Uranus Might Have Experienced a Freak Event When Voyager 2 Visited

Our understanding of Uranus might have been all wrong for nearly 40 years.

In January 1986, NASA’s Voyager 2 spacecraft zoomed past Uranus as part of a grand tour of the outer solar system. That flyby, lasting just five hours, remains to date the only visit to Uranus by a spacecraft from Earth, with much of our understanding of the planet coming from that brief encounter.

Peculiarly, as Voyager 2 soared about 50,000 miles above the planet, it found that Uranus was quite different from other worlds in the outer solar system. In particular, its protective magnetic field, known as a magnetosphere, was devoid of plasma, something prevalent around other planets.

“We observed this empty magnetosphere,” said Jamie Jasinski, a space plasma physicist at NASA’s Jet Propulsion Laboratory. And its radiation belts, regions of the planet’s magnetic field that trap high-energy particles, were surprisingly intense, something that “breaks current radiation belt theory,” Dr. Jasinski said.

Now we may know why. In a paper published Monday in the journal Nature Astronomy, Dr. Jasinski and his colleagues suggest that Voyager 2’s visit flyby occurred during an exceptional increase in solar activity, which caused shrinking of the planet’s magnetosphere. That created conditions at Uranus that occurred just four percent of the time in the data that the team analyzed.

“If we had arrived a week earlier, we would have had a completely different picture of Uranus,” Dr. Jasinski said.

Fran Bagenal, an astrophysics and planetary science professor at the University of Colorado, Boulder, and a member of the Voyager program’s plasma science team, said it was a “big surprise” when she saw Dr. Jasinski present the research at a conference this summer. “Why didn’t we see this?” she said. “I was kicking myself. It was completely out of the blue.”

The research could also spark important questions as scientists plan a possible return to this fascinating world by a NASA space probe.

When Voyager 2 swooped past Uranus, Dr. Jasinski believes the planet was being hit by something called a co-rotating interaction region. This is a burst of activity from the sun’s surface that sweeps out into space as the star rotates, ejecting long streams of plasma to the edge of the solar system. This phenomenon has been observed affecting other planets, such as Saturn. But it hadn’t been considered for Uranus.

“Their result is solid,” said Adam Masters, a space and planetary scientist at Imperial College London. “Everyone was focused on data taken near to the planet, but they looked back at data taken when the spacecraft was approaching and leaving the Uranus system.”

Dr. Bagenal added that “it explains why the density that we saw was so low.”

The solar activity would have increased the pressure of the solar wind on Uranus’s magnetosphere by 20 times as much, Dr. Jasinski said, squashing it to just a fifth of its normal volume. This would explain both the decreased amount of plasma detected near Uranus, which would be trapped closer to the planet inside the magnetosphere, and the intensity of the radiation belts, which would have been filled with energetic electrons from the sun.

NASA is working on a mission to return to Uranus next decade. It plans to launch a spacecraft by 2032 that would orbit the planet for the first time and send a probe into its atmosphere. The new paper highlights just how little we know about the planet, and how eager scientists are to return.

And scientists say there are good reasons to study Uranus and its fellow ice giant, Neptune: they provide a point of comparison for understanding many similar worlds around distant stars.

“The reason we care about Uranus and Neptune, and their quirky magnetic fields and interiors, is because the most populous class of exoplanets are super-Earths and sub-Neptunes,” said Heidi Hammel, an astronomer and planetary scientist at the Association of Universities for Research in Astronomy.

And many planetary scientists are enthusiastic about better understanding Uranus, which is particularly unusual in that it orbits on its side relative to the other planets, possibly the result of a giant impact early in its life. “It’s got extreme seasons, and the magnetic field is rotating at a very weird angle,” Dr. Masters said. Other questions abound, he added: Where did Uranus form? How does its atmosphere work? Do its moons conceal oceans like those around Jupiter, Saturn and Neptune?

“We have lots of mysteries we want to resolve,” he said.

Perhaps the planet, though, is slightly less unusual than Voyager 2’s flyby suggested. “This is why we need to go back,” Dr. Bagenal said.

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