Mercury has shrunk by up to 30 percent more than previously estimated, according to a study published on September 10, 2026, in Geophysical Research Letters. Researchers found that debris from asteroid and comet impacts concealed the planet’s surface wrinkles, masking the true extent of its cooling contraction.
How Asteroid Debris Hid Mercury’s Massive Contraction
The solar system’s smallest planet is losing its grip on size. Formed about 4.5 billion years ago, Mercury has continuously leaked heat into space from its fiery origins. As its metallic core cools, the entire planet shrinks like a grape turning into a raisin in the sun, causing its outer crust to buckle and crack into massive cliffs and mountainous ridges.
For decades, planetary scientists relied on visible tectonic records to measure this shrinkage, estimating that the planet’s diameter had reduced by between 2.5 and 10 miles. But a new analysis reveals that Mercury may have lost up to 14.5 miles of diameter since its formation. That represents a contraction rate up to 30 percent greater than previous calculations indicated.
The discrepancy comes down to cosmic violence. A barrage of meteors has pocked Mercury’s surface over the eons, hurling giant shattered rock across the landscape. This debris created a fresh gravel spread that obscured the planet’s telltale wrinkles. Rugged regions contained the fewest visible wrinkles, and around major impacts like the double-ring Rachmaninoff basin, tectonic cracks disappeared almost entirely under thick layers of rubble.
Rewriting the Thermal Evolution of a Metallic Core
To uncover the hidden contraction, a team of researchers combined previous maps of Mercury’s geology with newer maps accounting for its roughened surface. We compared a global map of Mercury's surface roughness with maps of shortening structures and contraction,
Gaku Nishiyama, a planetary scientist at the German Aerospace Center Institute of Space Research, said in a statement.
Nishiyama noted that the corrected figures align with planetary physics.
This mathematical correction resolves a long-standing tension where observed shrinkage lagged behind what physics predicted. Paul Byrne, a planetary scientist at Washington University in St. Louis who was not involved in the study, praised the novel approach of accounting for billions of years of impact bombardment.
Byrne previously presented findings at a 2013 conference indicating that Mercury had shrunk by about 11 kilometers, whereas the new study pushes that figure to about 23 kilometers, or roughly 14 miles. Byrne is confident that other celestial bodies such as the moon and Mars may show similar shrinkage patterns.
What an Oversized Core Tells Us About Mercury’s Birth
Understanding a faster shrink rate fundamentally alters what astronomers understand about the planet’s interior composition. A more drastic contraction suggests Mercury holds a much larger metal core containing fewer light elements like silicon than previously assumed.
That massive iron core strongly implies the planet survived a catastrophic collision with another body in its ancient history, an event that would have stripped away most of its rocky mantle. An oversized core could also explain how such a tiny world maintained the interior heat necessary to drive its global magnetic field.
Even these revised estimates might still represent an underestimate. The researchers noted that existing surface data remain incomplete due to the intense glare of the sun and the inherent difficulties of observing the innermost planet from afar.
BepiColombo Arrives to Verify the Math
Scientists will not have to wait long to test the new models with fresh data. BepiColombo is due to arrive at the planet by the end of 2026, concluding an eight-year journey that began with its 2018 launch on an Ariane 5 rocket.
The mission represents a joint effort between the European Space Agency and the Japan Aerospace Exploration Agency. Only two prior missions have ever visited Mercury: NASA’s Mariner 10 and the MESSENGER spacecraft, the latter of which ended its orbital mission in 2015. Equipped with advanced lasers, BepiColombo will enter orbit to scan the surface and detect fine details in geological features that earlier missions missed.
