Mercury Shrank Up to 23 Kilometers, New Study Finds

New research indicates that Mercury has shrunk considerably more than earlier estimates suggested, potentially by as much as 23 kilometers in diameter since the planet formed. The paper, published September 10, 2026, in the journal Geophysical Research Letters under the title “Underestimation of Planetary Contraction due to Obscuration by Surface Roughness: The Case of Mercury” (DOI 10.1029/2026GL12406), is led by planetary scientist Gaku Nishiyama, identified by phys.org as a researcher at the Institute of Planetary Research at the German Aerospace Center (DLR), while wire coverage picked up by Canada’s CP24 and the US outlet Local10 describes him as a researcher at Hokkaido University in Japan. The finding lands at a pointed moment: two months before the European-Japanese BepiColombo probe enters orbit around Mercury and attempts to check the shrinkage measurement with a laser instrument.

Mercury, the smallest planet in the solar system and the closest to the sun, formed roughly 4.5 billion years ago in a series of collisions between rocks and asteroids around the young star. That energy heated the planet, and its iron core, oversized relative to the planet itself, has been cooling and contracting ever since, pulling the mantle and crust down with it. As the interior compresses, the outer layer wrinkles and cracks, forming tectonic features known as scarps and ridges. Earlier estimates put Mercury’s diameter loss in this process at somewhere between 4 and 16 kilometers. The degree of shrinkage is not just a geological curiosity: it helps researchers gauge the size of the planet’s metallic core, the share of light elements such as silicon it contains, and Mercury’s starting temperature.

How the team arrived at the new estimate

For the new paper, Nishiyama and his team combined earlier maps of the geological traces of shrinkage with new maps of surface roughness across the entire planet, a measure of how uneven the terrain is. For the first time, they showed that Mercury’s roughest regions have the fewest visible wrinkles. “That led us to think there’s a process that obscures these features,” Nishiyama said, describing the team’s conclusion that debris thrown out by impact craters likely buries the wrinkles in those areas, much like fresh gravel covering the ruts in a road. Correcting the planet-wide estimate for that effect, including the rough terrain where shrinkage traces are hidden, pushed the team’s number 10 to 30 percent higher than the previous figure.

The sources diverge somewhat on the exact number. In its own lead, phys.org puts the loss at close to 19 kilometers of total diameter since the planet formed. The Associated Press wire story, carried by Canada’s CP24 and the US outlet Local10, says the shrinkage could run as high as 23 kilometers, noting that Mercury’s diameter today is just under 4,900 kilometers (about 3,000 miles). The paper itself, as described in the body of the phys.org piece, sets the upper bound of the new range at 14.5 miles, or 23 kilometers, against the earlier estimate of 2.5 to 10 miles (4 to 16 kilometers). The gap between the 19 and 23 kilometer figures across sources is not explained, but both numbers point to the same conclusion: the estimate is larger than previously thought, and by a wide margin.

Nishiyama told the wire service: “We’re pretty excited, it feels like we’re getting closer to the true picture of how Mercury evolved.” He offered a more specific assessment of his own finding to phys.org:

“30 percent is a little surprising, but the corrected amount of shrinkage actually makes sense to me.”

According to the original reporting, the corrected estimate lines up more closely with what would be expected from the physics of how a planet this size cools, making the new finding more consistent with existing models of Mercury’s interior.

The data behind the estimate comes mainly from MESSENGER, the NASA mission that studied Mercury until 2015 and could reliably measure features larger than five kilometers. Before that, the only spacecraft to image Mercury was Mariner 10 in the 1970s, meaning only two earlier missions have visited the planet.

What to expect from the BepiColombo probe

In November 2026, BepiColombo, a joint project of the European Space Agency and the Japan Aerospace Exploration Agency, is due to enter orbit around Mercury. Nishiyama is part of the mission’s science team and expects its laser instrument to deliver a more precise confirmation of how much Mercury has actually shrunk as it cools from the inside. Higher-resolution images than any taken before should start arriving as soon as the probe reaches orbit, and researchers like Nishiyama are hoping they will reveal scarps, ridges and impact craters in more detail than ever.

What this research does not prove

The paper does not measure Mercury’s shrinkage directly, it estimates it through a model: the team assumed that rough regions, if not covered in debris, would carry roughly the same share of visible wrinkles as smoother terrain, and used that assumption to work out how many traces are likely hidden beneath the surface. That is not the same as a laser-measured change in diameter, which is what BepiColombo is meant to provide. Nishiyama himself says the corrected estimate could turn out to be even higher than what his team calculated, meaning 23 kilometers is closer to the current upper limit of what is known than a final number. Until the laser data from BepiColombo arrives, the exact scale of Mercury’s shrinkage remains an estimate drawn from visible geological traces, not a measured fact.