Carl Sagan wrote, “The universe is a pretty big space. It’s bigger than anything anyone has ever dreamed of before. So, if it’s just us, seems like an awful waste of space, right?” Perhaps Sagan would be cheered by a new estimate: In our galaxy (the Milky Way) fifteen percent of all solar systems are like Earth’s. That estimate wasn’t (quite) pulled out of a hat.

Astronomers have been finding stars with planets since 1995. As of January 6, 2010 they have located 422 planets – exoplanets. The number goes up frequently. We’re finally coming to the point where some statistical inference is possible, although the sample is still small.

One group, Microlensing Follow-up Network (MicroFUN), uses a method called gravitational microlensing to detect planets. As seen from Earth, when one star crosses in front of another, the near star can magnify the light – like a lens – from the more distant star. The magnification of light is enhanced if there are planets orbiting the lens star. These minute differences in lighting can be detected, leading to confirmation of planets – usually of the very big gas giant type similar to Jupiter and Saturn in our solar system.

Based on the last four years of MicroFUN data, only one solar system like Earth’s has been discovered. If every star had a similar solar system, six should have been found by now. So if the one in six ratio holds, about 15% of solar systems are like ours.

This estimation is stretching available data (arguably, way too much); but in a way, it’s a quibble. What it shows is that the other solar systems of the galaxy, and presumably the universe, are of various types – many similar to our own. Our solar system is not unique, in fact, in the Milky Way alone there are somewhere between 200 and 400 billion stars. At 15% that makes about 75 billion solar systems like ours. If only 3% of those had life-friendly planets, there would be over 2 billion planets with some kind of life.

At this point, the numbers look good for life on other planets. We may find life elsewhere even in our own solar system. As knowledge advances, locally or universally, the chances for life other than on Earth look good – really good.

“Now we know our place in the universe,” said Ohio State University astronomer Scott Gaudi. “Solar systems like our own are not rare, but we’re not in the majority, either.”

[Source: Ohio State University]