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Why is the Phoebe dust ring the largest planetary ring in the solar system?

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Saturn Saturn and its spectacular rings, in a natural-color composite of 126 images taken by the Cassini spacecraft on October 6, 2004. The view is directed toward Saturn's southern hemisphere, which is tipped toward the Sun. Shadows cast by the rings are visible against the bluish northern hemisphere, while the planet's shadow is projected on the rings to the left.

The Phoebe dust ring is considered the largest planetary ring in the solar system because of its immense spatial extent, stretching from 7.3 to 11.8 million km (4.6 to 7.4 million miles) from Saturn.12

It is important to clarify that the ring's size is defined by its vast radial distance rather than its density or mass. The ring is composed of dust shed from impacts on the moon Phoebe, which orbits Saturn at a much greater distance than the planet's other major moons. Because this material originates from Phoebe and follows the moon's orbit, the ring occupies a massive region of space far beyond the planet's main ring system. Despite its enormous scale, the ring is extremely tenuous, with an optical depth of only $2 \times 10^{- 8}$ , making it visible primarily in infrared light.12