It is the planet with the wildest, weirdest weather in the solar system. Its monster storms and equatorial winds of up to 900mph bewilder scientists.

... and how it was two years ago

Astronomers knew almost nothing about Neptune until 1989 when the Voyager probe flew past the planet. It revealed a blue gas-giant planet.

Now a series of co-ordinated observations made with the Hubble Space Telescope and the ground-based Nasa infra-red telescope in Hawaii have revealed more about Neptune's extraordinary weather.

Blending a series of Hubble images together, scientists from the University of Wisconsin-Madison have constructed a series of time-lapse movies allowing them to watch the ebb and flow of its weather systems.

Just where cold and distant Neptune gets the energy for such a remarkable weather system is a mystery.

Inhabiting the dark outer reaches of the solar system Neptune is bathed in dim sunlight that is barely one thousandth as bright as it is near Earth.

According to Neptune expert Lawrence Sromovsky: "It is an efficient weather system compared to the Earth. It seems to run on almost no energy."

From the new observations it has been possible to measure the atmospheric circulation and view a "strange menagerie of variable, discrete cloud features and zonal bands".

Some of the new features appear to go deeper into Neptune's atmosphere.

Storm the size of Earth

When Voyager passed the planet nine years ago it saw a great dark spot, a storm system nearly the size of the Earth itself. Two years ago Hubble revealed that it had disappeared and had been replaced by another dark storm.

"The great dark spot was a feature we have not seen on any other planet," said Mr Sromovsky. "They seem to come and go, and then dissipate."

Another strange aspect of the distant planet's weather are bands of clouds that run parallel to its equator.

It appears that Neptune has changed a lot since Voyager visited it. Mr Sromovsky points out that "the character of Neptune is different from the time of the Voyagers, the planet seems stable yet different".

Until next year it is Neptune and not tiny Pluto that marks the farthest planet from the Sun. Pluto's orbit sometimes brings it closer than Neptune although there is no chance of a collision between the two.