Mercury Has Wrinkles That Show It’s Shrinking. Now, Scientists Say the Planet Has Shriveled Far More Than Previously Thought

A new study suggests that our solar system’s smallest planet has shrunk by up to 30 percent since its birth roughly 4.5 billion years ago

Sara Hashemi | Daily Correspondent

September 22, 2026 3:12 p.m.

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Mercury, our solar system’s smallest planet and the one closest to the sun, is covered in wrinkles. They tell scientists that the teensy world has shrunk as it cooled over the past 4.5 billion years. Its birthplace was piping hot, after all.

But Mercury may have shriveled even more than previously thought, at around 10 to 30 percent, researchers report in a study published the journal Geophysical Research Letters on September 10. That’s a loss of up to 14.5 miles in width—far more than past estimates of around 2.5 to 10 miles—a fairly significant chunk, considering the planet’s current diameter of merely 3,032 miles. The findings could unlock new insights about Mercury’s interior and evolution, in turn shedding light on other rocky worlds.

“There are lessons about the formation and evolution of large, rocky bodies like our Earth, that can be learned on Mercury better than anywhere else in our solar system,” says Hannes Bernhardt, a planetary geologist at the University of Maryland, College Park who wasn’t involved in the study, to Ashley Strickland at CNN.

Mercury is a difficult planet to study. Its proximity to the sun makes it tough for a spacecraft to reach; one trying to enter Mercury’s orbit must constantly brake to fight the gravitational pull of our star, and the planet’s surface temperatures are too hot to allow spacecraft to get very close. It’s also hard to observe from Earth, given the brightness of the sun. Only one spacecraft has entered this little world’s orbit: The one used in NASA’s MESSENGER mission, which circled Mercury from 2011 to 2015.

In the new study, researchers combined MESSENGER data with a digital model of Mercury’s terrain based on images to create a map of the planet’s surface roughness. They found that rougher areas on Mercury had fewer visible wrinkles—signs of the planet’s contraction—which led them to believe something was obstructing their view of the folds. Perhaps rocky debris kicked up from impacts covered up the ridges formed by shrinkage.

“The reason why the estimation remains difficult is that we have to rely on topographies that are visible on the surface of the planet,” says study co-author Gaku Nishiyama, a planetary scientist at the German Aerospace Center’s Institute of Space Research, to K.R. Callaway at the New York Times.

Accounting for those obstructed areas helped the team to calculate the new shrinkage estimate. Up to “30 percent is a little bit surprising, but the corrected amount of contraction actually makes sense to me,” Nishiyama says in a statement. The updated numbers allow observations of Mercury’s cooling and shriveling to align more closely with physics-based predictions.

“The authors have thought about an additional angle to figuring out Mercury’s geological history from afar, which is the effect of surface roughness from billions of years of impact bombardment,” says Paul Byrne, a planetary scientist at Washington University in St. Louis who wasn’t involved in the study, to Mary Randolph at Scientific American. “If their results hold—and I’m confident they will—then we’ll have taken a step further in investigating planets we can’t yet visit in person.”

Understanding Mercury’s shrinkage opens the door to more insights about the mysterious planet. “Every planet is an odd little duck that has a story to tell,” says Sam Birch, a planetary geoscientist at Brown University who also wasn’t involved in the work, to the Times. “If you can understand how fast Mercury is shrinking, you can understand how fast it’s cooling, what size it started at and what it’s made out of.”

The updated amount of contraction already hints that Mercury might have a larger metal core, with fewer light elements such as silicon mixed in or a higher starting temperature than previously thought, Nishiyama says in the statement. Additionally, he thinks the new numbers could still be an underestimate.

Thankfully, more data should be coming soon. The European Space Agency and Japan Aerospace Exploration Agency’s BepiColombo mission is slated to enter Mercury’s orbit later this year and begin scientific operations in April 2027.

“New data from BepiColombo will open a door for understanding how Mercury has been shaped up to now,” Nishiyama tells Jeffrey Kluger at Time.

Sara Hashemi is a science writer and fact-checker currently based in New York City. Her work has appeared in Sierra, The Body, Maisonneuve magazine and more.