Just about any amateur astronomer can tell you the basics about Jupiter. It's the fifth planet from the sun. It's got a Great Red Spot on its lower half. And it's encircled by two prominent brown stripes. Well, check your telescope tonight and you'll find that one of those stripes has gone missing — and scientists aren't entirely sure why.
Amateur astronomers raised the alert about the fading stripe last fall. The giant planet ducked behind the sun for a few months over the winter, and when it came back to the morning sky, the dark band in the Southern Hemisphere was gone.
"This is not the first time this has happened," says Kelly Beatty, senior contributing editor for Sky and Telescope magazine. He tells NPR's Guy Raz that particular stripe goes missing every 10 years or so. In fact, it's disappeared about 18 times since the turn of the 19th century.
"We have some confidence that the belt will come back," Beatty says. "We just don't know why."
He suspects that the stripe may not actually be missing at all. Unlike the Earth, he explains, Jupiter doesn't have a solid surface. "What we see when we look through a telescope is a planet-wide cloud deck surrounding the entire place. So these two bands, which are kind of like racing stripes around the midsection of Jupiter, are dark bands that have a different composition than the other clouds around them.
"What scientists think has happened is that some kind of disturbance has taken over in the Southern Hemisphere and created cirrus clouds, maybe, that [have] completely enveloped the planet and covered this band with a high, thin blanket that will eventually go away," he says.
So Jupiter's southern stripe might just be hiding. How long until it reappears, nobody knows. "It could be six months from now," Beatty says. "It could be two years from now."
One thing's for certain, he says: "There are amateur astronomers around the world with their eyes glued to their telescopes in the hope that they will be the first to be able to see the beginning of the return of the south equatorial belt."
Jupiter's disappearing belt wouldn't have been noticed so quickly without those hobbyists, Beatty says. In fact, in astronomy, the pros depend on the amateurs to sound celestial alerts.
"There aren't enough professionals to keep track of everything going on in the universe all the time," Beatty says. "So in a sense, they rely on amateur astronomers — who have very good equipment, by the way — to actually keep an eye on things."
"When they see something, they notify the professionals, and the big guns get swung over to take a look."
If you'd like to join the watch, Jupiter's easy to spot just before dawn. "If you're just eyeballing the sky," Beatty suggests, "it's a bright star in the eastern sky. It's the only star that bright anywhere nearby; it's very obvious."
"If you have a pair of binoculars," he adds, "you'll see that Jupiter's actually a little disk. If you have a small telescope, you'll be able to see not only this disk, but the two stripes across it — or what were the two stripes. You'll only see one."
Beatty admits he kind of misses the second stripe. "I kind of miss the symmetry of it, because it tells me that there is order on Jupiter."
"Right now, the fact that that one belt is missing, it's like a missing tooth. There is disorder on Jupiter — and we just don't know why."