For decades we thought we understood Uranus pretty well. According to our best measurements, the penultimate planet has a number of features. And one of the most mysterious is its magnetic field. According to measurements taken by NASA’s Voyager 2 during its flyby in 1986, Uranus’ magnetosphere is a hot mess; It is off-center and irregular, unlike anything else in the Solar System.

This has shaped how we think about Uranus ever since, making the planet’s history a mystery that scientists are trying to unravel. There’s just one problem: Uranus’s magnetic field is probably not weird most of the time, according to space plasma physicist Jamie Jasinski of NASA’s Jet Propulsion Laboratory at Cal Institute of Technology.

Jasinski and his team conducted a new analysis and found that Voyager 2’s flyby likely occurred during a brief period when solar activity was affecting Uranus, making unusual observations.

“The scientific picture we have of Uranus since Voyager 2’s flyby is that it has an extremely magnetospheric environment. But I always thought Voyager 2’s flyby of Uranus might have occurred during some strange activity, but it didn’t. overall,” Jasinski told ScienceAlert.

“I started looking at solar wind data on Uranus, and when I saw that Voyager 2 had measured a sharp increase in the dynamic pressure of the solar wind just before the flyby, I realized that the magnetosphere must have been compressed by 20 percent of its volume. Shortly after a flyby that would impact the discoveries we made with Voyager 2 before!”

The importance of Voyager 2’s measurements of Uranus and its surroundings cannot be underestimated. These are our best and closest observations of the planet to date, and are vital to our understanding of the stinky exoplanet. Because Uranus is so far away from Earth, so diverse and difficult to reach, research efforts often focus on closer targets.

But our solar system is not static, so any observations we make must be interpreted in the context of the vagaries of space weather occurring at the time. Voyager 2 revealed a magnetosphere of Uranus unlike anything we’ve seen before, with intense radiation belts and far less plasma than we would normally expect to see based on other planets.

Following his work on NASA’s MESSENGER mission exploring the planet Mercury, Jasinski believed that this context might be missing from observations of Uranus.

“In one in a thousand orbits around the planet over a four-year period, we experienced rare, random moments when the Sun’s activity completely destroyed the entire magnetic field,” he explained.

“I really emphasized that if we had observed during one of these events, we would have a very different picture of Mercury. So, could we be observing Uranus at strange times? “Voyager 2’s transit of Uranus took only five days, so I thought we were observing Uranus at the wrong time.”

This prompted the team to go back and look at data collected by Voyager 2 a week earlier near Uranus. The researchers were on the money: The dynamic pressure of the solar wind increased by a factor of 20 just before the flyby.

In the team’s estimation, this means that fluxes from the Sun—continuous streams of particles flowing out in the solar wind—increase during the flyby, creating an environment around Uranus that is present less than 5 percent of the time.

“We knew this meant that Voyager 2’s flyby occurred under very specific conditions. The magnetosphere would have been compressed to about 20 percent of its original volume in the few days before it entered the magnetosphere, resulting in increased activity and dynamics.” Jasinki said.

“I was surprised it happened, but it was also a moment of clarity when all of Voyager 2’s strange measurements finally made sense to me.”

If the solar wind were moving at its normal speed, Uranus’ magnetic field would be more similar to the magnetic fields of the other gas giants in the Solar System, Jupiter, Saturn, and Neptune. This is a discovery with many implications. We may have to rethink how Uranus works. Based on Voyager 2 measurements, scientists have suggested that the interior of Uranus, where the magnetic field is produced, is something unique in the Solar System. This assumption may need to be revised.

A short flight never provides enough data to fully understand how a planet works. Scientists persisted in their mission to study Uranus and Neptune; It will give us the data we need to confirm Jasinski and his team’s conclusions and learn more about not only Uranus but also its lunar system.

“The dramatic compression of the magnetosphere could push all the water out of the system right before Voyager 2 gets there and destroy any evidence of satellite activity, which could change the discoveries we make during the flight,” Jasinski told ScienceAlert. “If Voyager 2 had arrived a week earlier, it would have completely would observe a different magnetospheric environment.

“It shows how dynamic this system is. It is such a mysterious planet. “We really need to go back there and investigate further.” Nature Astronomy.