The aliens would be the size of a bear, according to mathematical studies

With thousands of planets outside the Earth's solar system, there is a good chance that some of them will have the necessary conditions for life. If there is extraterrestrial life, scientists are not quite sure what it would look like, but they might be able to see how much these strange "beings" can weigh. Most of these creatures would be large, weigh about 700 pounds. (314 kilograms), says a cosmologist.

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

But while Simpson's mathematical experiment can get scientists and other people thinking about the possibilities of extraterrestrial life, some researchers say some of their statistical assumptions may not be true.

The estimation of the size of the alien

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

As an analogy, we must consider the populations of the Earth. If you had to choose a single person from Earth, that person would be more likely to be from China (1 in 5 chance) than New Zealand people (a probability of 1 in 1600). However, there are many more New Zealand countries of larger size than those in China, so if you were to pick up random country names, you would be much more likely to choose a Spain- or Mozambique-sized country than a Russia-, or China- United States by size.

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

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

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

The last part of Simpson's analysis focused on the size of other life forms. Earth animals have a widely known relationship between size and number of individuals, the smaller the species, the more individuals tend to exist. For example, in a quest for alien life on Earth it would be far more likely to encounter a mosquito than a blue whale.

However, the relationship between size and population can also be plotted against a probability curve, which predicts that the average weight of an alien would be around 692 pounds. (314 kg), approximately the size of a bear or a moose. Therefore, based on the results of this model, about half of the extraterrestrial creatures would weigh more than that, and the other half would weigh less.

It may sound contradictory that large creatures are from smaller planets, but it is not: Remember that populations of small planets, on average, would be small in relation to the 7 billion human beings living on Earth.

Statistical warnings

However, some scientists say that this mathematical prediction has some serious warnings. Michael Kopp, a professor of theoretical and evolutionary biology at the University of Aix-Marseille in France, said he is not sure about the statistical argument, as it is unclear whether humans are a random sample of intelligent beings. It is also quite possible that humans on Earth could be on the median of all civilizations - in other words, in the grand scheme of the universe, Earth is more comparable to a country like Canada and in terms of population of India Or China.

"The prediction that most civilizations contain less than 50 million people 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 that this is so"

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 relationship between size and population seems to be fairly consistent. However, he added that it is not necessarily true that the size distribution among intelligent species followed the type of curve Simpson has modeled.

Seth Shostak, a researcher at the SETI Institute, said Simpson is unlikely to be exactly right, especially about foreign body size. "Whatever is great, it is very likely that it was in the water," he said. While whales are probably very intelligent, for the purposes of the search for extraterrestrial intelligence, radio telescopes or lasers are needed, because that is the easiest way to detect interstellar distances. "You can not do an underwater radio telescope"

There is also the question of how intelligent life would develop. One of the reasons that humans and other animals became smarter was to find food. "Such a big animal will not have much trouble getting dinner," Shostak said. He added that it could work against the development of large brains and, therefore, of intelligence.