"Rare earths" -- maybe not

One of the stars of "Expelled" is an astronomer who argues that Earth is a rare beast -- a planet sufficiently fine-tuned for life as to be very improbable. Thus, more than chance is at work.

He says.

Another researcher thinks the sun, at least, is not that uncommon.

There's nothing special about the Sun that makes it more likely than other stars to host life, a new study shows. The finding adds weight to the idea that alien life should be common throughout the universe.

"The Sun's properties are consistent with it being pulled out at random from the bag of all stars," says Charles Lineweaver from the Australian National University (ANU) in Canberra. "Life does not seem to require anything special in its host star, other than it be close."

Some previous studies of the Sun's vital statistics have concluded that it is unusual among stars, for instance, by having a higher mass than average. Such atypical properties might somehow help explain why the Sun seems to be unique, as far as we know, in having an inhabited planet.

With his ANU colleague José Robles and others, Lineweaver has now analysed 11 features of the Sun that might affect its ability to have habitable planets. They included its mass, age, rotation speed and orbital distance from the centre of the Milky Way.

Then they compared these with well-measured statistics for other stars to answer the question – overall, does the Sun stand out from the crowd any more than some other randomly chosen star would?

The Sun did stand out in two ways: it is more massive than 95% of nearby stars and its orbit around the centre of our galaxy is more circular than those of 93% of nearby stars.

Very ordinary

But when all 11 properties were taken on board, the Sun looked very ordinary. Robles's team calculates that there would be only about one chance in three that a star selected at random would be "more typical" than the Sun.

They conclude that there are probably no special attributes that a star requires to have a habitable planet, other than the obvious one – the planet must be within the star's habitable "goldilocks" zone, orbiting at a distance where the temperature is not too hot for life, nor too cold, but just right.