New research has shown Mercury, the planet that orbits closest to the sun, has been slowly shrinking since the first time its crust solidified four billion years ago.

According to BBC News, scientists have been aware of the occurrence since the 1970s when the Mariner 10 probe observed the planet. Now Mercury has shrunk an estimated seven kilometers in its entire history.

The shrinking is caused by rapid cooling over time and began when the planet's crust first solidified. In the process, its surface cracks and wrinkles. A new study, published in the journal Nature Geoscience, has used recent data from NASA's Messenger satellite to suggest it is shrinking at a quicker rate than previously surmised.

"With Messenger, we have now obtained images of the entire planet at high resolution and, crucially, at different angles to the sun that show features Mariner 10 could not in the 1970s," study co-author Steven A. Hauck, II, a professor of planetary sciences at Case Western Reserve University, said in a press release.

The Mariner noticed large cracks in Mercury's surface when it flew by in 1974 and again in 1975, evidence of how the planet's body was retracting. At the time, scientists guessed the planet had shrunk in radius by about one to three kilometers, a figure highly disputed. Messenger's data is believed to be much more accurate and far more severe at a loss of seven kilometers in radius.

"Some of these lobate scarps are enormous," study lead author Paul Byrne, of the Carnegie Institution, told BBC News. "There's a structure called Enterprise Rupes in the southern hemisphere that is a single scarp system. It's 1,000 kilometers long and in places has three kilometers of relief. Imagine standing in front of it. It's Mercury's version of a mountain belt."

Mercury is an estimated 4,000 kilometers in diameter, but has a defining iron core protected by only about 400 kilometers of a rocky blanket.

A probe called the BepiColombo, a collaboration between Europe and Japan, is set to launch in 2016 to follow up on Messenger's findings.

"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," the probe's principal investigator Dave Rothery, from the U.K.'s Open University, told BBC News. "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."