17 billion worlds and counting

In what must have been music to the ears of Seth Shostak, it was announced yesterday that the latest estimate of stars in our own galaxy with Earth-like planets circling them may be as high as 17 billion. The figure makes a lot of sense. Stars are formed as dust clouds gradually come together to form a central massive object which is surrounded by an "accretion disc", where secondary less massive objects form and begin to orbit the central mass, or star. That's the way our solar system formed 4.6 billion years ago, and it has to be the way stars and their satellites form everywhere. Most of these planets will be lifeless balls of gas or rocky deserts, but statistically some will form, like our own Spaceship Earth, right in the optimal, or "Goldilocks" zone where there is at least a chance of life developing. In our own case, there are only eight planets plus some rubble; yet it happened here.

It's all a question of maturity. 4.2 billion years ago Earth was struck by a Mars size proto-planet which has been named "Thea". Fortunately it hit us at an oblique angle (a direct hit would have vapourised the whole planet) and a huge slew of material was blown off into space where it hardened and formed our moon. It took a few hundred million years for the whole "double planet" system to cool down, during which we were struck repeatedly by asteroids and comets rich in water. And the evidence suggests that no sooner had the Earth cooled to the point where this water could exist in a liquid state, life began to form: algae and bacteria to begin with, then more complex life-forms, leading eventually to our good selves. It's a wonderful story, better in fact than anything the creationists could ever come up with. Good for you Seth. Seems like you were on the right track after all, and, like you say, it's only a question of when, and not if, we make contact with another life form out there. And that, let me tell you, will be one helluva day...