This article in New Scientist describes the criteria that is currently used to predict the most likely place to find alien life; stars that are stable and are have Earth-like planets.
Personally, I think this criteria is wrong; here's why:
I believe that any lifeform that evolves to our current intelligence level will figure out fairly quickly how to create digital life. Digital life is similar to biological life except that its mind can be encoded digitally, providing profound benefits such as immortality, speed-of-light travel, and the ability to continually upgrade both its mind and its hosting evironment. For more thoughts on digital life, see this blog entry.
Digital life is the natural evolution of biological life. Digital life can live in any host sufficiently advanced to contain and execute its mind. In once sense, these hosts are like computers, but far more advanced than anything we have built so far; under a microscope, even the most primitive of these hosts would look as complex as a human brain.
It's interesting to ponder what a digital lifeform would be like, and what it would do with its life.
Biological minds live inside biological bodies that require organic food for fuel, air to breathe, gravity to maintain muscle tone, and a ton of other constraints. Biological lifeforms evolved on planets over billions of years and therefore planets are their natural habitat. They can certainly survive in space, but they have a lot of biological baggage that they have to deal with.
Digital lifeforms, on the other hand, can host themselves in custom bodies adapted for a variety of environments. Such bodies could be built out of advanced inorganic materials that could operate happily on a planet, inside a star, on in deep space. Given the range of possible options, I think that living on a planet would be pretty boring for a digital lifeform when compared with alternatives such as exploring the universe at near-light speeds.
My guess is that digital lifeforms will usually leave their planet of origin and travel through deep space to explore the wide reaches of the universe. Their biological creators would most likely also explore space, but to a much lesser degree due to both physical and mental constraints.
If I'm correct, Earth-like planets might be the most likely place to find biological lifeforms, but not the best place to look for their digital offspring; these would have left a long time ago and traveled to places they find more "interesting".
This begs another question; what kind of place would a digital lifeform find interesting? That's a topic for a future blog entry!

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