In 1781, German-born British astronomer William Herschel made Uranus the first planet discovered with a telescope. But, 243 years later, we still do not know much about this cold planet, the third largest in our solar system. And some of what we thought we knew about it turns out to be untrue.
Much of the knowledge about Uranus was collected when NASA's robotic spacecraft Voyager 2 flew past it in 1986. But scientists have since learned that the craft visited at a time of unusual conditions. An intense solar wind event at the time led to misleading observations about Uranus, and especially its magnetic field.
Observations in a solar wind event
The solar wind is a high-speed flow of charged particles coming from the sun. The researchers took a second look at eight months of data from around the time of Voyager 2's visit and found that it flew near Uranus just a few days after the solar wind had reduced the planet's magnetosphere to about 20 percent of its usual size.
A magnetosphere is an area of space surrounding a planet that is controlled by the planet's magnetic field. The magnetosphere protects the planet from solar and cosmic particle radiation.
The Voyager 2 observations led scientists to think Uranus' magnetosphere lacked in plasma and had very intense belts of highly energetic electrons.
Plasma is often called the fourth state of matter after solids, liquids and gases. In the night sky, plasma gives out light in the form of stars, nebulas, and even the auroras above the north and south poles. Plasma is a common feature in the magnetosphere of other planets, so scientists did not understand why there would not be much of it observed around Uranus.
Jamie Jasinski studies space plasma at NASA's Jet Propulsion Laboratory. He is the lead writer of the study published recently in the publication Nature Astronomy.
"We found that the solar wind conditions present during the flyby only occur 4 percent of the time. The flyby occurred during the maximum peak solar wind intensity in that entire eight-month period," Jasinski said. "We would have observed a much bigger magnetosphere if Voyager 2 had arrived a week earlier."
Such a visit would likely have shown that the Uranus magnetosphere is like those of Jupiter, Saturn and Neptune, the solar system's other large planets, the researchers said.
Future exploration
Uranus looks blue green because of the methane contained in its atmosphere. The planet is big enough to fit 63 Earths inside it. Among the solar system's eight planets, only Jupiter and Saturn are larger.
Uranus orbits almost 20 times further from the sun than Earth does. And it has 28 known moons and two sets of rings.
Data collected by Voyager 2 had suggested that its two largest moons - Titania and Oberon - often orbit outside the magnetosphere. But the new study shows that they usually stay inside the protective space, making it easier for scientists to magnetically detect possible subsurface oceans.
"Both are thought to be prime candidates for hosting liquid water oceans in the Uranian system...," Jet Propulsion Laboratory planetary scientist and study co-writer Corey Cochrane said.
Scientists want to learn whether subsurface oceans on moons in the outer solar system have conditions that could support life. So, on October 14, NASA launched a spacecraft on a mission to Jupiter's moon Europa to try to answer 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.