Jupiter's Great Red Spot seems to be on a cosmic diet, shrinking rapidly before our eyes.

Astronomers using the Hubble Space Telescope calculate that the spot, a giant long-lasting storm, is narrowing by more than 900 kilometres a year, much faster than before.

In the late 1800s the red spot was an oval 41,000 kilometres wide. Now it's a circle that's 16,500 kilometres across.

Michael Wong, a scientist at the University of California, Berkeley, said the spot is a mystery. Astronomers don't know why it's red or shrinking, or what will happen next. If this pace continues, in 17 years the spot could be gone. Or it could stop at a smaller size.

Wong said one theory is the spot eats smaller storms, and that it is consuming fewer of them.

Jupiter is the largest planet in the solar system and has a diameter 11.2 times that of Earth.



