Based on our own experience, we know of at least three Filters and can strongly guess as to another.
(1) That life develops on a world whose mass allows achieving orbital spaceflight using relatively low-technology means (i.e. at least partially chemical)
(2) That advanced life develops on a world with massive energy reserves easily accessible with low technology (e.g. coal, oil)
(3) That no environmental cataclysm occurs during the advanced life/industrialization gestation time period (e.g. climate change, asteroid)
To this, we can pretty strongly add another:
(X) That, during the brief window between energy surplus (and advanced technology) and environmental collapse (from energy exploitation), interplanetary colonization is launched, supported, and made self-sufficient
There's no law that says the window for something remains open forever, when you're talking about planet-scale economic requirements.
(1) That life develops on a world whose mass allows achieving orbital spaceflight using relatively low-technology means (i.e. at least partially chemical)
(2) That advanced life develops on a world with massive energy reserves easily accessible with low technology (e.g. coal, oil)
(3) That no environmental cataclysm occurs during the advanced life/industrialization gestation time period (e.g. climate change, asteroid)
To this, we can pretty strongly add another:
(X) That, during the brief window between energy surplus (and advanced technology) and environmental collapse (from energy exploitation), interplanetary colonization is launched, supported, and made self-sufficient
There's no law that says the window for something remains open forever, when you're talking about planet-scale economic requirements.