We Don’t Know Neptune at All

You don’t really hear about Neptune, do you?

Not as often as the other planets, certainly. Space robots regularly provide snapshots of the surface of Mars and the clouds of Jupiter. Mercury is a frequent scapegoat for astrology-minded folks having a bad day (even though Mercury being in retrograde is actually just an optical illusion) For 13 whole years, the Cassini spacecraft orbited Saturn before plunged into the planetending its glorious streak of observations. And planetary scientists recently announced that NASA should prioritize sending a probe to Uranus in the next decade. Indeed, Neptune’s brief foray into the news cycle last week, because of a new study about what makes Neptune so blue, was a rare appearance.

And even that finding was an accidental discovery, according to Patrick Irwin, a planetary physicist at Oxford University and the lead author of the study. Irwin told me that he and his team had set out to study the atmosphere of both Neptune and Uranus, not to investigate the specific mystery of Neptune’s lovely appearance. The two ice giants—so called because scientists believe the planets were originally glommed together from icy materials—are often studied in this way, as a pair. They have so much in common: They’re about the same size—bigger than Earth, but smaller than Jupiter and Saturn. They are surface-less worlds, with atmospheres of hydrogen, helium, and a splash of methane. And deep in their interior, scientists suspect, the pressure is so intense that carbon atoms compress into diamonds.

Scientists already knew that Neptune and Uranus get their general bluish appearance from the methane in their atmosphere, which absorbs incoming sunlight’s red hues, leaving blues and greens for our eyes to see. But Irwin and his colleagues found that a particular layer of methane haze is twice as thick on Uranus as it is on Neptune. “These atmospheres are naturally blue if there were no haze,” Irwin told me. “Adding haze makes them paler.” The researchers suspect that Neptune, which has a more turbulent atmosphere, is better at churning up methane particles and thinning out this layer. That’s why Uranus is a soft aquamarine, and Neptune is cerulean, the bluest planet in our solar system—the perfect distinction for our most neglected planet.

As the farthest planet from the sun, Neptune should, in theory, be able to bask in its place in the cosmic lineup. But everyone’s still hung up on whether Pluto, which lost its place in the final slot in 2006, counts as a fully fledged planet or not. And when planetary scientists aren’t squabbling about that, they’re searching for Planet Nine, a hypothetical world that supposedly circles the sun beyond Neptune, whose existence could explain the strange orbits of some faraway celestial bodies. In the years ahead, if NASA takes up the science community’s recommendation and dispatches a spacecraft to Uranus, Neptune will become the only planet that humankind has not visited on a dedicated mission.

Side-by-side images of Uranus and Neptune, taken by the Hubble Space Telescope
Uranus, at left, and Neptune, as seen by Hubble (A. Simon / ESA / NASA; MH Wong / University of California / OPAL)

Neptune has always been a bit of an outlier. Astronomers realized it was there only when they noticed that Uranus, discovered by telescope in 1781, was being tugged around in its orbit by the gravity of some invisible celestial body. Neptune was eventually spotted in 1846, right where astronomers had predicted. Many years and technological leaps later, in 1989, NASA’s Voyager 2 spacecraft came along, zooming past Neptune, the final stop on a grand tour of the outer planets. The flyby gave us a close-up look of a gloriously blue world, its moons, and its rings. (Yes, Neptune has rings! They’re not as glamorous as Saturn’s, but they’re therefashioned from small pieces of rock and dust.) Voyager captured deep-blue patches in the atmosphere that turned out to be powerful storms, and scientists named the biggest one they saw—about the size of Earth—the Great Dark Spot.

No spacecraft has visited Neptune since. Or Uranus. Planetary scientists, in their recent recommendation to NASA, decided to go with one ice giant over the other simply because Uranus is closer, and would take our space robots less time to reach. Such is the challenge of exploring worlds that take so long to circle the sun—84 years for Uranus, and a whopping 165 years for Neptune. A dedicated mission to Uranus, departing sometime in the early 2030s, would no doubt enrich our understanding of both ice giants. But that’s not really giving Neptune its due. The last planet in the solar system has its own story and its own idiosyncrasies—fascinating and mysterious in their own right, and worthy of our focused attention.

For example, “Neptune has loads of internal heat left over from formation, which it is currently radiating away to space,” Irwin said. Neptune gives off 2.5 times as much heat as it absorbs from the sun, while Uranus, even though it’s closer to our star, does not—which is rather strange, considering the ice giants are so similar in composition. All of that excess makes Neptune a stormy, windy place. (Jupiter might have the prettiest-looking storms in the solar system, but Neptune has the fastest winds.) And scientists are eager to understand how Neptunian weather works. The Great Dark Spot that Voyager 2 found? It had completely vanished by the time the Hubble Space Telescope observed the planet in the 1990s. Other dark spots have similarly come and gone. “There’s a lot of interesting variability in the atmosphere,” Imke de Pater, an astronomer at UC Berkeley, told me. “And particularly in the past few years, we don’t see much cloud activity at all—which is very odd.”

Also odd: Neptune’s seasonal temperatures. It’s currently summer in the planet’s southern hemisphere, and has been for nearly two decades. (On Neptune, each season lasts about 40 years.) So when planetary scientists recently examined telescope observations spanning that time, they expected to see signs of the planet steadily warming up. “Goes should be warmer in summer,” Naomi Rowe-Gurney, an astronomer at NASA’s Goddard Space Flight Center who worked on the research, told me. But Neptune, she said, is cooling down instead. “There is something happening that we really don’t understand,” Rowe-Gurney said.

Neptune has one of the most intriguing moons in the solar system, which it swiped millions of years ago from a nearby region populated by icy objects: Triton, a planet-size world, with a smooth, icy surface. When Voyager 2 swung past the Neptunian system, it detected plumes of nitrogen gas spewing from cracks in the moon’s terrain. Scientists believe there’s a whole ocean churning beneath Triton’s frozen crust, which makes it a potentially promising candidate in the search for alien life. “That’s another place in the solar system life could have started, and maybe is still there,” Kunio Sayanagi, a planetary scientist at Hampton University, told me. NASA recently considered a mission concept for a dedicated probe to Triton, but the space agency decided to fund two spacecraft to Venus instead.

So are we ever going to Neptune? Not with astronauts, of course, but with a spacecraft specifically designed to explore the wonders of our eighth planet? The world’s newest space observatory, the James Webb Space Telescope, is scheduled to observe the ice giant later this year, and should provide unprecedented data about the nature of its atmosphere. But that’s not the same as being there, in orbit. By that measure, we don’t know Neptune at all, and we won’t for some time. A recent mission concept for a Neptune orbiter suggested launching in 2033 and arriving in 2049. But going to Uranus first would mean setting that timeline back by a decade, perhaps even longer. The bluest planet may have to wait until well into the 2050s for its own spacecraft companion.

Rowe-Gurney said that it was Neptune, with its “beautiful, deep-blue color,” that first sparked her interest in planetary science. But Uranus, with its funky spinning-on-its-side quality, is her favorite planet now. Actually, none of the planetary scientists I spoke with for this story said Neptune was their favorite planet, which doesn’t really mean anything—what kind of sample size is four people? (They questioned the prompt, too; Irwin said “that’s like asking a parent to choose their favorite child!”) But still, I couldn’t help thinking, especially after we’d gone over all the wonderful things about Neptune: Ouch! Later this month, five planets will be visible in the Northern Hemisphere’s night sky, in the hours before dawn: Mercury, Venus, Mars, Jupiter, and Saturn, shining like tiny jewels. Uranus is too far to be seen by the naked eye, of course, and so is Neptune. Perhaps someday, deep in the future, Neptune will seem as familiar to us as the worlds closer in. For now, though, when I look for the twinkle of the planets in the dim morning sky—your favorite likely among them—I’ll spend an extra thought on the bluest one.


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