Science | November 5, 2024

The Six Most Amazing Discoveries We’ve Made About Neptune

Despite the lack of a dedicated mission to the planet, scientists have learned plenty through ground observations and space telescopes

A photo of Neptune, taken by the Voyager 2 probe, with the colors rebalanced to represent its truest appearance NASA / Voyager 2 / PDS / OPUS / zelario12 (washed out color image) via Wikipedia under CCO

Shi En Kim

Reporter

When it came to discovering Neptune, scientists didn’t need to see to believe. The eighth planet in our solar system was detected, not with telescopes, but through math. In 1846, scientists had observed irregularities in Uranus’ orbit, as if an invisible counterweight were tugging on the planet from the far side. From these observations, researchers calculated the position of a hypothetical planet, then pointed their best telescopes there. Thus Neptune was found, plucked from a sea of stars in the night sky.

About 30 times farther out from the sun compared with Earth, Neptune sits at the fringes of our solar system and barely within range of our ground-based telescopes. The planet is so remote that the Voyager 2 mission, the only spacecraft to have paid a visit, took 12 years to arrive there from Earth. For four months in 1989 it skimmed that distant world and gathered valuable data on Neptune’s bizarre features.

Neptune belongs to the class of ice giants whose membership also includes its neighbor, Uranus. As with the gas giants, Neptune’s outer shell is primarily made up of hydrogen and helium. Deeper within, the mantle and core are no longer gassy but instead rich with compressed exotic ices.

Since the Voyager 2 flyby, ever-improving terrestrial telescopes, the Hubble spacecraft and the James Webb observatory have brought Neptune into clearer view, allowing scientists to advance their understanding of its skies, meteorology and moons. Neptune is a world that’s constantly remaking itself and enthralling its observers. Here are some of the most exciting discoveries that scientists have made of that ocean-colored planet.

Neptune’s true colors aren’t deep blue

Neptune's Colors — In a study published in early 2024, scientists reprocessed the images of Neptune from the Voyager 2 mission to reveal the planet’s true colors. Patrick Irwin

Neptune is named after the god of the ocean for its azure coat, but, in truth, its colorings aren’t really close to sea blue. The sapphire hues in the Voyager 2 mission photos were exaggerated by image processing to showcase the swirling cloud patterns more prominently. New research published this year rebalanced Neptune’s saturation levels and revealed that the planet actually has a pale green-blue wash similar to Uranus.

Still, Neptune is a tad bluer. Its teal tones come from methane gas in its hydrogen- and helium-rich atmosphere. Methane is adept at absorbing red wavelengths in the visible spectrum and scattering blue light. Uranus has methane, too—more, in fact—but the strength of its blue is diluted by haze in its upper atmosphere. In contrast, Neptune is better at clearing its skies because it has a more active airspace. Winds on Neptune churn up methane gas from the deep, and they glom onto aerosol particles in the atmosphere, causing haze to rain out as snow.

The traces of methane on Neptune do more than give the planet a cobalt coat. They reveal the planet’s origins—that Neptune likely migrated inward sometime during its early history. Neptune’s methane gas comes from methane ice, and scientists think such ice only forms on colder worlds that are much farther out from the sun. Scientists think that a young Neptune lost speed and allowed itself to be reined in by the sun’s gravity, leaving behind its methane cache as a clue of its far-flung birthplace.

Crazy weather graces Neptune’s skies

Diamond Rain on Neptune — An artist’s illustration of the diamond rain in Neptune’s skies Greg Stewart / SLAC National Accelerator Laboratory

The gas giants might be famous for having the largest storms in the solar system, but Neptune holds the title for having the fastest winds. Supersonic gales blow at 1,200 miles per hour, around five times faster than the strongest gusts ever measured on Earth.

Scientists are scratching their heads over what drives these powerful tempests. Being the farthest planet from the sun in our solar system, Neptune doesn’t receive enough sunlight to sustain its windy tendencies. Instead, some pundits finger Neptune’s inner heat as the power source. Nick Teanby, a planetary scientist at the University of Bristol in England, says scientists are still trying to figure out what is happening inside the planet. “There’s something going on with this deep interior,” he says.

Neptune’s relatively large size—it’s four times girthier than Earth—has allowed it to retain its primordial heat, so much so that it radiates twice as much warmth than it receives from the sun. In fact, Neptune runs hotter than Uranus, Voyager 2 revealed.

Besides the squalls, it rains on Neptune, too—not liquid droplets but diamonds. To be clear, scientists haven’t directly observed rock rain on Neptune, but they think it’s plausible, because Neptune is covered in a thick gassy sheath, whose inner pressures are crushing enough to forge diamonds from methane in the atmosphere. Laboratory experiments by scientists since 1999 have recreated the conditions inside Neptune, and they confirmed that the pressures are indeed capable of squeezing and heating organic compounds into stone. But mining Neptune’s deep interior is impossible, because no robot can survive at the planet’s extreme pressures.

The moon Triton tends to let off steam

Neptune and Triton — A photo montage of Neptune peeking behind its largest moon, Triton NASA / JPL

Among Neptune’s 16 known moons, the largest, Triton, is an oddball. It has fewer things in common with its other lunar siblings than it does with Pluto. That’s because, like the former planet, Triton is a captured Kuiper Belt object—it originally belonged to a legion of icy shards that prowled the edges of the solar system before being snagged on Neptune’s gravity. When Triton first entered orbit, it likely caused chaos among the other moons such that they collided with each other and reconfigured themselves. As further evidence of its oddity, Triton encircles its parent in the opposite direction compared with the other moons.

But running counter to Neptune’s spin is an unsustainable slog. Neptune’s gravity is slowly sapping energy from Triton, causing the moon to inch closer to Neptune. Scientists predict that Triton might one day wander too close and end up becoming gravitationally sheared into fragments.

Triton’s peculiarity doesn’t end there. When Voyager 2 dropped by the Neptunian system, the spacecraft observed five-mile-high plumes bursting out of Triton. The plumes rain out as snow on the moon’s surface or form the moon’s rich ionosphere, a lunar sheath of charged particles. Some scientists suspect that the plumes come from the sun heating through Triton’s ice cap until a pressure buildup causes a watery explosion, similar to the mechanism of popcorn. In recent years, scientists have proposed an even more outlandish explanation: The plumes are hydrothermal ejecta from an underground ocean.

“In the ’80s, we had no room in our imagination that there could possibly be liquid water that far out in the solar system,” says Abigail Rymer, a space physicist at the Johns Hopkins University Applied Physics Laboratory. But discoveries of ocean worlds on Saturn’s Enceladus and Jupiter’s Europa, both of which have spouted similar-looking plumes, have made scientists cautiously hopeful that the Neptunian moon could also be harboring a sodden secret.

Ever-changing spots mottle Neptune’s surface

Jupiter boasts a Great Red Spot, but its counterparts exist on Neptune. During the Voyager 2 flyby, the spacecraft observed a vortex amid Neptune’s clouds that scientists later dubbed as the Great Dark Spot. Heidi Hammel, a planetary astronomer at the nonprofit Association of Universities for Research in Astronomy, recalls tracking this storm cloud with ground telescopes on Hawaii’s Mauna Kea when she was a graduate student. But when the Hubble Space Telescope turned its gaze toward Neptune’s dark eye in 1994, the spot had disappeared.

Since then, at least five more shadowy spots have peppered Neptune’s face. Unlike Jupiter’s long-lasting vortex, Neptune’s blemishes come and go within five years or so. Their bruised appearances may come from upper clouds parting to reveal darker layers underneath, or perhaps a dark cloud tower looming in the stratosphere. Through Hubble, scientists have also observed these smudges zigzagging across Neptune’s hemispheres. Moreover, these dark spots are often flanked by bright streaks, a product of winds flowing over the storms and freezing out methane ice crystals that glint.

This phenomenon shows just how incredibly complex Neptune is, Hammel says. “This planet is really changeable. It’ s not like a leopard, right? It changes its spots.” But how it conjures up its storms is still a tantalizing mystery—“we’ll need another mission someday to Neptune to truly figure that out,” she adds.

The magnetic field is a mess

Most planets in the solar system have a magnetic field whose spheres of influence look like the symmetric lobes of an hourglass. But the ice giants deviate from their planetary brethren. Neptune “has got a screwy magnetic field,” Hammel says.

For starters, the planet’s magnetic field lines look like the head and arms of a flailing jellyfish. Whereas other planets have a magnetic field axis that skewers the core and is slightly tilted from the spin axis, Neptune’s magnetic axis not only completely misses the center but also doesn’t go all the way through. In other words, the magnetic pole is buried inside the planet, somewhere between the cap and the equator.

Neptune’s mantle is a mix of superionic ices that’s neither quite liquid nor solid. The mobile ions here are responsible for drumming up Neptune’s magnetic shield. Given that its magnetosphere factory lies in the mantle layer rather than the core, the planet pays no heed to the centrality rules that govern typical magnetospheres.

Such a wonky magnetosphere would probably give rise to peculiar auroras unlike what we see on Earth. “There have been a lot of open questions about what the interaction of the solar wind with Neptune’s magnetosphere looks like,” says Carol Paty, a magnetosphere scientist at the University of Oregon. Earth’s telescopes aren’t strong enough to view Neptune’s auroras directly, but scientists think that they blossom in weak patches close to the equator, rather than trace circular streams near the poles.

Neptune’s magnetosphere is large enough to envelop Triton. In fact, scientists suspect that the magnetosphere coaxes Triton’s energetic particles to cross over to Neptune’s atmosphere and excite auroral flares on the planet.

Like the other planetary giants, Neptune has rings

Neptune's Rings — When viewed in infrared wavelengths, Neptune’s rings become apparent. NASA, ESA, CSA, STScI

Neptune joins the ranks of Jupiter and Saturn in that it also has rings, albeit a set that isn’t quite so prominent. Scientists first detected Neptune’s gossamer halo in 1984 using ground-based telescopes. When Neptune crossed in front of a star, the planet seemed to block some of the starlight even before reaching a full eclipse, suggesting that some wispy mass drifted beyond the geographical boundary of the planet—a ring, perhaps. Even then, scientists disputed the ring’s existence for years because of inconsistent observations. Finally, Voyager 2 put an end to those arguments once and for all, after it saw Neptune’s ring in its full glory close up. It turns out, Neptune’s ring is clumpy, so some sections are harder to detect than others. The arcs have shifted slightly since their first detection, leading scientists to believe that the rings are young and still settling in.

Since Voyager, scientists have advanced ground-based telescope technology to reliably discern the rings’ faint signal. But the James Webb Space Telescope takes the cake at revealing Neptune’s wreath in unprecedented detail, and the image it captured is a showstopper. When Hammel first saw these new visualizations in 2022, “I actually started crying,” she says.

Shi En Kim | | READ MORE

Shi En Kim is a Washington, D.C.-based freelance science journalist. Her work has appeared in National Geographic, Scientific American, the Atlantic, Popular Science and others. In 2021, she interned at Smithsonian magazine as an AAAS Mass Media Fellow.