Mercury, already our smallest planet, is shrinking. 

The news comes as NASA marks the third year of MESSENGER — a robotic spacecraft orbiting Mercury that gets its name from its functions, MErcury Surface, Space ENvironment, GEochemistry, and Ranging. According to a press release posted on the MESSENGER site, the anniversary is marked by a number of new papers explaining the significance of data gathered by MESSENGER since 2011. One of those papers, published in the scientific journal Nature Geoscience, reveals that Mercury's core has gotten significantly smaller than scientists first predicted. 

Mercury is now 4.4 miles narrower than it was 3.8 billion years ago, when the planet's core started to cool down. Until now, scientists thought the planet had contracted by only 0.5 to 2 miles since that time.

Mercury currently measures 3,032 miles across, so either change is relatively small, but scientifically significant because, in a roundabout way the research confirms an old scientific theory (since debunked) about how things work on Earth. 

Discovery.com explains the topographic model that led to Mercury's shrinking is actually how scientists used to suspect the Earth was built, before plate techtonics was discovered: 

In that theory, the Earth's mountains were thought to be a lot like the wrinkles that form on an apple's skin when it dries out and shrinks. But the idea just didn't fit the geology of Earth particularly well. Now a modern theory -- plate tectonics -- explains our planet's features far more accurately. Mercury, on the other hand, has only one solid shell for a crust, rather than Earth's many plates that shift about. 

So when observers noticed similar shrinking on Mercury they realized that old theory was correct just misapplied. Lead author Paul K. Byrne and his team write in the paper's abstract:

The tectonic features on Mercury are consistent with models for large-scale deformation proposed for a globally contracting Earth — now obsolete — that pre-date plate tectonics theory. We find that Mercury has contracted radially by as much as 7 km, well in excess of the 0.8–3 km previously reported from photogeology and resolving the discrepancy with thermal models. 

According to Byrne, their findings correct inconsistencies between scientific models and observed evidence of Mercury's shrinking. "These new results resolved a decades-old paradox between thermal history models and estimates of Mercury's contraction," adding

Now the history of heat production and loss and global contraction are consistent. Interestingly, our findings are also reminiscent of now-obsolete models for how large-scale geological deformation occurred on Earth when the scientific community thought that the Earth only had one tectonic plate. Those models were developed to explain mountain building and tectonic activity in the nineteenth century, before plate tectonics theory.

So while Earth's mountainous ranges are not actually a sign of a shrinking core, Mercury's are. As the planet's molten iron center cooled over billions of years, its protective shell — the surface crust — contracted to accommodate its smaller size. The cracks in the rock MESSENGER spotted on Mercury are evidence of the planetary contraction, just as scientists initially suspected. Between this and Harvard-Smithsonian scientists' finding of the "first direct evidence of cosmic inflation," it's been a pretty great day for space science.