A new study suggests Mercury has shrunk far more than earlier estimates indicated. Fresh BepiColombo observations could reveal how the planet’s hidden scars record its long cooling history.

Mercury may have shrunk more than scientists previously estimated, with the planet’s diameter potentially reducing by as much as 23 km, a study has revealed.

Researchers said the revised estimate could be about 30 per cent higher than earlier calculations—a significant difference for a planet that is only about 4,900 km across.

The findings, published in the journal Geophysical Research Letters, suggest that Mercury’s battered and uneven surface may have concealed some of the evidence of its contraction.

The study comes a week after the European and Japanese BepiColombo spacecraft moved closer to Mercury ahead of entering orbit in November.

The spacecraft are expected to separate at that stage for a more detailed survey of the planet.

Gaku Nishiyama, the study’s lead author and a participant in the BepiColombo mission, said the spacecraft’s laser instrument could help determine how much Mercury has contracted as its interior cools.

He said the planet may have shrunk even more than the team’s estimate based on measurements taken by NASA’s Messenger spacecraft in the 2010s. Before Messenger, the only other spacecraft to visit Mercury was NASA’s Mariner 10 in the 1970s.

Mercury, the smallest and innermost planet in the solar system, has been contracting since its formation about 4.5 billion years ago. Its large, hot iron core has been cooling and contracting over time, causing the mantle and crust to tighten and the planet’s surface to buckle.

Nishiyama and his team compared maps of faults and other geological features linked to contraction with newer maps showing the roughness of Mercury’s surface. They found that the roughest regions had fewer visible contraction wrinkles, suggesting that some may have been buried beneath debris ejected by impact craters.

After accounting for these obscured areas, the researchers arrived at a higher estimate of Mercury’s overall contraction. They said the findings could also help scientists better understand how other planetary bodies contract as they cool.

“We are quite excited,” Nishiyama said in an email, adding that it feels like “we are approaching the reality of Mercury’s evolution”.

Fresh observations from BepiColombo are expected to provide a clearer picture of how much Mercury has contracted and what its changing interior can reveal about the planet’s evolution.