University of Barcelona cosmologist Fergus Simpson has been thinking deeply about alien size. He recently published a paper entitled "The Nature of Inhabited Planets And Their Inhabitants." He relies on a mathematical model that says other planets conserve energy just like we do. "Throughout the animal kingdom, species which are physically larger invariably possess a lower population density, possibly due to their enhanced energy demands," he says. Bigger beings consume more and release more energy than do smaller beings. Earth, he says, "constitutes a simple random sample amongst inhabited planets." We have more smaller beings than larger ones. He doesn't believe, however, that Earth is a fair sample. Instead, he says that "most inhabited planets are likely to be closer in size to Mars than the Earth." If that is true, then "since population density is widely observed to decline with increasing body mass, we conclude that most intelligent species are expected to exceed 300kg." Yes, that would be 650 pounds. More via CNET.