Some of Uranus’ apparent oddities might be due to bad timing.

In 1986, the Voyager 2 spacecraft flew past the planet, recording mysteries of its magnetic field. Turns out, Uranus may have just been in an unusual state. A solar wind event days before the flyby compressed the giant planet’s magnetosphere, researchers report November 11 in Nature Astronomy. That compression could explain several long-standing puzzles about Uranus and its moons, and could inform planning for future missions (SN: 4/20/22).

“We just caught it at this freak moment in time,” says Jamie Jasinski, a space plasma physicist at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. “If you had known that going in, you would have questioned everything that Voyager 2 measured.”

Voyager 2 found that Uranus’ magnetosphere, the bubble of magnetism surrounding a planet, was bizarre. It seemed to lack plasma, a common component of other planets’ magnetospheres. And it had inexplicably intense belts of energetic electrons.

Jasinski and colleagues looked back at data Voyager 2 collected months before the flyby (SN: 2/1/86). The team found that the density and speed of the solar wind, a stream of charged particles emanating from the sun, increased steadily for days.

The pressure from that solar wind would have compressed Uranus’ magnetosphere, shrinking its extent from an estimated 28 times Uranus’ diameter to more like 17 times it within a week. The compression could account for both the lack of plasma and the intense radiation belts, Jasinski says.

In fact, Uranus is in the state in which Voyager 2 found it only 4 percent of the time, the team calculates. That means much of what we know about Uranus’ magnetosphere does not represent a typical day there.

“We don’t really know anything about Uranus, because it was a single flyby,” says Corey Cochrane, a space physicist also at JPL.

On the plus side, the new finding means it might be easier for some future mission to search for oceans beneath the surface of Uranus’ moons Titania and Oberon.

Astronomers can detect oceans on icy moons if they orbit inside the magnetosphere (SN: 10/8/24). Salty water responds to the magnetic field around it and produces its own magnetic field, which spacecraft can pick up. If Uranus’ magnetosphere is normally bigger than documented by Voyager 2, those moons should be well within it — and therefore good sites to search for subsurface seas.