets are not as isolated in their histories as once imagined, but part of a shared story of cosmic recycling.
A Broader Implication for Planetary Science
The significance of this study extends beyond Mercury. If grazing collisions between similarly sized protoplanets were common in the early solar system, they may have shaped other worlds as well. Understanding how these interactions stripped, mixed, or redistributed material could help explain not only Mercury’s metallic nature but also broader questions about planetary differentiation and diversity.
“This model can be extended to investigate the formation of other rocky planets and contribute to our understanding of differentiation processes and material loss in the early solar system,” Franco says.
The next step will be to compare these simulation results with geochemical data—from meteorites, from Earth’s rocks, and from space missions that have studied Mercury directly. The European Space Agency and the Japan Aerospace Exploration Agency’s joint BepiColombo mission, currently on its way to Mercury, will provide crucial data. Its detailed measurements of the planet’s surface and magnetic field could help confirm or challenge this new collision model.
Mercury’s Future in Exploration
Despite being one of the closest planets to Earth, Mercury remains the least explored of the inner worlds. Its proximity to the Sun makes missions technically challenging, as spacecraft must fight the star’s immense gravity and withstand extreme heat. Only a handful of missions—NASA’s Mariner 10 and MESSENGER, and now ESA-JAXA’s BepiColombo—have dared to visit it.
But interest in Mercury is growing. The planet is not just a curiosity—it is a laboratory for understanding how rocky planets form and evolve. Its oversized core, strange surface chemistry, and unique magnetic field all hold secrets about the early solar system. As Franco emphasizes, “Mercury remains the least explored planet in our system. But that’s changing. There’s a new generation of research and missions underway, and many interesting things are yet to come.”
A Planet Forged by Chance
The story of Mercury is one of cosmic violence and survival. Born in a crowded field of planetary embryos, it may have been shaped by a glancing collision that tore away much of its outer layers, leaving behind the dense metallic world we see today.
This perspective makes Mercury not an outlier, but a survivor—a remnant of the chaotic dance of creation in which planets collided, merged, and transformed. Its existence reminds us that our solar system was once a place of fire and fury, where the destinies of worlds could change in an instant.
And perhaps, in Mercury’s enduring metallic heart, we see not only the scars of its past but also a key to understanding our own planet’s origins. For in every collision, every fragment, every strange survivor like Mercury, lies part of the larger story of how our solar system came to be—and how we, billions of years later, came to wonder about it.
More information: Patrick Franco et al, Formation of Mercury by a grazing giant collision involving similar-mass bodies, Nature Astronomy (2025). DOI: 10.1038/s41550-025-02582-y. On arXiv: DOI: 10.48550/arxiv.2503.02826
