While astronomers bicker over whether our solar system has 8 planets or 10—Does puny Pluto count? What about that upstart Xena?—here's a radical thought: Our solar system has only one true planet, and its name is Jupiter.
Jupiter's mass is greater than all the other planets combined and doubled. It is so enormous it could swallow 1,400 Earths. Everything else is pretty much debris in comparison. In short, if you want to understand what a planet is, you need to understand Jupiter. And yet, we don't. We've sent five probes whizzing past it and parked one in a long-term orbit around it, but we just keep running into bigger puzzles.
Take Jupiter's most famous feature, its Great Red Spot. The "spot" is actually a hurricane twice the size of Earth, and its color varies but is typically more of a salmon pink. Why pink? Planetary scientist Tony Del Genio of NASA's Goddard Institute for Space Studies vaguely blames "impurities in the clouds." Which impurities? No one can say, even though the Galileo spacecraft studied Jupiter and its moons at close range for eight years. "We really don't know, because spectrographs work well with light from gases, not droplets of liquid like those in the Jovian clouds," says Caltech planetary scientist Andrew Ingersoll.
How did the Red Spot form? Another blank, although astronomers just got a lucky break. In 2000, three smaller, white storms merged to form the second-largest vortex on Jupiter; a few months ago, that giant white spot started to turn ruddy. At some point, maybe Jupiter's long-lived storms become powerful enough to dredge up colorful compounds from deep down in the atmosphere.
All around those spots, Jupiter is covered with bumblebee stripes of darker clouds called belts and lighter clouds known as zones. Each runs parallel to Jupiter's equator, spun out by the planet's breakneck rotation—its 89,000-mile-wide bulk completes a full turn in just under 10 hours.
For nearly a century, scientists had assumed Jupiter's weather worked like Earth's, with light zones forming around updrafts just as white clouds condense around rising currents here. Images from Galileo and more recently from Cassini (which captured the picture of Jupiter and its moon Io, above) proved otherwise. "The spacecraft saw lightning only in dark bands, which means that these are areas of rising gas," Ingersoll says.
So what makes the zones light and the belts dark? "It's an open research question," Del Genio says. When Galileo shot a probe into Jupiter's atmosphere in 1995, it apparently had the bad luck to pass through one of the planet's relatively dry, cloud-free regions. "Jupiter's belts crackle with lightning, and only water can make thunderstorms," Del Genio insists. Everyone is sure the water is there; they just can't find it.
At least Jupiter presents no mystery to the backyard observer. All summer it dominates the heavens after nightfall. Look up, spot the brightest point of light, and you've found it. In the city, the king of the planets often stands by itself, staking its place in the sky even when bright urban lights have blotted out everything else. Jupiter alone—somehow that just seems right.
Saturn passes through the Beehive star cluster, close to the horizon in the western evening sky.
Mars joins Saturn in the Beehive, passing just a half degree from Saturn on the 17th.
Mercury is the luminous orangish "star" in evening twilight, well below Saturn and Mars.
Pluto makes its closest approach of 2006, but you'll need a large telescope and a star chart to find it.
Summer begins at 8:26 a.m., EDT.
Jupiter lords it over the constellation Libra.