We have yet to confirm that there's life on another planet, and at this rate, we might not want to: Intelligent aliens might be as big as bears.
At least that's what one cosmologist from the University of Barcelona speculates in a recent paper. With the help of Bayesian statistics, a branch of math that helps estimate probabilities, Fergus Simpson took as a starting point the assumption that Earth is an average inhabited planet (which is a pretty big assumption, since we know of no other inhabited planets). From there, he estimated that an alien population would likely be smaller than 50 million. That's because up to 95 percent of planets are likely the size of Earth or smaller, he says, so one with an intelligent species may not be able to sustain a population as large as ours.
Simpson then estimated the size of the average alien in this species. By plotting the relationship between the average weight of an animal versus the size of its population, he estimated that this average alien would weigh in at 314 kilograms, or about 692 pounds. That's the size of a grizzly bear, a tiger, or a nurse shark.
It doesn't initially make sense for a large animal to be from a small planet, but when you compare our hypothetical alien to some of Earth's species, it becomes clearer. Smaller animals need fewer resources, so there are more of them. For instance, there are a lot of tiny bugs on Earth, but few pandas or giant squids.
But this is a highly speculative paper, and the math doesn't account for a range of variables that might affect an animal's size, such as a planet's gravity.
In case you're worried about giant bear-aliens invading Earth, don't be: Scientists say that a species that size might not be very intelligent. After all, we're not the biggest animals on Earth, and we run this place. As SETI researcher Seth Shostak pointed out to Newsweek, “Polar bears are large but do not write great literature and build radio towers.”
Besides, if aliens decide to come knocking, we can always send our own bears after them.