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Planet Mercury, which is 4,880km wide, has become 7km smaller today than when its crust first solidified around four billion years ago. The innermost world shrunk as it cooled gradually. 

The phenomenon was first identified by scientists in the mid-1970s when the Mariner 10 probe was whizzed by the planet, reported BBC. Mariner managed to photograph 45 percent of the planet by crossing it twice in 1974 and 1975. 

Latest images from the US space agency's Messenger satellite enabled researchers to further refine their estimates about the contractions. 

"Some of these lobate scarps are enormous," Dr Paul Byrne from the Carnegie Institution in Washington DC and lead author of the paper told BBC News.

"There's a structure called Enterprise Rupes in the southern hemisphere that is a single scarp system. It's 1,000km long and in places has 3km of relief. Imagine standing in front of it. It's Mercury's version of a mountain belt. It utterly dominates the topography and it is astounding given the diminutive size of Mercury."

Researchers said planet's metal core is the defining feature which is covered by a thin rocky veneer. A joint mission to Mercury is expected by Europe and Japan. 

"People used to think the Earth was shrinking - which it is a little bit, but we can't see it because of the way tectonic plates are created and destroyed on the Earth," explained Dr Dave Rothery from the UK's Open University, who will be one of the personal investigators in the follow up Messenger's observations, according to BBC. 

"Before we understood plate tectonics, people thought mountain belts on Earth were because the planet was shrinking and forcing stuff upwards, and areas of thick accumulation of sediment were where the crust was being forced down by contraction. We now know that's broadly speaking wrong, but this is the process on Mercury because it's a one plate planet."

The report is detailed in the journal Nature Geoscience.