What is Jupiter's Great Red Spot, and does Jupiter really shield Earth?

The Great Red Spot is a giant storm in Jupiter's atmosphere, a persistent high-pressure region that makes the largest anticyclonic storm in the Solar System. Winds up to 496 kilometres an hour swirl around it, and its red-orange colour comes from a sunburn effect, in which chemical compounds high in the atmosphere are blasted by solar radiation. Whether Jupiter also shields Earth from comets is a separate question, and the honest answer is that astronomers disagree.

The storm may be younger than you think. A similar spot was seen from 1665 to 1713, which would make the storm at least 361 years old if it is the same one. But a 2024 study suggests it is not: the original spot disappeared, and later another formed, the one seen today. Robert Hooke is often credited with the first sighting in 1664, but his spot was likely in a different belt and in the shadow of a moon. The Great Red Spot has been frequently observed since 1831.

Why can a storm last so long? Jupiter has no solid planetary surface to cause friction, so the swirling eddies have nothing to slow them. Still, the spot is shrinking: in 2004 its length was about half what it was a century earlier, when it was about three times the diameter of Earth.

The shield idea comes from Jupiter's size. Its mass is nearly 2.5 times that of all the other planets combined, and it takes about 200 times more asteroid and comet impacts than Earth, so it has been called the Solar System's vacuum cleaner. Scientists used to believe it partially shielded the inner system, but 2008 simulations suggest it does no net blocking, and the topic remains controversial.