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Mercury may be getting smaller—and scientists now think they underestimated just how much the tiny planet has contracted. A new study suggests that Mercury could have shrunk by as much as 14.5 miles (23 kilometers) in diameter since its formation, potentially 30% more than previous estimates.

The discovery is giving planetary scientists a fresh look at what is happening inside the solar system’s smallest planet and could help explain how Mercury evolved over the past 4.5 billion years.

Mercury Is the Solar System’s “Incredible Shrinking Planet”

Mercury has been known to be shrinking for decades.

Scientists first found evidence of Mercury’s contraction during NASA’s Mariner 10 mission in the 1970s. Later observations from NASA’s MESSENGER spacecraft provided much more detailed evidence.

The reason is relatively straightforward: Mercury’s interior has been cooling since the planet formed.

As its enormous iron-rich core gradually loses heat, the interior contracts. The mantle and crust respond to this reduction in volume, forcing the planet’s surface to compress.

That compression produces enormous geological features called lobate scarps and wrinkle ridges—essentially planetary “wrinkles” created as Mercury’s crust is squeezed.

NASA has previously documented these features as evidence that Mercury’s interior has cooled and contracted over billions of years.

Scientists May Have Missed Some of Mercury’s Wrinkles

The new research, led by scientists including Gaku Nishiyama of the German Aerospace Center’s Institute of Space Research, takes a closer look at why previous estimates may have been too low.

The problem is Mercury’s surface.

Unlike Earth, where erosion, weather and plate tectonics continually reshape the landscape, Mercury has been heavily bombarded by asteroids and other space debris. Over billions of years, countless impacts have covered portions of the older surface with debris.

That debris may have hidden some of the geological evidence of Mercury’s contraction.

Researchers compared maps of Mercury’s contraction-related geological features with maps showing the planet’s surface roughness. They found that areas with rougher, heavily cratered terrain tended to show fewer visible signs of contraction.

The implication is important: the wrinkles may not actually be missing—they may simply be buried beneath material from ancient impacts.

Mercury Could Have Shrunk Up to 23 Kilometers

Previous estimates suggested that Mercury’s diameter had decreased by roughly 2.5 to 10 miles (4 to 16 kilometers) over its lifetime.

The new analysis indicates that Mercury may have contracted 10% to 30% more than previously estimated, potentially reducing its diameter by as much as 14.5 miles (23 kilometers).

That might not sound enormous compared with the size of a planet, but Mercury is only about 3,000 miles (4,900 kilometers) across.

A change of more than 14 miles represents a significant amount of contraction when viewed across an entire planetary body.

Importantly, scientists are not saying Mercury is suddenly shrinking dramatically today. This is a process that has occurred gradually over billions of years as the planet has cooled.

Why Does Mercury Have Such a Large Iron Core?

One of the biggest mysteries about Mercury is its unusual interior.

Mercury is the smallest planet in the solar system, but it has a disproportionately large metallic core. That massive core makes Mercury particularly interesting to planetary scientists trying to understand how rocky planets form and evolve.

The amount of contraction observed over Mercury’s history provides another clue about its interior.

If Mercury has contracted more than previously believed, scientists may need to refine their models of the planet’s internal structure, temperature and composition.

The new estimate may actually bring observations more closely into agreement with what physics predicts for a cooling Mercury.

MESSENGER Helped Reveal the Mystery

Much of what scientists know about Mercury’s surface comes from NASA’s MESSENGER mission, which became the first spacecraft to orbit Mercury in 2011.

MESSENGER spent years photographing and mapping the planet before its mission ended with a planned impact in 2015.

The spacecraft provided an unprecedented view of Mercury’s scarred surface, revealing thousands of geological features associated with the planet’s history.

However, MESSENGER’s data also had limitations. The spacecraft could not resolve every small geological feature, meaning some evidence of Mercury’s contraction may have remained hidden.

The new study attempts to account for those missing features statistically by examining the relationship between surface roughness and visible contraction structures.

BepiColombo Could Provide the Answer

Scientists won’t have to wait forever to test the new estimate.

The BepiColombo mission, a joint project involving the European Space Agency and Japan’s JAXA, is approaching Mercury and is expected to begin its Mercury science mission in November 2026.

BepiColombo is particularly important because it will provide scientists with new and more detailed measurements of Mercury’s surface.

Its instruments should allow researchers to examine the planet’s topography and geological structures at higher resolution than was possible with previous missions.

A laser instrument aboard the spacecraft could help scientists determine how much Mercury has actually contracted and whether the new estimate is accurate.

Could Mercury Still Be Shrinking Today?

Possibly.

Scientists already have evidence that Mercury has experienced relatively recent geological activity.

MESSENGER observations revealed small fault scarps that appear young on geological timescales. Because Mercury’s surface is constantly bombarded by meteoroids, small features should eventually be erased or degraded.

Their survival suggests that Mercury’s interior may still be cooling and contracting today.

That raises an intriguing possibility: Mercury may not be a completely geologically dead world.

Instead, the planet could still be slowly changing beneath its heavily cratered surface.

A Tiny Planet With a Big Scientific Story

Mercury may be small, but it provides scientists with a remarkable laboratory for studying planetary evolution.

Its huge iron core, extreme temperatures, magnetic field and heavily cratered surface make it very different from Earth. Yet the basic physics of a cooling planetary interior can provide important clues about how rocky worlds change over time.

The new research also demonstrates how scientists can discover something new without collecting new data. By looking at older spacecraft observations in a different way, researchers found evidence that may have been hiding in plain sight.

And BepiColombo could soon provide an even clearer picture.

The Bottom Line

Mercury may have shrunk significantly more than scientists previously thought.

A new analysis suggests that impact debris has hidden some of the planet’s geological “wrinkles,” causing researchers to underestimate how much Mercury contracted as its interior cooled.

The planet could have lost up to 14.5 miles (23 kilometers) from its diameter over approximately 4.5 billion years.

Now, scientists are waiting for BepiColombo to provide a closer look.

If the spacecraft confirms the new measurements, our understanding of Mercury—and the mysterious interior of the solar system’s smallest planet—could change once again.

Mercury may be small, but it still has plenty of secrets left to reveal.