Mercury, the smallest planet in the solar system, may have shrunk far more than previously estimated, according to new research published Sept. 10 in the journal Geophysical Research Letters. The study suggests that Mercury’s diameter has decreased by as much as 30% more than earlier calculations indicated, potentially losing up to 14.5 miles since its formation.

Scientists have long known that Mercury has contracted over time, a process evidenced by the distinctive wrinkle-like features—called lobate scarps—visible across its surface. These formations result from the cooling and shrinking of Mercury’s interior, which causes its outer rocky layers to crumple and crack. The planet originally formed amid intense heat generated by violent collisions in the early solar system, and as that heat dissipated, Mercury has steadily contracted.

However, the new research points out that the extent of Mercury’s shrinkage has been underestimated due to the presence of impact craters from asteroids and comets. These collisions have created debris and depressions that obscure some of the planet’s shrinking-related surface features, making the full scale of contraction difficult to assess. The study combined previous geological maps with more recent data accounting for Mercury’s roughened terrain, revealing a more pronounced planetary shrinkage than formerly recognized.

Lead author Gaku Nishiyama, a planetary scientist at the German Aerospace Center’s Institute of Space Research in Berlin, noted that a greater degree of shrinkage implies several possibilities for Mercury’s interior composition and history. This could indicate a larger metal core, a lower proportion of light elements such as silicon in the core, or a higher initial temperature when the planet formed. Nishiyama also acknowledged that the new estimates could still underestimate the planet’s total contraction.

The analysis draws on surface data primarily from NASA’s Messenger mission, which orbited Mercury until 2015 and provided detailed images and measurements of the planet’s landscape. While Messenger’s instruments are capable of detecting relatively small features, their resolution limits the full assessment of Mercury’s shrinkage.

Looking ahead, the European-Japanese BepiColombo mission, launched in 2018, is expected to arrive in Mercury’s orbit by the end of 2026. This mission will carry two orbiters equipped with advanced instruments designed to provide new insights into Mercury’s geology, interior structure, and thermal evolution. Researchers anticipate that BepiColombo’s observations will help clarify many of the remaining questions about the planet’s history and the processes driving its contraction over billions of years.