Astronomer Dr. Alan Duffy is on the search for exoplanets in our distant universe & explains how we'll find extraterrestrial life.

HERE’S a real-estate glut on a galactic scale: Want to skip Earth? Well, scientists say there are four life-capable planets in our galaxy for every living Australian. That’s 100 million worlds.

International astronomers have published new research in the journal Challenges of Astrobiology assessing the wealth of new information flooding into their labs from an ever-increasing array of telescopes and sensors.

Taking all the facts and figures they have at hand into account, they set out to produce an empirical figure on just how much life was likely to be out there.

The result: 100 million worlds in our own home galaxy of 300 billion stars.

“That’s a staggering number,” Australian astronomer Dr Alan Duffy.

“That means that on a typical night in the Outback where you can see 5000 stars, 50 of them will have very habitable alien worlds around them.”

Still don’t grasp the scale?

“In terms of that number it’s as many footballs (depending on your code) that you need to fill the McG stadium to the brim,” Dr Duffy, of the Swinburne University of Technology, said.

However, the figure is not all that big when you take into account that there is actually likely to be 17 billion Earth-sized planets. But not all of them live in the orbital “sweet spot” that is not too cold, and not too hot, to support liquid water.

And then there’s all the moons.

In the word of the astronomers: “the view that the evolution of complex life on other worlds is rare in frequency but large in absolute number,”

The team from the Department of Biological Sciences, University of Texas at El Paso developed a new formula for determining the probability of life called the Biological Complexity Index.

This categorises planets and moons according to what we know about their likely features. It then produces a likely number of bodies capable of sustaining life before estimating the “relative probability that complex, macro-organismic life forms could have emerged on other worlds”.

There are a few hitches.

Only 11 of the 1700 worlds so far discovered are likely to be more habitable than Europa - the moon of Jupiter regarded as the next-best place for life in our own solar system.

And we still don’t know how to get there.

Then, when we get there, they may already be occupied.