Astronomers have discovered the smallest planet outside our Solar System, and the first that is definitely rocky like Earth. In September 2010, the 4th planet of the Red Dwarf star Gliese 581, appeared to be the best known example of a possible terrestrial exoplanet orbiting near its home star.

An extrasolar planet, or exoplanet, is a planet outside the Solar System.  Extrasolar planets became an object of scientific inquiry in the nineteenth century. Many astronomers supposed that they existed, but there was no way of knowing how common they were or how similar they might be to the (more…)