Jupiter's Great Red Spot — one of the most iconic and well known features in the solar system — is shrinking, and nobody knows why.

The Great Red Spot seen at three different years and sizes: in 1995, 2009, and 2014. Image: NASA and ESA

The Great Red Spot is a gigantic cyclone swirling around in the atmosphere of Jupiter. Winds whip at several hundred miles per hour inside this great storm. The first definitive observation of the spot in Earth telescopes was done by Samuel Heinrich Schwabe in 1831, so we know its been around at least that long, but it may have been observed as far back as the late 1600s. When Schwabe saw it, the Great Red Spot was gauged to be more than 25,000 miles across, large enough for three planet Earths to sit side by side within it.

But sightings since the 1930s have shown the spot shrinking. A recent Hubble photo (seen above) observes the Great Red Spot at its smallest size yet —- just over 10,000 miles across, barely big enough for 1.3 Earths to fit inside. Scientists are studying small eddies at the edge of the storm that may somehow be sapping it of its strength. Will this monstrous cyclone continue to downsize? Researchers can't say for sure.