NASA's Kepler team announced the discovery of Kepler-10b, the smallest exoplanet yet identified. The planet is only 1.4 times the diameter of Earth, but it's 4.6 times as dense and a lot hotter than Earth since it orbits close to its star.
With a potentially small planet in their sights, follow-up observations using the Keck telescope in Hawaii have confirmed the planet and helped figure out its mass. The Keck is able to detect the Doppler shifts that occur as the planet pulls its host star ever so slightly closer or further away, depending on where it is relative to the line of sight from Earth. These measurements can provide an indication of the planet's mass. Kepler 10-b weighs in at about 4.6 times the mass of Earth, making it rather dense; NASA calls its density "similar to that of an iron dumbbell."