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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 in our night sky). For 13 whole years, the Cassini spacecraft orbited Saturn before plunging into the planet, ending 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.

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