"Spring" is arriving on distant Neptune. The Hubble Space Telescope has detected an increase in the planet's cloud cover in the southern hemisphere, considered a sign of seasonal change, say astronomers.
By Dr David Whitehouse
BBC News Online science editor
Observations of Neptune - the most distant gas giant planet from our Sun - made over six years, show an increase in the brightness of banded cloud features.
The clouds are brighter now...
"Neptune's cloud bands have been getting wider and brighter," says Lawrence Sromovsky of the University of Wisconsin-Madison, US.
"This change seems to be a response to seasonal variations in sunlight, like the seasonal changes we see on Earth."
Neptune is known for its remarkable weather systems. It has massive storms and ferocious winds that sometimes gust to 700 km/h (435 mph).
The new Hubble observations are the first to suggest that the planet undergoes a change of seasons.
Images taken in 1996, 1998 and 2002 showed progressively brighter bands of clouds encircling the planet's southern hemisphere - findings consistent with coarser ground-based observations.
"In the 2002 images, Neptune is clearly brighter than it was in 1996 and 1998," says Sromovsky.
"The greatly increased cloud activity in 2002 continues a trend first noticed in 1998."
Four (long) seasons
Like the Earth, Neptune would have four seasons: a warm summer and a cold winter, with spring and autumn being transitional seasons.
Because it takes Neptune 165 years to orbit the Sun its seasons will last for decades. If what scientists are observing is truly seasonal change, the planet will continue to brighten for another 20 years.
What is remarkable, say researchers, is that Neptune exhibits any evidence of seasonal change at all, given that the Sun, as viewed from the planet, is 900 times dimmer than it is as seen from Earth.
...than they were a few years ago
There seems, Sromovsky says, to be a "trivial amount of energy available to run the machine that is Neptune's atmosphere".
"It must be a well-lubricated machine that can create a lot of weather with very little friction."
Near-infrared observations at the US space agency's Infrared Telescope Facility on Mauna Kea, Hawaii, are planned for this summer to further monitor the changes on Neptune.