An extrasolar planet is one which is not in our Solar system. According to The Extrasolar Planets Encyclopaedia, at the 19th of June 2009, 353 extrasolar plants had been discovered. The rate of discovery is increasing very quickly.
All 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. They’re defined in the newspaper Free Floating Planets by Annemarie Hagenaars, Ilja Rosenbrand and Charlotte de Valk as a thing that does not orbit a star and has a mass less than 13 times that of Jupiter. (This upper mass limit is to differentiate them from brown dwarf stars, and is roughly the mass below which sustained nuclear fusion occurs.) I would also put a lower limit of mass on what we would think about a Planet.
Naturally, nearly all of the planets so far discovered are much bigger than the Earth. This is because larger things are easier to find, and doesn’t imply that Earth sized planets are rare. The array of sizes of extrasolar planets found suggests that there’ll be plenty of Earth sized planets out there.
In 1992, a potential planet (Planet: PSR 1257+12 b) was discovered which is just a bit 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 could be liquid on at least portion of the planet, at least part of the time. A planet also should be big enough to hold a reasonable atmosphere.
A planet also should not be too large, 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 celebrities, about 30 larger planets have. We can expect there to also be Earth sized planets.
Earth Sized Planets
I’ve been using this term loosely. Maybe we ought to consider what sized planet we can live on. One as little as Mars, if it had sufficient water and was the right distance from it star 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 had been of lower density which our Earth, it would have a larger diameter and its surface gravity would not be quite as high. From the limited sample we have available, it seems that the Earth is a high density planet. 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 many planets will have moons, and that these will be of a huge array of sizes.
The 30 so big planets in the habitable zones of the celebrities might have Earth sized moons. There is nothing impossible about a really big planet, like lots of the ones found so far, having more than one Earth sized moon.
There could be an additional type of habitable moon. If a significant 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 in the moons of Jupiter.
No definite signs of life have been found on any extrasolar planet, but at present our techniques for observing them are not good enough to inform. These techniques are rapidly improving.
There are certainly plenty of places in our galaxy which could support life like the Earth’s. We still don’t know if extraterrestrial life exists.