A new study suggests that Mercury, the smallest planet in our solar system, may have shrunk by 10% to 30% more than previously thought. This would mean its diameter could have decreased by as much as 14.5 miles (23 kilometers) since the planet formed, compared to earlier estimates of a 2.5 to 10-mile (4 to 16-kilometer) reduction. The research, published in the journal Geophysical Research Letters on September 10, 2026, was led by Gaku Nishiyama, a planetary scientist at the German Aerospace Center (DLR) Institute of Space Research. The study suggests that debris from impact craters has hidden evidence of Mercury’s contraction, making it harder to fully understand how much the planet has shrunk over time. Mercury formed around 4.5 billion years ago through the violent collisions of rocks and asteroids orbiting the sun. The heat from these impacts caused Mercury to be extremely hot initially, and as it has cooled over billions of years, its interior has contracted. This shrinkage caused the outer layers of the planet to crumple and crack, forming tectonic features such as scarps and ridges. Understanding the degree of Mercury's contraction is key to learning about its interior structure and evolutionary history. Greater shrinkage could imply a larger metal core, fewer light elements like silicon in the core, or a higher initial temperature, according to Nishiyama. Impact craters leave depressions and cover the surface with debris, making the landscape rougher and obscuring signs of contraction. Nishiyama and his team combined older maps of geological features with new maps of surface roughness to show that the roughest areas on Mercury have fewer visible wrinkles. They believe that debris from impact craters in these rough areas may be covering up the signs of shrinkage, much like fresh gravel might hide ruts on a road. By examining contraction features in less disrupted areas, they estimated that the missing features in rougher regions could account for the additional shrinkage. Mercury’s current diameter is about 3,032 miles (4,880 kilometers). The new findings indicate that the planet’s contraction is more significant than previously thought, offering new insights into its composition and formation. Nishiyama noted that the updated figures might still be an underestimate, as data from NASA’s MESSENGER mission, which ended in 2015, can only reliably measure features larger than about 3 miles (5 kilometers) across. The BepiColombo mission, which began collecting higher-resolution scans of Mercury’s surface in November 2026, is expected to provide more detailed information on scarps, ridges, and impact craters. Nishiyama, who is part of the BepiColombo mission science team, hopes the new data will refine the estimates of Mercury’s contraction and offer a clearer picture of the planet’s evolution.