An extrasolar planet is one that is not in our Solar system. According to The Extrasolar Planets Encyclopaedia, as of the 19th of June 2009, 353 extrasolar plants were discovered. The rate of discovery is increasing very quickly.
All of the planets so far confirmed happen to be orbiting round stars. Planets not orbiting stars do exist, but discovering them is difficult although at least three possibilities are found.
These are sometimes called free floating planets. (This upper mass limit is to differentiate them from brown dwarf stars, and is roughly the mass below which sustained nuclear fusion takes place.) I would also place a lower limit of mass on what we would think about a Planet.
Naturally, nearly all the planets so far discovered are much bigger than the Earth. This is because bigger things are easier to find, and doesn’t suggest that Earth sized planets are rare. The array of sizes of extrasolar planets discovered suggests that there’ll be a lot of Earth sized planets out there.
In 1992, a possible world (Planet: PSR 1257+12 b) was discovered which is just somewhat more than a fifth of the mass of the planet, so we can expect increasing numbers of small planets to be discovered.
For all of us, or similar creatures, to live on a planet it generally needs to be orbiting a star in the fairly narrow region where water can be liquid on at least portion of the planet, at least part of the time. A planet also needs to be large enough to hold a sensible atmosphere.
A world also should not be too big, although there has been plenty of speculation about what type of life could live on gas giants.
Although no Earth sized planets have been found in the habitable zones of other stars, about 30 larger planets have. We can expect there to also be Earth sized planets.
Earth Sized Planets
Maybe we ought to consider what sized world we can live on. One as little as Mars, if it had enough water and was the right distance from it celebrity would do, although this is near the lower limit.
A planet much larger than the Earth might have a tendency to have an uncomfortably significant gravity. However, if it was of lower density which our Earth, it would have a larger diameter and its surface gravity would not be so high. It is probable that Humans could live on some planets with a larger mass than the Earth in addition to a larger surface area for living on.
Based of the limited information we have, we can expect that most planets will have moons, and that these are of a huge array of sizes.
The 30 so big planets in the habitable zones of their stars might have Earth sized moons. There is nothing impossible about a very major planet, like lots of the ones found so far, having more than one Earth sized moon.
There could be another class of habitable moon. If a big planet was orbiting farther out than the habitable zone of its star, tidal forces could warm its moons enough to melt water. This warming effect is clear from the moons of Jupiter.
No definite signs of life have been found on any extrasolar planet, but at present our techniques for observing them aren’t good enough to inform. These techniques are rapidly advancing.
We still don’t know if extraterrestrial life exists.