Mercury, the smallest and innermost planet in our solar system, is shrinking faster than scientists previously believed, according to a new study that suggests its contraction may be 30% greater than earlier estimates. The research, published in the journal Geophysical Research Letters, indicates that the planet’s diameter may have decreased by as much as 14 miles (23 kilometers) since its formation 4.5 billion years ago—a significant loss for a world barely 3,000 miles (4,900 kilometers) across .
The findings come at an opportune moment, just one week after the European and Japanese BepiColombo spacecraft shed its cruising platform and advanced toward Mercury. The linked craft are expected to enter orbit around Mercury in November before splitting up for a more detailed survey. BepiColombo’s laser instrument should help confirm how much Mercury is withering as a result of internal cooling, according to Gaku Nishiyama, the study’s lead author who is part of the space mission .
Why Mercury Is Shrinking
Mercury formed approximately 4.5 billion years ago through a series of violent collisions between rocks and asteroids orbiting the Sun. The energy from these impacts generated tremendous heat, which the planet has been losing ever since. As Mercury’s interior cools, it contracts, and the outermost rocky layers compensate for the planet’s changing size by crumpling and cracking to form tectonic features such as scarps and ridges—essentially wrinkles on the planet’s surface .
The degree of shrinking is crucial for understanding Mercury’s internal structure and evolution. “More shrinking means Mercury could have a larger metal core, less light elements like silicon mixed into the metal core, or a higher starting temperature,” explained Nishiyama, a planetary scientist at the German Aerospace Center’s Institute of Space Research . This suggests that Mercury’s interior is even more unusual than scientists previously thought, potentially featuring a larger inner core and higher initial temperature than earlier models indicated .
The Role of Surface Roughness
The challenge in accurately measuring Mercury’s shrinkage lies in its battered surface. Cooling should shrink a planet roughly uniformly, making “wrinkle” features—known as shortening structures—similarly common everywhere. However, the formation of new impact craters creates depressions and covers the landscape in debris, making the surface rougher and rendering the signs of shrinkage harder to spot among billions of years of geologic features .
Nishiyama and his team combined previous maps of geologic evidence for contraction with new maps of surface roughness for the entire planet’s surface. For the first time, they demonstrated that the roughest areas on Mercury have fewer visible wrinkles. “It made us think that there’s a process obscuring shortening structures,” Nishiyama said. He believes impact debris in rough areas could be covering up the shrinkage wrinkles, similar to how freshly added gravel hides ruts in a road .
A particularly clear pattern emerged when researchers examined relatively young impact craters. Around the Rachmaninoff crater, for example, shortening structures are less common in areas covered by rough ejecta—material thrown across the surrounding area during impacts. Some structures also become less visible closer to the crater, suggesting they may be partly buried by impact material .
Revised Estimates and Implications
After accounting for the relationship between surface roughness and visible tectonic structures, the estimated radial contraction of Mercury increased from 8.3 kilometers to 11.6 kilometers. The researchers suggest this figure may still be an underestimate and that actual contraction could be greater . The total change in the planet’s diameter could be up to 14.5 miles (23 kilometers) rather than the currently estimated 2.5 to 10 miles (4 to 16 kilometers) .
“The new estimates on Mercury’s radial contraction would tell a different scenario of Mercury’s thermal evolution,” Nishiyama noted. “So what would be the implication on Mercury’s evolution when this new estimate is used? This is the next question that we have to tackle” .
The implications could extend beyond Mercury. Paul Byrne, a planetary scientist at Washington University in St. Louis who was not involved in the study, noted that other celestial bodies such as the Moon and Mars may show similar shrinkage patterns. “Do those worlds next!” he said .
Future Observations
According to Nishiyama, the updated figures could still be an underestimate because existing Mercury data from NASA’s MESSENGER mission, which ended in 2015, can only be reliably used to measure features larger than about 3 miles (5 kilometers) across . Before MESSENGER, only one other spacecraft had visited Mercury: NASA’s Mariner 10 in the 1970s .
In November 2026, BepiColombo—only the third-ever mission to Mercury—will begin collecting higher-resolution scans of the planet’s surface. The BepiColombo Laser Altimeter (BELA) will measure surface roughness at much finer scales and could reveal relationships between much younger geological events and tectonic structures that have so far been difficult to detect completely .
“With the BepiColombo Laser Altimeter, we expect that Mercury’s topography can be captured with a better resolution, and then we would be able to get more reliable information on contraction features and roughness,” Nishiyama said. “This information will definitely corroborate the findings and may further refine the estimates” .
The same effect could also be relevant on other rocky worlds, particularly the Moon, whose surface is even rougher than Mercury’s . As scientists continue to unravel Mercury’s geological history, each new discovery brings them closer to understanding the complex processes that have shaped not only the solar system’s smallest planet but potentially other terrestrial bodies as well.
“We are quite excited,” Nishiyama said, adding that it feels like “we are approaching the reality of Mercury’s evolution” .
