(CNN) -- Is anybody out there? For millennia, humans have gazed at the night sky, asking this question. That's why scientists and NASA are eagerly searching for "exoplanets" -- that is, planets that orbit around stars other than our sun. Last week NASA's Kepler satellite reported the discovery of three Earth-sized exoplanets within the so-called "habitable zone," defined as the neighborhood of a star where liquid water -- essential for life as we know it -- can exist. In our solar system, only three planets lie in the sun's habitable zone -- Venus, Earth and Mars. Planets too close to the star they orbit, like Mercury, are too hot for liquid water. Even Venus turns out to be too hot, thanks to a runaway greenhouse effect caused by its carbon dioxide-rich atmosphere. Planets farther from their star, like Jupiter, are too cold, at least on its surface -- although Europa, one of Jupiter's moons, may have oceans of liquid water beneath an icy crust.
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People have always wondered whether there's life in outer space
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Meg Urry says last week's announcement by NASA shed new light on the question
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Kepler satellite found three planets in zones capable of supporting life, she says
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Urry: We can't journey to such far off planets but we can try to communicate with far-away life
For now, I can tell you that one @placeholder-year is 88 Earth-days, one Venus-year is 225 Earth-days, one Earth-year is 365 days, and one Jupiter year is about 12 Earth-years.
For now, I can tell you that one Mercury-year is 88 Earth-days, one @placeholder-year is 225 Earth-days, one Earth-year is 365 days, and one Jupiter year is about 12 Earth-years.