The Rings Of Neptune

When we think of the planets in our solar system, most of us quickly picture Saturn because of its elaborate ring system. Actually, there are four planets that have rings. Voyager 1 discovered rings on Jupiter on March 7, 1977, and on Uranus on March 10, 1977. Those were the only planets known to have rings, until July 22, 1984, when the rings of Neptune were discovered. Neptune’s rings, mainly consisting of five principal ones, were initially thought to be “arcs” and were discovered through simultaneous stellar occultation observations by Patrice Bouchet, Reinhold Häfner, and Jean Manfroid at the La Silla Observatory (ESO). Their program was proposed by André Brahic, Bruno Sicardy, Françoise Roques of the Paris-Meudon Observatory, and William B. Hubbard’s teams at Cerro Tololo Interamerican Observatory in Chile. A stellar occultation occurs when a bright star is obscured by a planetary ring system as seen from Earth. The rings were later imaged in 1989 by the Voyager 2 spacecraft. While parts of Neptune’s rings are as dense as the less prominent sections of Saturn’s rings, like the C ring and Cassini Division, much of Neptune’s ring system is faint and dusty, resembling Jupiter’s rings, in some ways. Neptune’s rings are named after notable astronomers: Galle, Le Verrier, Lassell, Arago, and Adams. Additionally, Neptune has a faint unnamed ring aligned with the moon Galatea’s orbit, and three other moons…Naiad, Thalassa, and Despina orbit between the rings.

Neptune’s rings are composed of very dark material, likely organic compounds altered by radiation, much like those in Uranus’s rings. Neptune’s contain a high proportion of dust (20% to 70%) but have a low to moderate optical depth, less than 0.1. The Adams ring stands out with its five distinct arcs: Fraternité, Égalité 1 and 2, Liberté, and Courage. These arcs occupy a narrow range of orbital longitudes and have remained surprisingly stable since their discovery in 1980. While the mechanism behind their stability is still debated, it’s likely linked to the resonant interaction between the Adams ring and its inner shepherd moon, Galatea.

The first mention of Neptune’s rings dates back to 1846 when William Lassell, who discovered Triton, Neptune’s largest moon, believed he saw a ring around the planet. However, his claim was never verified and was thought to be an observational error. The first confirmed detection came in 1968 through stellar occultation, though this went unnoticed until 1977 when Uranus’ rings were discovered. Following that discovery, a team led by Harold J. Reitsema from Villanova University started searching for Neptune’s rings. On May 24, 1981, they observed a star’s brightness dip during an occultation, but the dimming didn’t indicate a ring. Later, after Voyager’s fly-by, it was revealed that the event was caused by Neptune’s small moon Larissa, an incredibly rare occurrence.

In the 1980s, Neptune experienced far fewer significant occultations than Uranus, which was near the Milky Way and moving through a denser star field. On September 12, 1983, Neptune’s next occultation hinted at a possible ring detection, but ground-based observations were inconclusive. Over six years, around 50 other occultations were observed, with only about one-third yielding positive results. There was evidence of something around Neptune, likely incomplete arcs, but the ring system’s details were unclear. The Voyager 2 spacecraft solved the mystery during its Neptune fly-by on August 25, 1989, passing just 3,080 miles above the planet’s atmosphere. It confirmed that earlier occultations were caused by arcs within the Adams ring. Post-Voyager, earlier terrestrial occultation data was reanalyzed, revealing features of the arcs from the 1980s that aligned almost perfectly with Voyager 2’s findings.

Since Voyager 2’s fly-by, the brightest rings of Neptune, Adams and Le Verrier, have been captured by the Hubble Space Telescope and Earth-based telescopes, thanks to improvements in resolution and light-gathering capabilities. These rings appear slightly above background noise levels at methane-absorbed wavelengths, where Neptune’s glare is significantly reduced. However, the fainter rings remain below the visibility threshold for these instruments. Finally, in 2022, the James Webb Space Telescope (JWST) captured images of the rings, marking the first observation of the fainter ones since Voyager 2’s visit.

Neptune looks very different when viewed by JWST and Hubble. Let’s start with color. The Hubble Space Telescope observes Neptune in visible light, the same kind of light humans can see. In visible light, Neptune appears as a brilliant blue planet due to the methane in its atmosphere, which absorbs red light and reflects blue. Hubble captures this familiar blue hue. In contrast, the JWST uses infrared light, which is invisible to the human eye. Infrared shows Neptune in a completely new way, highlighting heat instead of color. In JWST images, Neptune looks white with a glowing, ghostly vibe. This happens because methane gas absorbs red and infrared light so strongly that the planet appears quite dark in near-infrared wavelengths, except in areas with high-altitude clouds.

One of the most amazing features of Webb’s image is the incredibly sharp view of Neptune’s rings, a sight we haven’t enjoyed with such clarity in over 35 years. The last time these rings were so clearly visible was in 1989, during Voyager 2’s historic flyby. Even powerful telescopes like Hubble have struggled to capture the faint rings due to their low reflectivity and Neptune’s vast distance from the Sun. As I write this, it occurs to me just how much I have to learn about astronomy, nevertheless, I guess my research has managed to increase my knowledge a little bit.