The planets were where you had to go, because they were the most likely places to look for the kind of precious thing the prospector could bring back and turn in to make his fortune-and, naturally, to add to the Corporation's.
It was easy to describe the kind of planets they were looking for. They were looking for another Earth. Or something enough like Earth, anyway, to support some form of organic life, because inorganic processes hardly ever produced anything worth the carrying space it took to bring it home.
The most disappointing planets were the closest. When the Heechee came to Earth's solar system they gave it a good looking-over, and some of the ships on the Gateway asteroid reflected that. They still had stored navigation codes for places so near that human be-
ings could have visited them on their own-if they wanted to. Some of them in fact had already been reached by the crude human rockets-places like Venus, the Moon, Mars's south polar ice cap. Some were hardly worth the trouble, like Saturn's moon, Dione.
The prospectors were after bigger game than that. They wanted planets no man or woman had ever seen. They found a bewildering array of them.
The planets they reached in the Magic Mystery Bus Rides came in all shapes and sizes. There were two basic types. There were the orbiting rocks (like Earth; solid and landable-on), and then there were the would-be stars (like Jupiter; the gas giants, that were just a bit too small to start nuclear fusion in their cores and turn themselves into suns). No Gateway prospector ever landed on a gas giant, of course. They had nothing solid enough to land on. (That was a pity, for a few of them were interesting anyway .. . but that's another story.)
It was the orbiting rocks that were prospected as vigorously as a few thousand scared, hurried human beings could explore them. There were plenty of the solid planets. Most of them had no apparent life at all, unfortunately. They were too far from their sun, so they were eternally frozen, or they were too close, so they were as scorched as the planet Mercury. Many of them had too little atmosphere (or none at all), like Mars (or the Moon). Some of them had satellites of their own, like the Earth's Moon. Some of the target objects were satellites, but big ones, big enough to retain atmospheres and to land on.
There were something over two hundred billion stars in our own galaxy, and a hellish lot of them possessed planets of one kind or another. Even the Heechee ships weren't programmed to set a course for all the possible planets to explore. There were hardly course settings for one planet in a hundred thousand, in fact. Still, that left plenty for the Gateway prospectors to visit-many more of them than a few thousand men and women could reach in the course of a few dozen years.
So the first discovery the Gateway prospectors made was that there were plenty of planets to choose from. Human astronomers were glad to know that, because they'd always wondered, and the Corporation didn't even have to pay a discovery bonus to find it out: all they had to do was add up the findings of the returning explorers. It developed that binary stars didn't ordinarily have planets. Solitary stars, on the other hand, generally did. Astronomers thought the reason for that probably had something to do with conserving rotational velocity. When two stars condensed together out of a single gas cloud they seemed to take care of each other's excess rotational energy. Bachelor stars apparently had to dissipate it on smaller satellites.
Hardly any of the planets were really Earth-like, though.
There were a lot of tests for that sort of thing that could be applied from a considerable distance. Temperature sensing, for one. Organic life didn't seem to develop except where water could exist in its liquid phase, which was to say in the narrow, 100-degree band between about 270 and 370 Kelvin. At lower temperatures the stuff was useless ice. At higher ones water wasn't usually there at afl, because the heat vaporized it and the sunlight-from whatever sun was nearby-split the hydrogen out of the water molecule and it was lost into space.
That meant that each star had a quite narrow area of possible planetary orbits that might be worth investigating. As planets didn't care whether or not they were going to be hospitable to life when they were condensing out of the interstellar gases, most of them took orbits inside that life zone, or in the cold spaces outside it.
Most alien life, like most Earthly life, was based on the chemistry of the carbon atoms. Carbon was the best of all possible elements for forming useful long-chain compounds, and happily it is so frequently found that it is the fourth most common element in the universe. Most alien life had something like DNA, too. That
wasn't for any panspermian reason, but simply because systems like DNA provided a cheap and efficient way for organisms to replicate themselves.
