There could be one hundred billion Earth-like planets in our galaxy. So says Dr Alan Boss of the Carnegie Institution of Science at the annual meeting of the American Association for the Advancement of Science in Chicago. According to a BBC report:
[T]elescopes have been able to detect just over 300 planets outside our Solar System. Very few of these would be capable of supporting life, however. Most are gas giants like our Jupiter; and many orbit so close to their parent stars that any microbes would have to survive roasting temperatures.
But, based on the limited numbers of planets found so far, Dr Boss has estimated that each Sun-like star has on average one "Earth-like" planet. This simple calculation means there would be huge numbers capable of supporting life.
"Not only are they probably habitable but they probably are also going to be inhabited," Dr Boss told BBC News. "But I think that most likely the nearby 'Earths' are going to be inhabited with things which are perhaps more common to what Earth was like three or four billion years ago." That means bacterial lifeforms.
Dr Boss estimates that NASA's Kepler mission, due for launch in March, should begin finding some of these Earth-like planets within the next few years. Recent work at Edinburgh University tried to quantify how many intelligent civilisations might be out there. The research suggested there could be thousands of them.
Well, maybe. Dr. Boss is an expert and I'm not, but the experts I've read would, I think, say that his optimism is unwarranted. The article makes it sound as if planets suitable for life are commonplace in our galaxy, but all the astronomical discoveries of the last couple of decades point to the conclusion that life-sustaining planets are probably very unusual if not very rare in our galaxy.
In the first place, for the opening sentence above to be true, each star in the galaxy, not just each sun-like star, would have to average one earth-like planet in it's gravitational field, but astronomers know that most stars are completely unsuited for sustaining life.
In order to support life in its solar system a star must be located within a fairly narrow region in the galaxy. It can't be too close to the center, where radiation would be intense, nor too far away where it would revolve at dangerous speeds around the galactic pinwheel. The star has to be rich in heavier elements, and has to be fairly remote from other stars in the galaxy. It has to be a middle-aged star of relatively constant luminosity, not too big and not too small, not too old and not too young.
In other words, stars suitable for sustaining life are relatively unusual in our galaxy, but this is just the beginning. The star has to possess a planetary satellite capable of generating and sustaining life and this means it has to have perhaps hundreds of precisely-tuned properties. The planet has to be just the right distance from its star which means it has to revolve around the star at just the right speed. It has to have a nearly circular orbit and the right tilt to its axis. It has to be just the right mass so that its gravity will hold oxygen in the atmosphere but not hold slightly lighter noxious gases like ammonia. It has to rotate on its axis at the right speed, lest the temperature differences between day and night be too great, and it must possess a shifting crust. It must also have ample water and carbon, among other things, and also a large moon which has to be at just the right distance from the planet to stabilize its wobble. It must also be in a solar system where it's protected from meteorites by large gravitational vacuum sweepers like Uranus, Neptune, Jupiter and Saturn, and so on.
As the number of parameters that must be just right in order for a planet to be able to support life increases the chances of such planets existing in great numbers in our galaxy decrease.
This is why Peter Ward and Donald Brownlee titled their book on this subject Rare Earth. It's why Guillermo Gonzalez and Jay Richards titled their book on a similar subject Privileged Planet. Life, so far from being widespread throughout the Milky Way, may well exist on only one planet in the entire galaxy, indeed in the whole universe --- ours.
RLC