A view of Mercury's rugged, cratered landscape is pictured in this Messenger Spacecraft. Reuters/NASA

Thanks to new evidence gathered by NASA’s Messenger spacecraft, astronomers have found that Mercury, the smallest planet in our solar system, is getting even smaller than previously believed.

According to the new findings, published in the journal Nature Geoscience on Sunday, the planet closest to the sun has shrunk up to seven kilometres (4.3 miles) in radius over the past four billion years, much more than what scientists had earlier estimated.

“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,” Steven A. Hauck, II, a professor of planetary sciences at Case Western Reserve University in Cleveland, Ohio, and the study's co-author, said in a statement.

The study, led by Paul K. Byrne and Christian Klimczak at the Carnegie Institution of Washington, examined detailed images and topographic data provided by Messenger and built a comprehensive map of tectonic features, which suggests that Mercury has shrunk substantially as rock and metal that comprise its interior cooled.

Mariner 10, the first spacecraft sent to explore Mercury, gathered images and data over just 45 percent of the planet’s surface during three fly-bys in 1974 and 1975. Messenger, which launched in 2004 and was inserted into orbit in 2011, continues to collect scientific data about the planet. The spacecraft will complete its 2,900th orbit of Mercury later this month.

In order to determine how Mercury may have shrunk, the researchers studied tectonic features, called lobate scarps, and wrinkle ridges on the planet, which result from interior cooling and surface compression.

Lobate scarps are cliffs caused by thrust faults that have broken the surface and reach up to a height of nearly two miles. Wrinkle ridges are caused by faults that don't extend as deep and tend to have lower relief. With the new data, the researchers were able to see a greater number of these faults and estimate the shortening across broad sections of Mercury’s surface, and determine the decrease in the planet's radius.

In the nineteenth century, some European geologists had proposed that the Earth had shrunk as its mountains formed, and the planet’s core contracted and the surface wrinkled in response. While the notion was later discarded in case of the Earth, now it seems that the idea was right, but was applied to a wrong planet, National Geographic reported.

“One striking aspect of the form and distribution of surface tectonic features on Mercury is that they are largely consistent with some early explanations about the features of Earth's surface, before the theory of plate tectonics made them obsolete—at least for Earth,” Hauck said, in the statement.

According to scientists, the new findings help better understand how planets cool. Scientists believe that by looking at Mercury, they will learn not only about planets in our solar system, but also about the increasing number of rocky planets being found around other stars.