In September 2010, astronomers working under the Lick-Carnegie Survey announced the discovery of the most Earth-like planet around another star yet: Gliese 581-g, one of several planets orbiting a red dwarf star about 20 light-years from the Earth.
Before the current announcement, Gliese 581 had already emerged as one of the most important targets of exoplanet research - the search for planets around other stars. Gliese has several features which make it attractive for those researchers. First, it's relatively close at just 20 light-years away - still far enough away that it would take thousands of years to send a space probe there, but well within the Sun's interstellar neighbourhood, so to speak. Second, it's a red dwarf star. This means that the so-called habitability zone or "Goldilocks" zone in which an Earth-like planet could plausibly support life, is very close to the star.
Given the limited set of tools and tricks scientists currently have available to detect exoplanets, close-in planets are much easier to spot. However, with a Sun like our own, close-in planets (especially those inside the orbit of Mercury) would be far too close to the Sun to support life. Red dwarf stars are much smaller, much cooler - and that means that a planet can orbit much closer to such a star and still keep warm without being fried to a crisp.
In previous years, astronomers had already discovered a half-dozen planets orbiting Gliese 581, some of which seemed moderately Earth-like (albeit considerably larger). The new announcement of the seventh planet in the system, however, seems to be precisely the holy grail that astronomers were searching for: a planet roughly Earth-sized, orbiting in the habitable zone. Gliese 581g fits the bill well. Gliese 581g has a diameter no more than twice that of Earth, its mass indicates that it is probably rocky, and it orbits within the habitable zone. (In Gliese 581's case, that zone is about one-sixth the distance from the star as Earth is from the Sun.) The lead investigator on the survey which discovered the planet, Steven Vogt, reportedly feels that there is a "100 percent" chance of primitive life on Gliese 58g: "I have almost no doubt about it."
While Vogt may be correct (and without interstellar travel, we may simply never know), it bears noting that Gliese 581g would not look much like Earth if we could somehow visit it. The planet orbits so close to its star that it completes one "year" in just 37 Earth-days. Because Gliese 581 is much cooler than our Sun, the temperature could still be balmy rather than dangerous. But, at that distance, Gliese 581g would be tidally locked - meaning that one side always faces the star, and one "day" lasts the same amount of time as one "year." The same process affects Earth's Moon - the Moon completes both one orbit and one rotation in about a month's time, so that we always see the same "face" of the Moon from the surface of the Earth.
For us, that simply means that we get a constant view of the same massive craters glinting out at us from the Moon's surface. For Gliese 581g, however, the consequences would be much more significant. One side of the planet always faces its star - and would therefore be heated to murderous temperatures. The other side is permanently exposed to the darkness of space - and would therefore be extremely cold. Only on the thin band in between, where the temperature meets somewhere in a comfortable middle, would life as we know it have a good chance of eking out a living.