For the last decade scientists have been studying the stars for signs of planets and they have had very good luck discovering gas giants and other very large stars, but at millions of miles away seeing planets the size of earth, or even close to the size of earth is very difficult and while finding gas giants around other stars are interesting the truth is that it just is not very useful to us, while discovering worlds that look like ours might give us hints to some of the most important questions in human history, such as are we alone in the universe. To date the best example of a planet which might look a little like earth is Gliese 581 g, one of a number of planets discovered around the star Gliese 581.
The planet was discovered by Steven Vogt, Paul Butler and other astronomers in the Lick-Carnegie Exoplanet survey.
Calling Gliese 581 g earth like is largely a sign that we have set the bar pretty low at this time. This is a planet that orbits its planet in 37 days, is 0.146 AU’s from its star(1 AU is approximately the distance between the earth and the sun) and has a mass of at least three times that of earth. In addition it is close enough to the star that it is likely tidal locked meaning that one side of this world is always going to be facing the sun.
All of this may dampen the excitement a bit but there is some good news as well. The size of this world means that it is very possible that this world has an atmosphere that is even denser than that of earths and the gravity would only be at most twice that of earth, and while tidal locking would make one half of the world very cold and the other very hot there would be a band of twilight between the two that could be at the right temperature for the type of life you might find on earth and there are scientists who believe that there could be bacterial life, or possibly even more on this world.
The truly exciting thing about Gliese 581 is not this planet itself, but the proof that there are earth like worlds around other stars and possible billions of them and with that many planets the likelihood of us finding very interesting things on them goes up considerably.