Laura Faye Tenenbaum

Apr 19th 2023

What We’ve Learned From Webb’s New Images of Neptune


The planet Neptune is far, far away — more than 2.7 billion miles from Earth to be exact. It’s the outermost planet of our solar system, orbiting in one of the dimmest and most remote areas. As such, images of Neptune are difficult to come by, but thanks to the James Webb Space Telescope (Webb), we’ve been able to take a closer look at the planet.

What Does Neptune Look Like?

Webb is currently helping us get a better view of Neptune. Typically, Webb looks into deep space to search for dark stars and supernovas. In September 2021, astronomers pointed Webb’s Near-Infrared Camera (NIRCam) at Neptune, where it captured the clearest view of the planet’s rings since 1989 when Voyager 2 became the first spacecraft to observe the distant planet. Webb’s cameras also clearly showed Neptune’s fainter dust bands for the first time.

The planet Neptune is characterized as an ice giant due to the chemical makeup of its interior. Compared with gas giants Jupiter and Saturn, which are predominantly hydrogen and helium, Neptune is richer in heavier elements and molecules, such as water, ammonia and methane. The icy planet’s atmosphere is made of hydrogen, helium and methane. It’s this methane gas that gives the ice giant its signature blue color in images taken with the Hubble Space Telescope’s visible wavelengths cameras.

However, Webb’s NIRCam uses the near-infrared range of the electromagentic spectrum. As a result, Neptune does not appear blue in these images. In fact, Neptune’s methane gas strongly absorbs red and infrared light, so the planet looks dark at these wavelengths, except where high-altitude clouds are present. Neptune’s methane-ice clouds, which appear as bright streaks and spots reflecting sunlight, are the most prominent aspect of Webb’s images of the planet.

Because Webb creates such stable and precise images, it’s also been able to detect crisp, clear views of several of Neptune’s bright, narrow rings. Some of these rings have never been seen at all, let alone with this amount of clarity. In addition to the new, clear views of Neptune’s rings, Webb has also detected the planet’s fainter dust bands. These dust bands show up as fuzzy particles in between the brighter, ice-dominated rings.

There’s Still Much More to Learn

While looking closely to answer the question, “What does Neptune look like?” astronomers found a thin line of brightness that circles the ice giant’s equator. Neptune has the highest measured wind speeds in the entire solar system, with average wind speeds of 700 miles per hour near the equator. Astronomers believe these glowing bands could be clouds and storms in Neptune’s atmosphere — in other words, visual evidence of the planet’s powerful winds, churning storms and turbulent weather.

The storm clouds are most likely produced when warmer gas flows toward Neptune’s midlatitudes as part of the planet’s global atmospheric circulation. When the gases in Neptune’s atmosphere descend and warm at the equator, they glow at infrared wavelengths more than the surrounding, cooler gases, which means they show up on Webb’s cameras as bright bands.

Images of Neptune, including its rings, dust bands, high-altitude clouds and weather, aren’t all that’s been captured. Webb also caught images of seven of Neptune’s 14 known moons: Galatea, Naiad, Thalassa, Despina, Proteus, Larissa and Triton. Triton is Neptune’s largest moon. It’s covered in a frozen sheen of condensed nitrogen and reflects most of the sunlight that hits it, which means it looks like a bright point of light in some images. Triton orbits Neptune in a backward orbit, leading astronomers to speculate that this moon was originally a Kuiper belt object that was gravitationally captured by Neptune. Additional Webb studies of both Triton and Neptune are planned in 2023.

Neptune is tilted at a 28-degree angle along its axis, so it experiences four seasons, just like Earth. But one Neptunian year, or the time it takes for Neptune to orbit the sun, is equivalent to 164 Earth years, so each season on Neptune lasts 40 years. Astronomers plan to use Webb to continue studying Neptune, and by comparing these recent images with future ones, they hope to learn more about Neptune’s seasonal effects and dynamic atmosphere.

Astronomers also plan to use Webb to conduct more planetary research within our solar system and beyond for as long as possible, so stay tuned. There’s always more to learn!

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