The last time Chris McCarthy spoke at Ask a Scientist, back in July 2005, I wrote: "In 1995 astronomers detected the first known planet outside of our own solar system — a gas giant (like Jupiter) orbiting a star called 51 Pegasi. Since then, about 150 extrasolar planets have been found around distant stars. Small, rocky, earth-like planets with life-friendly chemistry have not yet been identified — but the spanking fresh discovery of the smallest planet currently known has planet hunters saying such a find may not be far off."

Now, two years later, the story is begging for an update. The recent announcement of the discovery of 28 more planets outside of our solar system has brought the total number of known exoplanets to 236! Of particular interest is Gliese 581c, the smallest exoplanet discovered to date. Only fifty percent bigger and five times more massive than Earth, Gliese 581c orbits within its dim sun's habitable zone, the region around a star within which a planet's temperature can sustain liquid water on its surface.

While we don't yet know if there is water, let alone life, on Gliese 581c, its discovery is a milestone that has astronomers very excited. Chris will tell us why.

Speaker: Chris McCarthy; Assistant Adjunct Professor, San Francisco State University

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