Gigantic, chilly Jupiter has its peers across the universe
Shining brightly in the south at nightfall this month is the planet Jupiter. Unmistakable to the naked eye, it’s the closest thing our solar system has to the giant worlds that astronomers are turning up by the hundreds around other stars. Clearly, Jupiters are a garden-variety planet across the entire universe.
But, like snowflakes, no two worlds are alike. Our own Jupiter has chilled to an unearthly deep freeze, due to its distance from the sun. As a result, its atmosphere is veiled with white clouds of ammonia. Jupiter’s whiteness is broken by tan cloud belts and reddish storm swirls that gain their tints from more esoteric chemicals, some welling up from unknown depths below. Occasionally a break in the clouds gives us a look down into holes of clear blue hydrogen-helium air, the color of our own blue sky. Jet streams race in opposite bands east and west, shaping the cloud belts and, where they rub together, spinning off whirlpools as big as continents. And there’s lightning in the clouds.
Positioned to the upper left of Jupiter these evenings, by about four finger-widths at arm’s length as you look over the trees and rooftops, is the smaller planet Neptune. It’s four times farther off in the solar system’s outer darkness and thus is too dim to see with the unaided eye.
Up close, Neptune, like Earth, is a blue planet, not because it displays blue oceans, but because it consists of mostly cloudless gas. On Earth, we look up and see blue, sunlit air concealing the blackness of space beyond. When we look at Neptune, we see blue sunlit air concealing the blackness of the depths below.
Neptune, too, seems to represent a common type of world in the wider cosmos, at least in terms of size and mass. It has 17 times the mass of Earth, compared with Jupiter’s 318 Earth masses. Right now, planet-hunting technology can’t detect planets smaller than “super-Earths,’’ with about 5 Earth masses. These, too, are proving to be abundant, though our own solar system missed out on getting one.
And as for Earth-mass planets, Marses, and smaller? Astronomers now assume that such “terrestrial’’ worlds are also as common as rocks, though few, in all likelihood, will be the friendly kinds of places where you could walk around in street clothes with just oxygen for life support. Nevertheless, it’s certainly turning out to be a planet-rich universe.
At least that’s how it looks to modern eyes. Capricornus is one of the earliest known constellations. Since ancient Sumeria, it has supposedly been a creature with the front half of a goat and the back half of a fish, certainly an odd chimera for someone, somewhere, to have imagined for the first time.
To the Platonists of ancient Greece, Capricornus marked a special spot in the heavens. When you died and your soul ascended through the sky’s starry shell to the realms beyond, Capricornus was the exit door. You took a journey right through that laughing clown mouth, like a ride at a carnival.
Actually, the Platonists picked this spot for the fact that Capricornus was (at the time) the southernmost constellation of the zodiac. Newly incoming souls, by contrast, arrived from the outer realms down through Cancer, the northernmost spot on the zodiac. It’s unclear why the Platonists got the idea that souls always migrate through the cosmos from north to south. But it probably made a good story at the time.
Alan M. MacRobert is a senior editor of Sky & Telescope magazine in Cambridge (SkyandTelescope.com). His Star Watch column appears the first Saturday of every month. 
