The NASA scientists revealed, on Sunday, after analyzing a set of photos taken by the Messenger probe in January, that Mercury, the smallest planet in the Solar System, has become even smaller. The data from a flyby of Mercury in January 2008 indicates the planet has shrunk by more than one mile (1.5km) in diameter over its history. Scientists believe the retiring is due to the planet's core slowly cooling.

According to their estimative studies, the planet is very likely to continue getting smaller. Even if its metal core is still active, the cooling activity inside it determines the upper layers to solidify permitting their intrusion over the molten layers below.

"Cooling of the planet's core not only fuelled the magnetic dynamo, it also led to contraction of the entire planet," said Principal Investigator Sean Solomon of the Carnegie Institution of Washington, US.

The Messenger spacecraft’s pass within 200km (125 miles) of Mercury earlier this year, marked it as the first time to view the planet up close since Mariner 10's third and final fly-by in March 1975.

The entire study with a detailed analysis was introduced on July 3 in the journal “Science.” Additional info presented by the scientists refers to the planet’s core, which is filled with sodium, sulfur and silicon.

The Messenger was launched on August 3, 2004 having the mission to establish the elements that compound Mercury’s surface, its geologic history and the nature of the planet’s exosphere and magnetosphere. The mission implicates a long-term effort, but until present time the results of its investigations and tests have shown it is truly a suitable and useful investment.

The scientists working on the mission are preparing Messenger’s Mercury orbit insertion, which is due for March 11, 2011 and is expected to deliver a large volume of important data.