One thing people might like to chase up are the Drake equations, which work out the probabilities of habitable planets out there. Much of them are figures pulled out of empty space, though.

Important things to bear in mind are the habitable zone around each star, and the necessity of a large Moon as a co-orbitable, and that's only two.

Our habitable zone is very narrow, and, worse, is moving (help, I can't remember whether it's going out or in!), so that in time, Earth will fall outside it, and the temperature range will not be able to support our life. Mars and Venus are nowhere near it. If you extend the criteria to allow bacteria as what places are habitable for, with nothing else, there are more possibilities, but if you want a fully operational biosphere, you need rather more in the plant line, and what we have can be very fussy about light and CO2 levels.

A number of arguments have been raised about the Moon, such as tides enabling movement from sea to land, and the effect on moderating the axial tilt. Since it seems that the Moon is, in one way or another, a captured object (I simplify severely), the odds on having a planet, in the habitable zone, with a large co-orbital body are much steeper than those for iron rich stars having terrestrial planets.

As for the aliens out there - if they can get here, it behoves us to keep very quiet indeed. Or broadcast Morris dancing and bodhran music.do you want to attract the Conquistadors?

Penny