Astronomy sleuths have found more than 50 planets circling other stars, yet have never seen even one of them. The planets themselves are so small and dim that they are completely hidden amid the starlight. The identifications come from an analysis of subtle variations in the light of each planet's parent star.
What science cannot deliver, however, imagination can. You can ponder the invisible planets while gazing at the suns they orbit, several of which are visible even to the naked eye. During May evenings, fourth-magnitude Tau Boötes lies high in the west. By autumn, anyone under dark skies can spot the slightly brighter Upsilon Andromedae. And in winter, third-magnitude Epsilon Eridani— the nearest star with a known planet, just 10 light-years away— is easily seen east of Orion's right foot.
But most people want to do more than stare at a bright dot in the sky. They want to see what the new worlds look like. Space artists have obliged with illustrations that attempt to balance drama with plausibility, but their task is not easy. Astronomers have detected only huge planets, especially those in tight, fast orbits about their stars. Even the Neptunes of the universe remain unnoticed, and forget about Earth-sized bodies. For now inventory is limited to heavyweight planets ranging from roughly Saturn-class to 17 times the mass of Jupiter. The majority are so close to their stars that they just roast. What can artists do with that?
So far they've painted them mostly as if they were Jupiter, complete with belts, oval storms, thin rings, and lots of big moons. "The starting point is that they're similar to Jupiter, although that leaves a lot of latitude," says space artist Lynette Cook. It's not a crazy guess. Giant planets would almost certainly have thick atmospheres, like Jupiter. And they probably form rotating quickly, which would cause the atmosphere to organize into belts and storms— again like Jupiter.
On the other hand, most of the new planets lie very close to their stars— less than one tenth Jupiter's distance from our sun. Intense heat would touch off far more animated chemical reactions than those in Jupiter's pink-and-white clouds. What the resulting witches' brew would look like is anybody's guess. And in the one instance that astronomers have watched a planet pass in front of its star (known only as HD209458), there were no signs of large satellites in tow. "We have to be careful not to cast the new planets as Jovian twins," warns artist Michael Carroll.
The danger in judging a planet's appearance from a few vital stats is demonstrated in our very own solar system. By chance, nature has provided two sets of planet-twins, roughly matched in size, composition, density, and distance from the sun. The first pair, Venus and Earth, could hardly look more different: One is a blank white, the other dynamic and blue. As for the second, Uranus is almost featureless green, whereas Neptune is pale blue, blemished with navy-blue and white storms.
Still, we're learning more every day. Observations of the star HD209458 have provided the first measures of the diameter and density of one of the new "hot Jupiters." Future studies and new instruments may soon reveal their general composition as well. But when we get actual images of the alien worlds— which could take decades, possibly much longer— we'll surely be in for a surprise. "Nature is riotous in its variety," says Carroll. "Whatever we come up with in our imagination is nothing compared with the reality that's waiting for us."