In the crowded early Solar System, young planets frequently collided and reshaped each other. But Mercury stood out. It formed unusually close to the Sun. Mercury’s days are longer than its years. It lacks an atmosphere and despite being closes to the Sun, it’s not as hot as Venus. But the strangest thing about Mercury is its core. Mercury shouldn’t exist—or at least, not like this.
Mercury is tiny, barely bigger than the Moon. Its metallic core makes up 70% of the planet’s mass, vastly exceeding Earth’s 32% and Mars’ 25%. It’s unlikely that the core actually formed like this. Instead, researchers suspect that Mercury must have formed like a bigger planet but somehow lost some of its mass.
For years, the best guess was that a massive impact had stripped away most of Mercury’s outer layers. But new research suggests the answer may lie in a subtler, more likely type of collision: a cosmic sideswipe between planetary siblings.
A Mystery in Plain Sight
The so-called “Mercury Problem” is an annoying one. Back in the 1960s, ground-based radar observations started shedding new light on the planet. Some observations suggested an abnormally large core, but it wasn’t until NASA’s MESSENGER mission (2010–2015) that scientists could measure it precisely.
MESSENGER studied Mercury’s chemical composition, geology, and magnetic field. MESSENGER never “saw” Mercury’s core directly. It had no instruments that could do that. Instead, it worked like a detective, using a combination of gravity mapping, spin tracking, and magnetic field measurements to infer the size and structure. This analysis revealed a huge, partly molten core making up about 85% of Mercury’s radius.
The canonical explanation is an ancient, catastrophic collision with a much larger body. But this story had a problem of its own. “Most studies assume a binary collision between bodies of substantially different masses, which seems to be unlikely according to N-body simulations,” write Patrick Franco and colleagues in Nature Astronomy.
Put simply, simulations show that such unequal-mass smashups are rare. To hit a small body like proto-Mercury with something six times smaller or larger, and do it just right to strip away the rocky mantle but leave the metal core intact, would require orbital gymnastics that don’t often occur.
So Franco and his team tried something different.
The Grazing
Instead of imagining Mercury as a battered survivor of a freak accident, the researchers explored a more commonplace event: a grazing impact between two similarly sized protoplanets.
“Through simulation, we show that the formation of Mercury doesn’t require exceptional collisions,” Franco explained in a press release. “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.”
Using a technique called smoothed-particle hydrodynamics (SPH), the team simulated hundreds of planetary collisions. The method, popular in astrophysics, models materials like rock and metal as millions of fluid-like particles that interact under gravity and pressure. Their target was to see if they could strip away enough rock to produce a Mercury-like world—without violating known physics, or the odds.
They succeeded. One particularly promising simulation involved a proto-Mercury with 2.36 times Mercury’s current mass, colliding at an angle of 32.5° with a slightly smaller planet. The aftermath: a remnant just 5% larger than Mercury, with a metal-to-silicate ratio almost identical to the real thing.
Where Did the Debris Go?
In earlier impact models, most of the rock ejected during the collision would fall back onto Mercury, diluting its metal-rich core. That didn’t fit the observations. But in Franco’s model, much of the debris escapes altogether.
“Depending on the initial conditions, part of the material torn away may be ejected and never return,” Franco said. “If the impact occurred in nearby orbits, one possibility is that this material was incorporated by another planet in formation, perhaps Venus.”
This idea matches what we know about planetary dynamics in the early Solar System. Simulations show that these types of grazing collisions, known as hit-and-run impacts, were common during the first 100 million years of planet formation. Planetary embryos jostled and slammed into each other like cosmic billiard balls, competing for space and mass.
“It was a planetary battlefield,” Franco stated. “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.”
Highly Probable
What makes this new scenario stand out is its statistical plausibility. We don’t have any “smoking gun” type evidence, but it just seems more plausible. Unlike the rare giant-impact hypothesis, which requires highly eccentric orbits and a nearly impossible set of conditions, grazing impacts between similar-sized objects occur in up to 20% of planetary formation simulations.
Even better, the new models don’t need to assume a strange chemical environment. “We assumed that Mercury would initially have a composition similar to that of the other terrestrial planets,” Franco said. The collision alone is enough to strip away 60% of the mantle and leave behind a core-heavy world.
The researchers tested dozens of configurations, tweaking the impact angles and speeds. They found a narrow band of “Goldilocks” collisions—not too direct, not too shallow—that consistently produced Mercury analogues. In one simulation, a remnant formed with a mass of 0.056 Earth masses and a core comprising 68% of its weight. Mercury’s actual mass? 0.055. Its core? Roughly 70%.
But the reality of it is that even as simulations paint a clearer picture, we’re still just starting to understand Mercury.
What about the planet’s volatile elements—chemicals like potassium and sulfur that shouldn’t have survived a massive impact near the Sun? MESSENGER found them on the surface, confounding scientists. The current study doesn’t solve that riddle but opens a pathway.
“Even if most of the volatile content was removed by a giant impact,” the study notes, “Mercury could have experienced subsequent non-erosive impacts from comets or leftover planetesimals, which delivered volatile material to its surface”.
And there’s more to come. In 2026, the European-Japanese mission BepiColombo will arrive at Mercury, equipped with instruments that can peer deep into the planet’s gravity and magnetic fields, possibly revealing the full structure of its enigmatic core.
“Mercury remains the least explored planet in our system,” said Franco. “But that’s changing. There’s a new generation of research and missions underway, and many interesting things are yet to come.”
