When it comes to planets outside our solar system, smaller is better. That's because smaller planets are more likely to resemble Earth. So the 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.
The detection comes courtesy of a technique called microlensing. Employed by consortia that operate global networks of telescopes (the Optical Gravitational Lensing Experiment and Microlensing Observations in Astrophysics), microlensing allows continuous monitoring 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.
At the meeting, astronomer David Bennett of the University of Notre Dame in Indiana, 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.
The planet's host star is small as well. At roughly 6% the mass of our sun, the star is probably too small to sustain fusion reactions in its core, said Bennett, making it a dimly glowing brown dwarf.
Bennett said the microlensing technique holds promise for finding planets even closer in size to Earth. What's more, he noted, future orbiting telescopes may be able to image a planet distinct from its star and therefore search for signs of life.