Smaller is better because smaller planets are more likely to resemble Earth. So discovery of the smallest known extrasolar planet, announced today at the June meeting of the American Astronomical Society in St. Louis, Missouri, comes as good news. "It gives us hope of finding lots of habitable planets," says astronomer Sara Seager of the Massachusetts Institute of Technology in Cambridge, who was not involved in the discovery.
It was discovered by microlensing. Microlensing monitors for subtle brightening of far distant stars as a relatively nearby star passes in front of them. The nearer star's gravity can slightly bend,or lens, the background star's light toward Earth, temporarily brightening it. If a planet circling the nearer star also lines up and bends some starlight, the network picks up a secondary brightening.
Astronomer David Bennett speaking for the consortia, announced the microlensing discovery of a planet just three times the mass of Earth. Most of the 300 or so known extrasolar planets are many times the mass of Jupiter, or thousands of times the mass of Earth. The newly discovered planet, which the team is referring to as MOA-2007-BLG-192L, orbits its star at 70% of the sun-Earth distance. That means the planet probably formed with lots of ice and gases, Bennett said, more like Neptune in composition than Earth.
Super cool!I summarized this from article http://sciencenow.sciencemag.org/cgi/content/full/2008/602/1
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