Summary
An analysis of wrinkles across Mercury’s cratered surface suggests that the planet’s diameter has decreased by around 23 kilometres since it formed. The finding offers new constraints on how Mercury’s interior evolved over roughly 4.5 billion years.
Mercury’s diameter has decreased by around 23 kilometres since the planet formed, according to an analysis of wrinkles across its surface reported by Nature on 17 September 2026. The result provides new constraints on the evolution of Mercury’s interior over the past 4.5 billion years.
Mercury is the smallest planet in the Solar System. Like other planetary bodies, it has cooled over time. Cooling causes the planet’s material to contract, and that reduction in volume is expressed at the surface as distinctive wrinkles.
The reported estimate is a cumulative change since Mercury’s formation, rather than a present-day shrinking rate. It describes how much the planet’s diameter has reduced across its geological history.
How Mercury’s cooling leaves marks on the surface
As Mercury’s interior cooled and contracted, the planet’s outer rocky layer had to accommodate the reduction in volume. The resulting surface structures preserve evidence of that contraction. By analysing these features, researchers can estimate the total change in Mercury’s size.
That record is partly obscured. Mercury’s surface is heavily marked by impacts from comets and meteoroids, which have modified the terrain and covered some of the evidence of earlier contraction. Separating contraction-related structures from the later effects of impacts is therefore important when reconstructing the planet’s history.
The analysis reported by Nature indicates that the visible evidence is consistent with a total diameter reduction of about 23 kilometres. The estimate is described as a suggestion from the surface features rather than as a direct measurement of the planet shrinking today.
Why the estimate matters
Mercury’s contraction is a record of changes inside the planet. A better estimate of the total shrinkage helps constrain models of how its interior cooled and evolved. The result also shows why Mercury’s surface cannot be interpreted simply as an untouched record: impact activity has altered the evidence that planetary scientists use to infer its geological history.
The finding is therefore significant not because Mercury is undergoing a sudden change, but because its ancient surface retains measurable information about the planet’s long-term thermal evolution. The approximately 23-kilometre figure links present-day surface structures with processes that unfolded over billions of years.