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At current state of the art, our tools for finding extraterrestial planets are quite crude. There are only two ways of doing it and they both involve measuring the gravitational effect of the planet on the star. First, you can measure the doppler shift of the star repeatedly over a long period of time (years). As the planet rotates around the star, the star itself moves back and forth and if the plane of orbit of the planet is not perpendicular to the line to the earth, then we'll be able to measure it, if the planet is massive enough and if the orbit is rapid enough. The longer the orbit, the longer the observation needed to prove that it's happening. The lower the mass of the planet, the less wiggle there is and the less likely it is that we'll be able to even notice it is happening. The other approach is even more crude and consists of actually measuring the movement of the star against the stellar background. So it's no wonder that most of the planets found so far have been huge and in relatively close orbits to their stars. Still, a lot of them have been found. What's puzzling is that they seem to defy our knowledge of how stellar systems form. When a planet much more massive than Jupiter is found in an orbit tighter than that of Mercury, it is puzzling. Some of them are in immensely eliptical orbits. Some, however, have been found in orbits more like those found in the Solar system, which is comforting. Far more worrying is the result of studies which have tried to determine how the Moon was formed. As such things go, Luna is extremely unusual. None of the rocky internal planets except Earth have moons (in the classic sense; Phobos and Deimos are captured asteroids in unstable orbits). Are the characteristics of the Moon important to the process which made Earth conducive to life? It's possible. One reason is that the process by which the Moon was formed will have changed the Earth itself. Earth and Venus are actually near twins. They have nearly the same diameter, for instance. But Earth is much more massive, which is why it has a higher surface gravity. It also is volcanically more active. And of course Earth's atmosphere didn't form in anything like the same way as that of Venus, which is locked in runaway greenhouse effect so bad that lead would be molten on its surface. (The surface of Venus, planet of love, is the closest approximation in the Solar System of Dante's vision of certain parts of Hell. None of the Venera landers operated more than six hours before pressure, temperature and acid mists destroyed them.) So why is Earth so much more massive? It may not have been to begin with. The theory goes like this: initially Earth formed much like Venus. But after it formed, it was struck by a foreign body about the size of Mars. As a result of this collision, much matter was thrown out into space and ultimately collected to form the Moon. But this also means that Earth got most of the iron and heavy metals from the foreign body, while the Moon got most of the rock (and maybe some of the previous rock from Earth itself). As a result, the non-metallic crust of Earth is probably thinner than that of Venus, and Earth has a higher density. Earth therefore has more radioactives and its core would remain liquid more readily, especially given that it would be subject to tidal heating from the orbiting Moon. All of this makes the Earth much more volcanically active. On the other hand, the Moon may have served to remove excess atmosphere from the Earth at a much higher rate than would take place without it, which helped to thin the atmosphere down and prevent the kind of runaway effect that is baking Venus. So the atmosphere of the Earth may be heavily dependent on how the Earth/Moon system was formed. If Earth had an atmosphere like that of Venus, there would be no life here. (It's also possible that tidal shifting and mixing of Earth's oceans are vital to the process of creating and maintaining life on Earth.) Unfortunately, a collision like that is very unlikely. The body which struck the Earth must have come from outside the system. If it had made a near miss without actually hitting, it would not have cause the change, but it would have seriously perturbed the orbit of Earth, making life even less likely. So you need to form a body like Mars in one stellar system, gravitationally perturb it into an escape orbit without destroying it, have it wander interstellar space and enter another stellar system, and then score a direct hit on a Venus-like planet there. That's obviously an exceedingly low probability event, maybe down to the level of once-per-galaxy. Which means that planets, even Venus-like planets, may be common but Earth-like planets with Luna-like moons may be exceedingly rare. Of course, we don't know if an Earth-like planet is vital for formation of life. With the discovery of chemosynthetic ecosystems at the bottom of the ocean, that don't rely on light, the possibility exists t 
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