The majority of stars, at least in our galaxy, could host exoplanets.
So far we have identified somewhere around 760 planets outside of our solar system, with over 2,000 still awaiting confirmation. Almost everywhere we look we are discovering new planets, they seem to be very commonplace. From the data collected so far, it looks as though there could be as many as 1.6 planets per star, which means that there are around 160 billion exoplanets in our galaxy alone. That number is staggering, especially when we consider the size of the objects we're talking about. Now remember, planets appear to outnumber stars, just try counting all the stars.
There are countless stars in the night sky.
Most of the exoplanets we've identified so far are exceptionally large, often gas giants, but this is due to a bias in our observation techniques. New information indicates that small, rocky worlds, much like earth, are actually far more common. The reason that we see the larger planets better is because they have a bigger impact, let me explain. There are two common methods for identifying planets, transit photometry and radial velocity. Transit photometry waits for a planet to pass in front of its star, then measures the dip in light, allowing us to not only get a feel for the size of the planet, but also a reading on what its atmosphere is comprised of. Larger planets are obviously favored with this technique because they make bigger, and thus more easily detected, dips in light. The other method, radial velocity, looks at the "wobble" in a star's motion. A planet orbits a star because it is being pulled by that star's gravity, but the planet's gravity also has an equal pull on the star. Because of their comparative masses, a star's planet doesn't have much of an effect, but it can be large enough for us to measure, giving us another technique for identifying exoplanets.
Kepler-10b, the smallest exoplanet yet identified.
There exist several more methods for identifying exoplanets, but the two I've described are by far the most common. Most exoplanets cannot be viewed directly with a telescope, they reflect very little light and the glare from their parent stars usually washes them out. However, even given the obstacles, some exoplanets have been observed directly, usually appearing as tiny dots.
The little dot near the top of the image is the planet.
An exoplanet is usually viewed through the infrared spectrum when being observed directly because it reduces the glare from the parent sun.
Many questions still remain about exoplanets. Much of the search for bodies beyond our sun has been focused on finding suns like our sun and planets like our planet, all in the hopes of finding life beyond or tiny sphere. We don't know under what conditions life can exist, but we have a pretty good idea. The jackpot of exoplanets would be nearly the size of earth orbiting it's sun in the habitable zone, the golden area where the planet isn't too hot or too cold, just right for the existence of water. So far, all of our experience says that life needs water to exist, while this isn't necessarily true, it's our best guess when looking for life. If all goes well, we might just find the perfect planet. We'll all just have to cross our fingers and hope.
If a planet is too close to it's sun, it will not be able to sustain life.