How were Neptune’s rings detected before Voyager 2 observed them?

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Clouds in Neptune's atmosphere — Neptune A photograph of clouds in Neptune's atmosphere, taken by Voyager 2 in August 1989. The view is from below the planet's equator, and north is up. The Great Dark Spot (center left) is 13,000 km (8,100 miles)—about the diameter of Earth—in its longer dimension. Accompanying it are bright, wispy clouds thought to comprise methane ice crystals. At higher southern latitudes lies a smaller, eye-shaped dark spot with a light core (bottom left). Just above that spot is a bright cloud dubbed Scooter. Each of these cloud features was seen to travel eastward but at a different rate, the Great Dark Spot moving the slowest.

Before Voyager 2 provided direct imagery, Neptune’s rings were detected in the mid-1980s through stellar occultation studies conducted from Earth.1

During these observations, astronomers noted brief, inconsistent dips in a star’s brightness as the planet passed in front of it. Because these dips were observed only occasionally and never appeared symmetrically on both sides of the planet, scientists initially hypothesized that Neptune possessed partial rings, or ring arcs, rather than a complete ring system. It was not until Voyager 2 arrived that these features were identified as dense, bright regions within a continuous outermost ring known as Adams.1