From NASA Goddard:
Uranus is (very, very slowly) losing "weight." Our 7th planet's tiny mass loss was known only to Voyager 2 — until Goddard scientists Gina DiBraccio and Dan Gershman did a double-take on the satellite's data, more than 30 years later.
Uranus' twisted magnetic field allows tiny amounts of the planet's atmosphere to leak out into space, potentially taking billions of years for significant loss to occur. This behavior has been observed at other planets, like Venus, Earth and Jupiter, but never at distant Uranus until now.
How much of an effect has this had on Uranus? The data from Voyager 2 is the one time Uranus has gotten on the scale, so to speak, so that's a hard question to answer. But the remaining mystery is part of the draw. "It's why I love planetary science," DiBraccio said. "You're always going somewhere you don't really know." Image: Voyager 2 took this image as it approached the planet Uranus on Jan. 14, 1986. The planet's hazy bluish color is due to the methane in its atmosphere, which absorbs red wavelengths of light.
Photo Credit: NASA/JPL-Caltech