A series of observations of Neptune by Hubble Space Telescope show that a huge dark storm raging in the giant planet's northern hemisphere was moving south, but then inexplicably took a major U-turn, heading back north. Not only that, but it may have also spawned a baby dark storm in the process.
Neptune is what's called an ice giant, basically a gigantic ball of hydrogen and helium gas with loads of methane, ammonia, and other molecules in it (that for historical reasons, planetary scientists refer to as "ices" even if they're gaseous). At nearly four times the diameter of Earth, Neptune is farthest large planet from the Sun, 4.5 billion kilometers away.
When Voyager 2 passed by Neptune in 1989, the images it returned surprised scientists; it saw an immense oval dark storm in the planet's southern hemisphere as big as Earth itself! Called the Great Dark Spot, it had measured wind speeds of a staggering 2,100 kph, the fastest wind ever measured in the solar system.
The sizes of Earth and Neptune to scale. There's a decent gap there, but not in most exoplanet systems. Credit: NASA / jcpag2012 at wikimedia
But, when Hubble looked at Neptune in 1994, ...