Hi All

Here's a few news links to a newly published simulation of terrestrial planet formation...

http://spaceflightnow.com/news/n0312/10earthlike/

http://space.com/scienceastronomy/earth-like_planets_031211.html

...and here's the pre-print...

http://au.arxiv.org/abs/astro-ph/0308159

...the simulation assumes a Jovian planet formed first, roughly where Jupiter is, which isn't a bad assumption since a number of simulations suggest Jovians formed quickly from gravitational instability. The resulting terrestrials are a really mixed bunch with some very dry and some wetter than Earth, and a really broad range of masses and eccentricities. In roughly 25% a terrestrial formed in the habitable zone [0.9 - 1.1 AU], so that might go some way towards filling in Drake's Equation. Combined with the planet/metallicity finding a few months ago it might mean about 1/16 of sun-like stars with the right metallicity will have other Earths - but how many stars in total fulfill that requirement???

Taking a wild punt ~ habitable planets last ~ 10% of Star's age and about 10% of stars are suitable mass/lumninosity, and about 50% the right metallcity, then that's an Earth every ~ 3200 stars which places the next nearest habitable planet about 125 light years away in this part of the Galaxy, on average. But then Alpha Centauri - with the right age, metallicity and mass/luminosity still comes in at ~ 6.25% chance of another Earth.

Should be right to an order of magnitude ;-)

Adam