Syra was the homeworld of the Syrans and the Six Legs. It was rich in metals, specifically the ones required for hyperdrives and nanodrives, leading Ruslan to establish mines there.
The planet was destroyed by the Sonomancer and the hundreds of thousands of Syrans were saved and relocated to New Syra by Galactic Heritage thanks to River Song and Helen Sinclair. (The Sonomancer) The Eleven’s TARDIS deposited molten metal from the planet’s core into 15th century Prague, where it was used by the Clocksmith to make the Doomsday Chronometer. (The Eighth Piece)