With thousands of planets outside Earth's solar system, there's a pretty good chance that some of them have the conditions needed for life. If alien life does exist, scientists aren't quite sure what it would look like, but they might be able to see how much these foreign "beings" might weigh. Most of these creatures will be big — nearly 700 lbs. (314 kilograms), one cosmologist says.

Fergus Simpson, of the University of Barcelona, outlines his statistical argument on the prepublished site arXiv. The finding is based on a model called Bayes' theorem and a branch of mathematics called Bayesian statistics. The purpose of such techniques is to estimate the probabilities that change depending on the information available.

But although Simpson's mathematical experiment may get scientists and others thinking about the possibilities of alien life, some researchers say some of his statistical assumptions may not hold true. [7 Huge Misconceptions about Aliens]

Estimating alien size

Simpson started his calculation with the number of individuals who would most likely live in a given alien civilization, and came up with about 50 million or fewer individuals. He posited that there are many civilizations in the galaxy and that any individual alien would be more likely than not to be from a highly populated civilization. The population distributions across planets would follow a bell-shaped distribution but not a true bell curve, he said in the paper. That means most cultures would support an average number of people, with fewer populations holding very low or very high populations.

As an analogy, consider the populations on Earth. If you were to pick any single person from Earth, that individual would be more likely to be from China (1-in-5 chance) than from New Zealand (about a 1-in-1,600 chance). However, there are a lot more New Zealand-size countries than China-size ones, so if you were to pick country names at random, you'd be much more likely to pick a Spain- or Mozambique-size country than a Russia-, China- or United States-size nation.

The same idea applies to aliens. Assuming Earth is at the high end for the number of residents, a habitable alien planet would hold about 50 million aliens, Simpson found.

Using a similar argument, Simpson wrote that the size of the planet supporting extraterrestrial life is likely to be smaller than Earth, at least most of the time. In his model, he assumed that about 50 percent of Earth's diameter is at the lower limit, because if it were any smaller, it would be difficult for the planet to retain an atmosphere or water. Mars, for example, is about 53 percent the size of Earth.

Once again, each individual alien would be more likely to live on a big planet, Simpson wrote, because those planets are likely to support more people. But a whole species has better odds of coming from a small one, because there are more small planets than large ones. Simpson wrote that, 95 percent of the time, planets will have a radius of 1.4 times Earth's or less.

The last part of Simpson's analysis focused on the size of other life forms. Earth animals have a widely known relation between size and the number of individuals — the smaller the species is, the more individuals of that species tend to exist. For example, an alien seeking life on Earth would be far likelier to run into a mosquito than a blue whale.

However, the relation between size and population can also be plotted on a curve against probability, which predicts that the median weight of an alien would be about 692 lbs. (314 kg) — about the size of a bear or an elk. So, based on the results of this model, about half of extraterrestrial creatures would weigh more than that, and half would weigh less. [The 10 Strangest Places Where Life Is Found on Earth]

It might sound contradictory for large creatures to be from smaller planets, but it isn't: Remember that the populations from small planets, on average, would be small relative to the 7 billion humans who live on Earth.

Statistical caveats

However, some scientists say this mathematical prediction has some serious caveats. Michael Kopp, a professor of theoretical biology and evolution at Aix-Marseille University in France, said he isn't sure about the statistical argument because it is not clear if humans are a random sample of intelligent beings. It's also quite possible that humans on Earth could be about the median of all civilizations — in other words, in the grand scheme of the universe, Earth is more comparable to a country like Canada in terms of population than India or China.

"The prediction that most civilizations contain less than 50 million individuals is based on the assumption that the distribution of civilization sizes corresponds to the distribution of species sizes … but there are no particular reasons to believe this is so," Kopp told Live Science.

The argument that intelligent extraterrestrial life would tend to be larger would be less problematic, he said, because the size distribution of terrestrial species is similar and the relation between size and population seems to be pretty consistent. However, he added that it isn't necessarily true that the distribution of sizes among intelligent species follows the kind of curve Simpson modeled.

Seth Shostak, a researcher at the SETI Institute, said it's unlikely Simpson is exactly right, especially about alien body size. "Anything that large, and you're likely to be in the water," he said. While whales are probably quite intelligent, for the purposes of the search for extraterrestrial intelligence, you need radio telescopes or lasers, because that's the simplest way to be detected over interstellar distances. "You can't make a radio telescope underwater," Shostak noted.

There's also the issue of how intelligent life would develop. One reason humans and other animals got smarter was to find food. "An animal that big isn't going to have much trouble getting dinner," Shostak said, and that might work against the development of big brains and, thus, intelligence.

Nonetheless, Shostak said the paper gets scientists thinking about the possibilities of extraterrestrial life. "It should be applauded," he said.