The Hubble Space Telescope spent hours with its eye turned toward Jupiter, capturing new images of the spectacular atmosphere and mysterious Red Spot of our solar system's largest planetary inhabitant.

The photos show ongoing changes to the Red Spot, in actuality a long-lived atmospheric storm that has been swirling south of the giant planet's equator for at least 300 years, visible from Earth since the beginnings of modern astronomy.

The new images confirm the spot is shrinking as the storm weakens; in the last year, it has shrunk by 150 miles.

It is now half the width it was 100 years ago, when it was around 25,000 miles across.

It's also not as red as it used to be, astronomers say, with less intense colors at its center and newly-observed wispy filaments twisting and turning through it in 330 mph winds.

That filamentary feature, along with a rare atmospheric wave feature just slightly north of the equator, are new discoveries in Hubble's annual mapping of Jupiter, they report in the Astrophysical Journal.

"Every time we look at Jupiter, we get tantalizing hints that something really exciting is going on," says Amy Simon, a NASA planetary scientist with the agency's Goddard Space Flight Center in Greenbelt, Maryland. "This time is no exception."

The wave feature has been seen just once before, by the Voyager 2 space probe that passed Jupiter decades ago.

In the new images, the wave spans a region of the planet's atmosphere where cyclones and anticyclones are forming.

Known as baroclinic waves, they are sometimes seen in the Earth's atmosphere in regions where cyclones are known to be forming.

"Until now, we thought the wave seen by Voyager 2 might have been a fluke," says study co-author Glenn Orton at the Jet Propulsion Laboratory in Pasadena, Calif. "As it turns out, it's just rare!"

The new images were captured as part of the Outer Planet Atmospheres Legacy (OPAL) program, which sees Hubble dedicate some observation time each year to study the outer planets.

Along with Jupiter, Uranus and Neptune have also had their portraits taken as part of this year's OPAL observations.

Those images will soon be added to online archives available to the public, with Saturn to follow sometime later, the researchers say.

"The collection of maps that we will build up over time will not only help scientists understand the atmospheres of our giant planets, but also the atmospheres of planets being discovered around other stars, and Earth's atmosphere and oceans, too," notes study co-author Michael H. Wong of the University of California, Berkeley.

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