Gliese 581c

Gliese 581c is an extrasolar planet (or exoplanet) orbiting the red dwarf star Gliese 581 at 20 light-years from Earth. It appears to be in the circumstellar habitable zone (or ecosphere, or "Goldilocks Zone"), a notional spherical shell of space surrounding stars where the surface temperatures of any planets present might maintain liquid water. Liquid water is considered vital for extraterrestrial life because of its role as the solvent needed for (known) biochemical reactions. As such, Gliese 581c is one of the first truly "Earthlike" planets discovered outside of the Solar System.

The star it orbits
Gliese 581 is a spectral class M3 red dwarf, with a metallicity (heavy element abundance) around half of our Sun's. Since its planetary system formed from the same gas-and-dust cloud as the star, we can assume the planets orbiting it have a similar abundance of heavy elements.

The star is at least 7 billion years old, which would be more than enough time for any (hypothetically) life-bearing planets orbiting it to have evolved complex life. It only took Earth some 4.6 billion years to evolve us humans.

Gliese 581c compared to Earth

 * Gliese 581c has a radius 50% larger than the Earth.
 * Gliese 581c has 5 times the Earth's mass.
 * Gliese 581c has a gravitation field about 2.2 times stronger than that of Earth.
 * Gliese 581c is 14 times closer to its star than the Earth is to our Sun; its star's tidal effects are 850 times stronger than the Sun's are on Earth, so it's probably locked in synchronous rotation with the same side always facing its star.
 * Gliese 581c has an estimated surface temperature between 0 and 40°C (32-104°F), with similar temperatures to several earthly climes. Not much snow, but habitable.
 * Gliese 581c is one of three planets so far discovered in the Gliese 581 system, as opposed to the 8 planets in our solar system.
 * Based on geometrical formulae governing approximately spherical things (like planets), its average density is some 48 percent greater than Earth's. This presents an interesting problem considering that, from the star's composition noted above, one would expect the planet to contain a smaller proportion of metal than Earth, but the higher average density suggests that it (if it is solid) would contain quite a bit more metal and quite a bit less rock.

The planet was discovered by the team of Stephane Udry of the Geneva Observatory in Switzerland using the HARPS instrument on the European Southern Observatory 3.6 meter telescope in La Silla, Chile, and is the smallest extrasolar planet discovered to date. Udry's team employed the radial velocity technique, in which the size and mass of a planet are determined based on the small perturbations it induces in its parent star’s orbit via gravity.