The Webb Telescope's Grand Tour of the Solar System

The James Webb Space Telescope has revealed galaxies in the early universe, hidden star formation, and sniffed the atmospheres of exoplanets. But it's also exploring closer to home, imaging each of the giant planets in detail. The telescope can see aspects of the planets' compositions in ways that passing satellites typically can't, both thanks to its sensitivity and its spectral resolution.

At the same time, homing in on the planets has tested Webb's capabilities for tracking objects that are not only quite bright compared to distant galaxies, but also extended, rotating, and moving quickly across the plane of the sky. The planets are so bright that they can quickly saturate the detectors. Imaging also requires multiple exposures that are later combined into mosaics.

To make full use of Webb's technological advances, Leigh Fletcher (University of Leicester, UK) and colleagues are conducting the Giant Planet Atmospheres program, designed to image and take detailed data on each of the outer solar system giants. The team has even included provisions for multiple images taken in the event of an asteroid or comet strike on one of the planets.

Jupiter

Jupiter has had its share of visitors. Galileo orbited the giant from 1995 to 2003, Cassini explored the system for six months in 2000 to 2001, and Juno is still flying around the planet's poles. Now, Webb is taking its turn looking at the clouds, hazes, and circulation patterns in various atmospheric layers.

“The brightness here indicates high altitude — so the Great Red Spot has high-altitude hazes, as does the equatorial region,” explained Heidi Hammel (AURA) for the image above. “The numerous bright white ‘spots’ and ‘streaks’ are likely very high-altitude cloud tops of condensed convective storms.”

But this image is only the beginning. Webb has also taken spectra of Jupiter, spreading out its emission into an infrared rainbow. These spectra shed light on the upper atmosphere's properties, such as its composition and motions. Earlier this year, for example, Ricardo Hueso (University of the Basque Country, Spain) used these data to report the discovery of a strong equatorial jet stream.

“What we have always seen as blurred hazes in Jupiter’s atmosphere now appear as crisp features that we can track along with the planet’s fast rotation,” he noted in a press release.

Additional work is forthcoming from the Giant Planets program, including studies on Jupiter's Great Red Spot. Read more here about Webb's first image of Jupiter.

Saturn

After Cassini's 13 years of exquisite close-up data on this ringed planet, Webb is delving deeper, offering complementary data. As for Jupiter, the spectra Webb takes of Saturn's atmosphere reveal not only what it's made of but what it's actually doing.

Webb's instruments include integral field units that enable it to take an image, and every pixel of that image is its own spectrum. Astronomers start by looking for the presence and motions of specific molecules, such as propane, benzene and other hydrocarbons, Fletcher explained at the fall meeting of the Division for Planetary Sciences. "You can use all these stratospheric hydrocarbons to give you a sense of the seasonal overturning within Saturn's atmosphere."

"We see strong depletion of hydrocarbons in the northern hemisphere," Fletcher added, "and presumably enrichment in the southern hemisphere as equinox approaches." Saturn's equinox occurs in 2025.

Read more about Webb's first image of Saturn.

Uranus

Uranus is far away, faint, and frigid. Being the coldest planet (Neptune's warmer), Uranus doesn't emit much infrared. It's also a bit of a mystery, with only one spacecraft flyby by Voyager 2 in 1986. Prior to Webb, most of its infrared spectrum hadn't been explored.

"Essentially, we're pitting the most sensitive infrared telescope ever against the darkest planet," said Michael Roman (also at University of Leicester).

In the fall meeting of the Division for Planetary Sciences, Roman reported that Webb shows three vertical layers of aerosols in the planet: "We have an extended haze, a scattering layer around one to two bars that's responsible for the polar brightness, and then hints of a deeper cloud layer at two to five bars."

Roman and colleagues continue to work on understanding the distribution of temperature and chemistry both with depth and across the planet's disk.

The new, wide-field view captures changes in Uranus's atmosphere. It also shows the planet's icy moons, worthy of their own investigation:

Read more about Webb's first image of Uranus, taken earlier this year.

Neptune

Like Uranus, Neptune is fairly dark at infrared wavelengths, except where high-altitude clouds are present. While methane gas absorbs infrared light in the planet's disk, methane-ice clouds appear as bright streaks and spots, because they reflect sunlight before it is absorbed by methane gas.

Also like Uranus, Neptune remains fairly unexplored, with only Voyager 2's flyby in 1989 giving us detailed data on the planet. Voyager 2 noted Neptune's ring system is complete, but clumpy. Webb now reveals that ring system in surprising detail in just a short exposure. Individual exposures reveal arcs in those rings that don't quite match expectations.

In terms of its atmosphere, Neptune is more similar to Saturn than Uranus. Webb's shorter exposures reveal several features observed previously, such as the bright polar vortex and a clump of clouds known as the South Polar Feature, as well as a new ring of emission around the pole that hadn't been observed before.

The image also revealed some of Neptune's many moons. The biggest, Triton, reflects much more infrared light, appearing as the bright star to Neptune's upper left in the image above.

Read more about Webb's first image of Neptune here.