The NASA/ESA/CSA James Webb Space Telescope is showing off its capabilities closer to home with its first image of Neptune. Not only has Webb captured the clearest view of this peculiar planet’s rings in more than 30 years, but its cameras are also revealing the ice giant in a whole new light.
Most striking about Webb’s new image is the crisp view of the planet’s dynamic rings — some of which haven’t been seen at all, let alone with this clarity, since the Voyager 2 flyby in 1989. In addition to several bright narrow rings, the Webb images clearly show Neptune’s fainter dust bands. Webb’s extremely stable and precise image quality also permits these very faint rings to be detected so close to Neptune.
Neptune has fascinated and perplexed researchers since its discovery in 1846. Located 30 times farther from the Sun than Earth, Neptune orbits in one of the dimmest areas of our Solar System. At that extreme distance, the Sun is so small and faint that high noon on Neptune is similar to a dim twilight on Earth.
This planet is characterised as an ice giant due to the chemical make-up of its interior. Compared to the gas giants, Jupiter and Saturn, Neptune is much richer in elements heavier than hydrogen and helium. This is readily apparent in Neptune’s signature blue appearance in NASA/ESA Hubble Space Telescope images at visible wavelengths, caused by small amounts of gaseous methane.
Webb’s Near-Infrared Camera (NIRCam) captures objects in the near-infrared range from 0.6 to 5 microns, so Neptune does not appear blue to Webb. In fact, the methane gas is so strongly absorbing that the planet is quite dark at Webb wavelengths except where high-altitude clouds are present. Such methane-ice clouds are prominent as bright streaks and spots, which reflect sunlight before it is absorbed by methane gas. Images from other observatories have recorded these rapidly-evolving cloud features over the years.
More subtly, a thin line of brightness circling the planet’s equator could be a visual signature of global atmospheric circulation that powers Neptune’s winds and storms. The atmosphere descends and warms at the equator, and thus glows at infrared wavelengths more than the surrounding, cooler gases.
Neptune’s 164-year orbit means its northern pole, at the top of this image, is just out of view for astronomers, but the Webb images hint at an intriguing brightness in that area. A previously-known vortex at the southern pole is evident in Webb’s view, but for the first time Webb has revealed a continuous band of clouds surrounding it.
Webb also captured seven of Neptune’s 14 known moons. Dominating this Webb portrait of Neptune is a very bright point of light sporting the signature diffraction spikes seen in many of Webb’s images; it’s not a star, but Neptune’s most unusual moon, Triton.
Covered in a frozen sheen of condensed nitrogen, Triton reflects an average of 70 percent of the sunlight that hits it. It far outshines Neptune because the planet’s atmosphere is darkened by methane absorption at Webb’s wavelengths. Triton orbits Neptune in a bizarre backward (retrograde) orbit, leading astronomers to speculate that this moon was actually a Kuiper Belt object that was gravitationally captured by Neptune. Additional Webb studies of both Triton and Neptune are planned in the coming year.
The James Webb Space Telescope is the world’s premier space science observatory. Webb will solve mysteries in our Solar System, look beyond to distant worlds around other stars, and probe the mysterious structures and origins of our Universe and our place in it. Webb is an international program led by NASA with its partners, ESA and the Canadian Space Agency. The major contributions of ESA to the mission are: the NIRSpec instrument; the MIRI instrument optical bench assembly; the provision of the launch services; and personnel to support mission operations. In return for these contributions, European scientists will get a minimum share of 15% of the total observing time, like for the NASA/ESA Hubble Space Telescope.