Much of the knowledge about Uranus was gathered when NASA’s robotic spacecraft Voyager 2 made a five-day flyby in 1986.

In 1781, the German-born British astronomer William Herschel made Uranus the first planet discovered with a telescope. This cold planet, the third largest in our solar system, remains a bit of an enigma 243 years later. And some of what we thought we knew about it turns out to be wrong.

Much of the knowledge about Uranus was gathered when the robotic space probe Voyager 2 made a five-day flyby in 1986. But scientists have now discovered that the probe visited during a time of unusual conditions – an event of intense solar wind – which led to misleading observations. about Uranus and especially its magnetic field.

The solar wind is a high-speed stream of charged particles emanating from the sun. Researchers took a fresh look at eight months of data from Voyager 2’s visit and found that it encountered Uranus just days after the solar wind had crushed its magnetosphere – the planet’s protective magnetic bubble – by about 20% from its value. the usual volume.

“We found that the solar wind conditions present during the flyby only occur 4% of the time. The flyby occurred during the solar wind’s maximum intensity over the entire eight-month period,” said space plasma physicist Jamie Jasinski of NASA’s Jet Propulsion Laboratory. , lead author of the study published Monday in the journal Nature Astronomy, opens a new tab.

“We would have observed a much larger magnetosphere if Voyager 2 had arrived a week earlier,” Jasinski said.

Such a visit would likely have shown that Uranus’ magnetosphere is similar to those of Jupiter, Saturn and Neptune, the other giant planets in the solar system, the researchers said. A magnetosphere is a region of space surrounding a planet where the planet’s magnetic field dominates, creating a shielding zone against solar and cosmic radiation.

Voyager 2 observations gave the wrong impression of Uranus’ magnetosphere as being plasma-free and possessing unusually intense belts of highly energetic electrons.

Plasma – the fourth state of matter after solids, liquids and gases – is a gas whose atoms have been split into high-energy subatomic particles. Plasma is a common feature in the magnetospheres of other planets, so its low concentration observed around Uranus was surprising.

“The plasma environment of any planetary magnetosphere typically consists of plasma from the solar wind, plasma from any moons present inside the magnetosphere, and plasma from the planet’s atmosphere,” Jasinski said.

“On Uranus, we didn’t see plasma from the solar wind or from the moon. And the plasma that was measured was very weak,” Jasinski said.

Uranus, blue-green in color due to methane contained in an atmosphere composed mainly of hydrogen and helium, is about 31,500 miles (50,700 km) in diameter. It is big enough to fit 63 Earths inside it. Of the eight planets in the solar system, only Jupiter and Saturn are larger.

Its unusual tilt makes Uranus appear to orbit the sun like a rolling ball. Uranus, which orbits nearly 20 times farther from the Sun than Earth, has 28 known moons and two sets of rings.

Voyager 2 observations had suggested that its two largest moons—Titania and Oberon—often orbit outside the magnetosphere. The new study indicates that they tend to stay inside the protective bubble, making it easier for scientists to magnetically detect potential subsurface oceans.

“Both are considered prime candidates to host liquid water oceans in the Uranian system due to their large sizes relative to the other major moons,” said Corey Cochrane, a planetary scientist and co-author of the study at the Jet Propulsion Laboratory.

Scientists are keen to find out if the subsurface oceans on moons in the outer solar system have suitable conditions to support life. NASA launched a spacecraft on Oct. 14 on a mission to Jupiter’s moon Europa to address just that question.

“A future mission to Uranus is crucial to understanding not only the planet and magnetosphere, but also its atmosphere, rings and moons,” Jasinski said.