WASHINGTON (Reuters) - In 1781, German-born British astronomer William Herschel made Uranus the first planet discovered with the aid of a telescope.
This frigid planet, our solar system's third largest, remains a bit of an enigma 243 years later. And some of what we thought we knew about it turns out to be off the mark. Much of the knowledge about Uranus was gleaned when NASA's robotic spacecraft Voyager 2 conducted a five-day flyby in 1986.
But scientists have now discovered that the probe visited at a time of unusual conditions - an intense solar wind event - that led to misleading observations about Uranus, and specifically its magnetic field. The solar wind is a high-speed flow of charged particles emanating from the sun.
The researchers took a fresh look at eight months of data from around the time of Voyager 2's visit and found that it encountered Uranus just a few days after the solar wind had squashed its magnetosphere - the planet's protective magnetic bubble - to about 20% of its usual volume.
"We found that the solar wind conditions present during the flyby only occur 4% of the time. The flyby occurred during the maximum peak solar wind intensity in that entire eight-month period," said space plasma physicist Jamie Jasinski of NASA's Jet Propulsion Laboratory, lead author of the study published on Monday in the journal Nature Astronomy.
"We would have observed a much bigger magnetosphere if Voyager 2 had arrived a week earlier," Jasinski said. Such a visit likely would have shown that the Uranus magnetosphere is similar to those of Jupiter, Saturn and Neptune, the solar system's other giant planets, the researchers said.
A magnetosphere is a region of space surrounding a planet where the planet's magnetic field dominates, creating a protective zone against solar and cosmic particle radiation.
The Voyager 2 observations left a misimpression about the magnetosphere of Uranus as lacking in plasma and possessing uncommonly 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 magnetosphere of other planets so its low concentration observed around Uranus was puzzling.