If NASA adopts the recommendation of the scientific community in the coming years and sends a spacecraft to Uranus, Neptune will become the only planet that humanity has not visited during a special mission. That’s a shame, because Neptune harbors intriguing mysteries. And the planet has a moon with Triton where there could be life.
We don’t often hear about Neptune. Certainly not as often as on the other planets. Space robots regularly take snapshots of the surface of Mars and the clouds of Jupiter. Mercury is a frequent scapegoat for astrology-minded people. For 13 years, the Cassini spacecraft orbited Saturn and made glorious series of observations there. And planetary scientists recently announced that NASA should prioritize sending a probe to Uranus over the next decade. Neptune’s brief foray into the news cycle last week, because of a new study on what makes Neptune so bluewas a rare occurrence.
And even that finding was an accidental discovery. The astronomers were out to study the atmospheres of both Neptune and Uranus, not to investigate the specific mystery of Neptune’s lovely apparition. The two ice giants Uranus and Neptune — so named because scientists believe the planets were originally fused together from icy materials — are often studied this way: as a pair. They have a lot in common. They are about the same size: bigger than Earth, but smaller than Jupiter and Saturn. They are surfaceless worlds, with atmospheres of hydrogen, helium and a dash of methane. And deep inside, scientists suspect, the pressure is so intense that carbon atoms squeeze into diamonds.
Neptune has always been a bit of an outlier
Scientists already knew that Neptune and Uranus get their general bluish appearance from the methane in their atmospheres, which absorbs the red hues of incoming sunlight, leaving blues and greens before our eyes. But now it has been discovered that a certain layer of methane nebula on Uranus is twice as thick as on Neptune.
The researchers suspect that Neptune, which has a more turbulent atmosphere, is better at shaking up methane particles and thinning this layer. Therefore, Uranus is a soft aquamarine and is Neptune cerulean bluethe bluest planet in our solar system.
Neptune has always been a bit of an outlier. Astronomers didn’t realize Neptune was there until they noticed Uranus, discovered through a telescope in 1781, being dragged around in its orbit by the gravity of an invisible celestial body. Neptune was finally spotted in 1846, exactly where astronomers had predicted.
Voyager 2 and The Great Dark Spot
Many years and technological leaps later, in 1989, NASA’s Voyager 2 spacecraft passed by, the last stop on a grand tour of the outer planets. The flyby gave us a close-up view of a gloriously blue world, its mane and its rings. (Yes, Neptune has rings! They’re not as glamorous as Saturn’s, but they’re there, made of tiny bits of rock and dust.) Voyager captured deep bruises in the atmosphere that turned out to be powerful storms, and scientists named the largest they saw—about the size of Earth—the Great Dark Spot.
Since then, no spacecraft has visited Neptune. Or Uranus. Planetary scientists decided in their recent recommendation to NASA that Uranus should be visited simply because Uranus is closer and would take less time to reach. That’s 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 doesn’t really give Neptune what the planet deserves. After all, there are several things to investigate.
Neptune is cooling when the planet should be warming and no one has a clue why
For example: Neptune has a lot of internal heat left over from its formation, which the planet is currently radiating into space. Neptune gives off 2.5 times as much heat as the planet absorbs from the sun, while Uranus, although closer to the sun, does not. Which is quite strange, since the ice giants are so similar in composition. All that excess makes Neptune a stormy, windswept place. (Jupiter may have the most beautiful storms in the solar system, but Neptune has the fastest winds.) And scientists are eager to understand how Neptune’s weather works.
The big dark spot Voyager 2 found? She had completely disappeared by the time the Hubble Space Telescope observed the planet in the 1990s. Other dark spots have come and gone in the same way.
Also odd: Neptune’s seasonal temperatures. It is 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 of that time, they expected to see signs that the planet was steadily warming. But Neptune is cooling down instead. No one has any idea why.
Triton: a promising candidate for extraterrestrial life
Neptune also has one of the most intriguing moons in the solar system: Triton, a planet-sized world with a smooth, icy surface. As Voyager 2 passed by the Neptunian system, it discovered plumes of nitrogen gas spewing from cracks in the moon’s terrain. Scientists believe an entire ocean is churning under Triton’s frozen crust, making it a potentially promising candidate in the search for extraterrestrial life.
NASA recently considered a mission concept for a special probe for Triton, but the space agency decided to fund two spacecraft to Venus instead. Are we ever going to Neptune again? Not with astronauts, of course, but with a spacecraft specially designed to explore the wonders of our eighth planet? The world’s newest space observatory, the James Webb Space Telescope, will begin observing the ice giant later this year and should provide unprecedented data on the nature of its atmosphere. But that’s not the same as being there. By that measure, we don’t know Neptune at all, and we won’t for a while.
A recent mission concept for a Neptune orbiter suggested launching in 2033 and arriving in 2049. But going to first would mean moving that timeline a decade, maybe even longer. So the bluest planet may have to wait well into the 2050s for a visit.
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