By | September 22, 2022
Neptune and its rings come into focus with the Webb telescope

No spacecraft has visited Neptune since 1989, when the NASA probe Voyager 2 flew by on its way out of the solar system. Neptune, which is four times as wide as Earth, is it the most distant planet in our solar system. Voyager 2’s observations whetted the appetite of astronomers, who were eager to learn more about the ice giant.

Now we have returned. Approximately.

On Wednesday, the James Webb Space Telescope cast its powerful gold-plated eye on this distant world. The power of this infrared machine, the largest and most advanced telescope ever sent into space, has provided some of our best views of Neptune in 30 years.

“I’ve been waiting so long for these images of Neptune,” said Heidi Hammel, a NASA interdisciplinary scientist for the Webb Telescope and vice president for science at the Association of Universities for Research in Astronomy in Washington, D.C. “I’m so glad it has worked.”

Ground-based observatories and Hubble Space Telescope has taken many pictures of Neptune over the past three decades. But Webb’s view of Neptune, taken in July, provides an unprecedented glimpse of the planet in infrared light.

It took just a few minutes for the telescope to image Neptune up close and another 20 minutes to take a wider view, revealing not only the planet but countless galaxies behind it stretching into the cosmos. “It’s aesthetically fascinating to see these distant galaxies and get a sense of how small the ice giant seems,” said Klaus Pontoppidan, Webb project scientist at the Space Telescope Science Institute in Baltimore, which operates the Webb telescope.

Most prominent in the telescope’s view are Neptune’s rings, which are seen encircling the planet at a slight tilt given its orientation to Earth. The Webb telescope allows astronomers to measure the rings’ reflectivity, providing unprecedented insight into this distant spectacle. New images may reveal the size and composition of these thin bands, which are likely made of ice and other debris.

“The ring system was absolutely amazing to me,” said Dr. Hammer. “I haven’t seen it in that level of detail since the Voyager encounter in 1989. It just pops up instantly.”

All over the planet are bright spots believed to be clouds of methane ice, which rise high into the planet’s sky and can persist for days.

“No one really knows what these things are,” said Patrick Irwin, a planetary physicist at Oxford University. “They seem to come and go, a bit like cirrus clouds on Earth.” Future observations by the Webb Telescope could reveal how they form and what they are made of.

Web images also show seven of Neptune’s 14 moons. The brightest is Triton, the planet’s largest moon, which scientists suspect was captured by Neptune’s gravity early in the solar system’s history. In infrared images, Triton’s frozen nitrogen surface makes it shine like a star, brighter than Neptune itself, as methane dims the planet in infrared light. NASA recently declined to send a mission to study Triton, and not much can be gleaned about it from this image. But future Webb observations should hint at the composition of Triton’s surface and may show changes indicative of geological activity.

“Triton is a geologically active world,” said Dr. Hammer. “When Voyager 2 flew by, it saw cryovolcanoes erupting. So there’s a possibility that there are changes in surface chemistry over time. We’ll be looking for that.”

Dr Hammel also believes that a glimpse of Hippocamp, an eighth Neptunian moon, is pictured just above the planet. “It’s very faint, but it’s in the right place,” she said.

These images of Neptune are just the latest in Webb’s tour of the solar system. This week we were treated to the telescope first glimpses of Marswhile during the summer we saw amazing view of Jupiter. Much more of our solar system will come under the observatory’s roving eye, including Saturn, Uranus and even distant icy objects beyond Neptune – like the dwarf planet Pluto.

“It illustrates that we are an all-purpose observatory,” said Mark McCaughrean, a Webb telescope scientist and senior science advisor at the European Space Agency. “We can observe very bright things like Mars and Neptune, but also very faint things. Everyone now sees that it works.”

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