(BBC) - A US plane on a top-secret, two-year mission to space has returned to Earth and landed in California.

The aircraft, resembling a miniature space shuttle and known as the Orbital Test Vehicle or X-37B, spent 674 days in orbit around the planet.

It touched down at Vandenberg Air Force Base on Friday morning.

The purpose of the plane remains unclear - a theory that it was taking a look at China's space lab has recently been downplayed by experts.

Air Force officials have only told US media the aircraft performs "risk reduction, experimentation and concept-of-operations development for reusable space vehicle technologies".

It is the third mission for the programme, started in 1999 and currently run by the Air Force's Rapid Capabilities Office.

The first plane flew in April 2010 and returned after eight months. The second launched in March 2011 and remained in space for 15 months.

The current aircraft - built by Boeing - utilises solar panels for power in orbit, measures over 29ft (9m) long, has a wingspan of nearly 15ft and a weight of 11,000lbs (4,989 kg).

A fourth X-37B mission is said to currently be slated for launch from Cape Canaveral, Florida, in 2015.