A large team of scientists led by CfA astronomer Francois Fressin, and including ten other CfA astronomers, last week announced the dramatic discovery of two Earth-sized planets around another star. For the scientists, the discovery capped two and one-half years of searching, since the launch of the Kepler satellite in March 2009 began the production of a steady stream of data on exoplanets. For the rest of us, the discovery capped about two and one-half millenia of searching - roughly since Anaxagoras and Democritus argued from their philosophical principles that there ought to be other Earths in the cosmos.
The two planets, both discovered orbiting a sun-like star called Kepler-20, have sizes respectively of 0.87 and 1.03 Earth-radii. Their masses are more uncertain, but if they are rocky planets calculations estimate that they should have about three times the Earth's mass. Neither planet, however, is Earth-like. Unlike the larger Kepler-22b planet announced last month, which orbits its star in the habitable zone, these two Earth-sized objects orbit very close to their star, and so have surface temperatures exceeding 1000 Celsius.
The new paper describing the planets, which is scheduled to appear in the journal Nature, also discusses the orbital parameters of the objects and their likely composition, and speculates on the possibility that a stable atmosphere could be present. But whether or not the details are corroborated, it is clear that the new results usher in a dramatic new phase in the study of exoplanets.