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Quirks and Quarks·Analysis

James Webb Space Telescope offers spectacular new picture of Neptune's rings — but Voyager got there first

Bob McDonald's blog: Neptune's faint, dark rings are almost impossible to see using Earth-based telescopes. The best view we've had of them was from the 1989 Voyager 2 flyby. Now the James Webb telescope has produced a beautiful new image.

Bob McDonald's blog: The best picture we’ve seen since Voyager 2 first saw the rings 33 years ago

Bob McDonald · CBC Radio ·
An almost pearlescent planet seen with two bright rings circling around it, with white dots representing the moons seen intermingling with the rings.
As well as capturing the rings, the James Webb Space Telescope captured seven of Neptune’s 14 known moons. In this cropped version of the image, released on Sept. 21, 2022, the moons Galatea, Naiad, and Thalassa are seen on the left side of the planet, and Despina, Proteus, Larissa are seen on the right side. Neptune’s large and unusual moon, Triton, is out of frame. (NASA, ESA, CSA, STScI)

This week saw the release of an ethereal image of Neptune, the most distant planet from the sun, taken by the James Webb Space Telescope. The image captured seven moons and the planet's thin rings, which are very difficult to photograph from Earth.

In the mid-1980s, Earth-based images hinted at their existence, but the rings weren't fully captured until 1989 when they were photographed by the Voyager 2 spacecraft during the one and only close encounter with the planet.

Neptune appears quite different through Webb's eyes than in the Voyager images — like a glowing crystal ball with the ghostly rings wrapped around it. That's because the telescope sees in near infrared, which is reflected by clouds high in the planet's atmosphere.

Voyager, which photographed the planet in visible light, saw a beautiful blue ball streaked with white clouds and a dark storm the size of the entire Earth.

A blue planet against a black background with some light blue swirls wrapping around the center.
This picture of Neptune was taken by Voyager 2 less than five days before the probe's closest approach of the planet on Aug. 25, 1989. The picture shows what's called the Great Dark Spot — a storm in Neptune's atmosphere — and the bright, light blue smudge of clouds that accompanies the storm. (NASA/JPL-Caltech)

I was fortunate to be at the NASA Jet Propulsion Laboratory in August 1989 when Voyager flew past Neptune. Early observations from telescopes on Earth had suggested that Neptune's rings were incomplete arcs, rather than true rings reaching all the way around the planet. 

During occultation events — where a planet passes in front of a distant star, becoming backlit — the starlight would wink off and on as it passed one side of the planet, suggesting the presence of rings obscuring the starlight. But the same didn't happen on the other side of the planet. This led to the idea that Neptune was surrounded by separate arcs rather than complete rings.

Photographing the rings turned out to be a challenge because at a distance 30 times farther from the sun than Earth, the light out at the edge of the solar system is only one thousandth as bright. That meant Voyager's cameras had to take long exposures while flying past the planet at 90,000 km/h. On top of that, unlike the bright icy rings of Saturn, Neptune's rings are made of very dark and hard to photograph material. 

During a press conference back in 1989, one of the scientists said that photographing the rings of Neptune was like trying to take a snapshot of a black race car speeding by at night without using a flash. 

A black and white photo showing what appears to be static, with a white curved line going down the center.
The Voyager spacecraft was 8.6 million kilometers from Neptune when it took this 61-second exposure on Aug. 19, 1989. At the time, this image was considered proof that Neptune had incomplete arcs instead of complete rings circling the planet. (NASA/JPL)

A day or two before the close encounter, a friendly betting game emerged among the scientists who were debating on whether or not the planet had a ring system. As Voyager drew close to Neptune, an early, low-resolution image showed what appeared to be the sought after ring arcs.

The "Arc-ers" rejoiced. But the "Ringers" were still hopeful, believing that the images would improve as the spacecraft got closer and the arcs would grow longer, joining together into rings.   

It was not until Voyager flew past Neptune, entered the planet's shadow and looked back towards the sun that the rings finally appeared in their full glory and the Ringers were victorious.

I don't know exactly what the wagers were or how much was won or lost.  

The rings turned out to be made of fine dust that is best seen when light shines through it, like dust in your home that shows up floating before a window when a sunbeam is shining through. Voyager discovered four rings, some with clumps made of larger particles that have been grouped together by the gravity of small moons embedded within the rings. Those clumps are what looked like arcs from a distance.

A black and white photo, with a thick black stripe down the center covering something that appears to be a bright white ball. Several rings are circling the bright white ball.
Two exposures with Neptune blacked out, center, were used to make this image of Neptune's ring system. The rings were once thought to be incomplete arcs, but Voyager 2 discovered at least 3 new rings and imaged the complete rings. These images were made from a pair of 10-minute exposures while the Sun was behind Neptune, and faint ring particles were being lit from the back. (NASA)

Thanks to the Voyager mission, we now know that all four of the largest planets in our solar system have rings and they are all different from each other. 

Saturn is most famous for its brilliant icy rings first seen by Galileo in 1610. A new study has suggested that they might be the remains of a hypothesized moon known as Chrysalis. The idea is that Chrysalis was drawn too close to the giant planet around 160 million years ago and was torn apart by powerful tidal forces. 

Jupiter, by contrast, is surrounded by a gossamer-thin ring system made of particles as fine as smoke and tinted red, created by dust blown off of its closest moons.

Uranus has dark rings forming thin lines with dust between them and Neptune has clumpy rings believed to be the results of collisions between small moons.

Even Earth may have had a ring in its early days according to the giant impact hypothesis, in which a Mars-sized object collided with the protoplanetary Earth and formed a ring of debris that eventually coalesced into our moon.

A red and blue circle against a black background, with a thin orange line circling around it.
This image of Jupiter's rings was taken by Voyager 2 looking back towards Jupiter from 1.5 million kilometres away. The thin, Jovian ring makes up the two orange lines to the left of the image. The blue and red arc is the edge of Jupiter, backlit by the Sun. (NASA/Voyager)

Rings are dynamic structures that are continually influenced by the gravity of orbiting moons and the planet itself, but how they change over time is unclear.

Voyager only saw the rings of Neptune once and it took twelve years to get there. No spacecraft has visited the planet since.

But now that the James Webb telescope can see them clearly at any time, we can watch for the up to two decades the telescope is expected to last to see how those mysterious rings evolve.

ABOUT THE AUTHOR

Bob McDonald

Bob McDonald is the host of CBC Radio's award-winning weekly science program, Quirks & Quarks. He is also a science commentator for CBC News Network and CBC TV's The National. He has received 12 honorary degrees and is an Officer of the Order of Canada.