Red Jr was formed from several storms that merged (Image: Christopher Go)

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The gas giant is growing another red spot, which US space agency (Nasa) astronomers have nicknamed "Red Jr".

Both red spots are actually raging storms in Jupiter's cloud layer, but scientists don't yet know how they get their characteristic brick colour.

Red Jr is about half the size of the Great Red Spot and almost exactly the same colour, Science@Nasa reports.

The new storm goes by the official name of Oval BA. It was first observed in 2000, when three smaller spots collided and merged.

Changing colours

At first, Oval BA remained white - the same colour as the storms that combined to create it. But over the past few months, it has started to change in appearance.

"The oval was white in November 2005, it slowly turned brown in December 2005, and red a few weeks ago," said Christopher Go, an amateur astronomer from the Philippines who has been observing Red Jr.

Some scientists believe a similar merger to the one that created Red Jr may have created the Great Red Spot, which is twice as wide as our planet and at least 300 years old.

Dr Glenn Orton, an astronomer at Nasa's Jet Propulsion Laboratory who studies storms on Jupiter and other giant planets, said: "This is convincing. We've been monitoring Jupiter for years to see if Oval BA would turn red - and it finally seems to be happening."

No one is quite sure why the Great Red Spot is red.

One theory is that the storm dredges material from deep beneath Jupiter's cloudtops and lifts it to high altitudes. Here, ultraviolet rays from the Sun turn colour-changing compounds (or chromophores) red.

Oval BA may have strengthened enough to do the same, scientists say.