Artist's illustration gives an impression of how common planets are around the stars in the Milky Way. Image Credit: NASA, ESA, and M. Kornmesser (ESO)

Artist's illustration gives an impression of how common planets are around the stars in the Milky Way. Image Credit: NASA, ESA, and M. Kornmesser (ESO)

Sen—A group of university researchers estimate that there are some 100 million other worlds in the Milky Way Galaxy that could support complex life.

Previously it had been estimated that since five per cent of our Galaxy's stars are thought to be Sun-like, the number of habitable exoplanets would number at least one billion.

Now astronomers have developed a new computation method to examine data from planets orbiting other stars. Their study, published in the journal Challenges, provides the first quantitative estimate of the number of worlds in our Galaxy that could harbour life above the microbial level.

"This study does not indicate that complex life exists on that many planets. We're saying that there are planetary conditions that could support it. Origin of life questions are not addressed—only the conditions to support life," say the paper's authors Alberto Fairén, a Cornell research associate, Louis Irwin, of the University of Texas, Abel Mendez, of the University of Puerto Rico, and Dirk Schulze-Makuch, of Washington State University.

"This constitutes the first quantitative estimate of the number of worlds in our Galaxy that could harbour life above the microbial level, based on objective data," according to Irwin.

"Complex life doesn't mean intelligent life—though it doesn't rule it out or even animal life—but simply that organisms larger and more complex than microbes could exist in a number of different forms. For example, organisms that form stable food webs like those found in ecosystems on Earth," the astronomers explain in an auxiliary statement.

Planets (orange squares) and satellites (yellow squares) are plotted on a graph that compares worlds' Biological Complexity Index (BCI) to their Earth Similarity Index (ESI). The vast majority of exoplanets known to date are gas giants (green circles), but the ones with highest BCI values are likely rocky-water worlds (purple circles). Image credit: Challenges

The scientists surveyed more than 1,000 planets and used a formula that considers planet density, temperature, substrate (liquid, solid or gas), chemistry, distance from its central star and age. From this information, they developed and computed the Biological Complexity Index (BCI).

The BCI calculation revealed that 1 to 2 percent of the planets showed a BCI rating higher than Europa, the moon of Jupiter thought to have a subsurface global ocean that may harbour forms of life. With about 10 billion stars in the Milky Way galaxy, the BCI yields 100 million plausible planets.

The Milky Way is so vast that planets with high BCI values are very far apart, according to the scientists. One of the closest and most promising extrasolar systems, called Gliese 581, has two planets with the apparent, possible capacity to host complex biospheres. The distance from Earth to Gliese 581 is about 20 light years.

“On the one hand,” according to Irwin, “it seems highly unlikely that we are alone. On the other hand, we are likely so far away from life at our level of complexity, that a meeting with such alien forms is extremely improbable for the foreseeable future.”