The number of planets on which complex life could exist in the Milky Way may be as high as 100 million, according to a study just published by Louis Irwin from the University of Texas at El Paso and colleagues in the journal Challenges.

The group of researchers—which also includes Alberto Fairén from Cornell University, Abel Méndez from the Planetary Habitability Laboratory at the University of Puerto Rico at Arecibo, and myself—have made the first quantitative estimate of the number of worlds in our galaxy that could harbor life above the microbial level, based on objective data. First we surveyed the growing list of more than 1,000 known exoplanets. Using a formula that considers planetary density, temperature, substrate (liquid, solid, or gas), chemistry, distance from its central star, and age, we then computed a “Biological Complexity Index (BCI)”, which rates planets on a scale of 0 to 1.0 according to the number and degree of characteristics assumed to be important for supporting various forms of multicellular life.

The BCI calculation revealed that 1 to 2 percent of known exoplanets showed a BCI rating higher than Jupiter’s moon Europa, which has a subsurface global ocean that may be hospitable to life. Based on an estimate of 10 billion stars in the Milky Way, and assuming an average of one planet per star, this yields the figure of 100 million. Some scientists believe the number could be 10 times higher.

The study does not claim, however, that complex life exists on that many planets—only that the conditions to support it might. Also, complex life doesn’t mean intelligent life (though it doesn’t rule it out), or even animal life. It simply means organisms larger and more complex than microbes, which could exist in a number of different forms, and quite likely would form stable food webs like those found in ecosystems on Earth. Nevertheless, this is the first study that relies on observable data from actual planetary bodies beyond our solar system rather than making educated guesses about the frequency of life on other worlds based on hypothetical assumptions.

Despite the large absolute number of planets that could harbor complex life, the Milky Way is so vast that, statistically, planets with high BCI values are very far apart. One of the closest and most promising extrasolar systems, known as Gliese 581, has possibly two planets with the apparent capacity to host complex biospheres, yet Gliese 581 is still about 20 light years from us.

Most planets with a high BCI are much further away. If the 100 million planets in Irwin’s study were evenly distributed across the galaxy, they would average about 24 light years apart. And of course, the distance to planets with intelligent life would likely be significantly further.

On one hand, it seems highly unlikely that we are alone. On the other hand, we are likely so far away from other life forms of our complexity that a meeting with aliens might be improbable for the foreseeable future.