Astrobiologists have long discussed the idea that planets can only be capable of supporting life if liquid water exists on the surface. Obviously, this is only possible if the temperature on the planet is similar to Earth’s and this, in turn, implies a certain distance from the mother star.

The search for planets in this so-called habitable zone has intensified in recent years with the launch of space telescopes such as Kepler that is finding new exoplanets at a dramatic rate.

But the idea that there may be zones within the galaxy that are particularly conducive to life is a much newer idea. The thinking here is that planets capable of supporting life are much more likely to exist around stars in certain parts of the galaxy.

Convention has it that the galactic habitable zone is a torus about 30 lightyears in diameter around the centre of the galaxy. So habitable planets are unlikely to form close to the galactic centre or very far away from it.

Today, however, Michael Gowanlock at the University of Hawaii in Honolulu and a couple of pals, reveal a new map of the galactic habitable zone in which challenges this convention and suggests that the galactic habitable zone is much more complex than a simple torus.

The new map uses the latest findings about exoplanets to determine galactic habitability. In particular, astronomers have recently discovered that exoplanets are much more likely to form around stars that contain elements heavier than helium or hydrogen, a property call metallicity.

The first stars in the early universe were formed entirely from hydrogen and helium but generated heavier elements when they ran out of fuel and exploded. The next generation of stars formed from the debris of these supernovas and so have higher levels of heavier elements.

It’s these stars, the later ones, that seem more likely to have planets and therefore more likely to have a planet in the habitable zone. Clearly, these stars are most likely to form in areas where there are lots of supernovas and in our galaxy that’s near the centre (at distance of about 9 light years).

But that raises a potential problem. A supernova would devastate a planet orbiting a nearby star, blasting its atmosphere to kingdom come and destroying the conditions in which life could evolve.

So too many supernovas reduce the probability of life evolving.

The question that Gowanlock and co ask is how these processes balance out–the rate of planet formation, the number of supernovas and the time it takes for complex life to evolve (as determined by our one data point on Earth).

The answer they say is that habitable planets are so common towards the centre of the galaxy that even if many are wiped out by supernovas, there should still be plenty that survive for long enough for complex life to evolve.

Their model suggests that 2.7 per cent of stars in the inner galaxy should have habitable planets. And there should also be habitable planets further away too. Gowanlock and co say about 0.25 per cent of stars in the outer galaxy should have habitable planets.

That’s a significantly different prediction from the standard torus model and means that a significant fraction of stars throughout the galaxy are potentially interesting. “We predict that ∼1.2% of all stars host a planet that may have been capable of supporting complex life at some point in the history of the Galaxy,” say Gowanlock and co.

There is an important caveat, however. Their model also predicts that 75 per cent of these habitable planets will be tidally locked around their mother star.

That could be a problem. Astrobiologists are fiercely debating the nature of planets that project the same face towards their star. The debate has been prompted by the discovery of a superEarth around Gliese 581, which is close enough to the star to be in the habitable zone but is also probably tidally locked.

One side of this planet would burn under a scorching sun while the other would freeze. Whether conditions anywhere on such a planet could favour life is not known.

If Gowanlock and co’s predictions are anything to go by, in the very near future we’re going to be finding many more alien Earths and that most of these will be tidally locked. So the question of whether such planets can support life is likely to become the focus of a great deal more attention.

Ref: arxiv.org/abs/1107.1286: A Model of Habitability Within the Milky Way Galaxy

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