The evolution of ice giant science

Like most planets, our understanding of Uranus and Neptune has greatly evolved since the early days of planetary science.

Before photographic imagery became a common tool for studying the universe, astronomers spent long, cold nights hunched over telescope eyepieces, hand-drawing what their eyes saw.

Heidi Hammel, a Planetary Society board member and the executive vice president of AURA, a consortium of universities that operates astronomical observatories, told me some early Uranus drawings portrayed the planet with distinct atmospheric features. The advent of photography changed that, revealing the planet as a featureless, pale-green orb. This led to a theory that planets farther from the Sun exhibited less atmospheric activity.

Intuitively, this made sense: Jupiter has vibrant bands and swirls, Saturn is stormy but muted, and Uranus was featureless (Neptune was too far away to discern much of anything).

"When Voyager flew by Uranus, it was like, 'Well, okay, we were right,'" Hammel said. "There were no clouds to look at there. It was very bland, with maybe 10 discrete cloud features."

Neptune was expected to be equally dull. But when Voyager arrived three years later, that wasn't the case.

"Neptune was a huge shock," she said. "There was this huge dark spot, like a quarter of the size of the planet—really monstrous. And then all of these bright features, and eventually a second dark feature, and all sorts of clouds. I mean, this planet was just absolutely covered with storm systems."

Why, then, was Uranus so bland? The question went unaddressed for years, until Hammel saw an academic poster at a conference with long-exposure images of Uranus captured by the Hubble Space Telescope. The images, taken in support of a search for new moons, inadvertently revealed a set of features not seen by Voyager 2.

"I said, what is that?" Hammel recalled. The poster author replied, "Well, that's Uranus."

Uranus, famously, orbits the Sun tipped on its side. When Voyager flew past, the planet's pole was facing the Sun. Since then, Uranus' 84-year orbit around the sun has progressed enough to illuminate more of the equator. This has apparently had a drastic effect on the planet's atmosphere, activating a host of swirls and storms.

Follow-up observations with adaptive optics-equipped ground telescopes continued to reveal new features. As for the early Uranus photographs, Hammel said atmospheric distortion likely smeared out discernible features, meaning those original astronomical drawings may have been correct all along.

"Only now do we have the technology to reproduce what the human eye was able to see back in the eighteen hundreds," she said.