astroengine writes: "The term "Earth-like worlds" is a vastly overused, and hopelessly incorrect, term that is popularly bandied about to explain some recent exoplanet discoveries. Although some of the distant small worlds being discovered by the Kepler space telescope may be of Earth-like size, orbiting their sun-like star in Earth-like orbits, calling those worlds "Earth-like" gives the impression these alien planets are filled with liquid water. It turns out that we have only a vague idea as to where Earth got its water, and it will take a long time until we have any hint of this life-giving resource on worlds orbiting stars thousands of light-years away."
We warn the reader in advance that the proof presented here depends on a
clever but highly unmotivated trick.
-- Howard Anton, "Elementary Linear Algebra"