Long-time Slashdot reader fahrbot-bot shared this report from the New York Times: A decade ago, a band of astronomers set out to investigate one of the oldest questions taunting philosophers, scientists, priests, astronomers, mystics and the rest of the human race: How many more Earths are out there, if any? How many far-flung planets exist that could harbor life as we know it? Their tool was the Kepler spacecraft, which was launched in March 2009 on a three-and-a-half year mission to monitor 150,000 stars in a patch of sky in the Milky Way. It looked for tiny dips in starlight caused by an exoplanet passing in front of its home star. “It’s not E.T., but it’s E.T.’s home,” said William Borucki when the mission was launched in March 2009. It was Dr. Borucki, an astronomer now retired from NASA’s Ames Research Center, who dreamed up the project and spent two decades convincing NASA to do it. Before the spacecraft finally gave out in 2018, it had discovered more than 4,000 candidate worlds among those stars. So far, none have shown any sign of life or habitation. (Granted, they are very far away and hard to study.) Extrapolated, that figure suggests that there are billions of exoplanets in the Milky Way galaxy. But how many of those are potentially habitable? After crunching Kepler’s data for two years, a team of 44 astronomers led by Steve Bryson of NASA Ames has landed on what they say is the definitive answer, at least for now. Their paper has been accepted for publication in the Astronomical Journal… The team calculated that at least one-third, and perhaps as many as 90 percent, of stars similar in mass and brightness to our sun have rocks like Earth in their habitable zones, with the range reflecting the researchers’ confidence in their various methods and assumptions. That is no small bonanza, however you look at it. According to NASA estimates there are at least 100 billion stars in the Milky Way, of which about 4 billion are sunlike. If only 7 percent of those stars have habitable planets — a seriously conservative estimate — there could be as many as 300 million potentially habitable Earths out there in the whole Milky Way alone. On average, the astronomers calculated, the nearest such planet should be about 20 light-years away, and there should be four of them within 30 light-years or so of the sun… “The new result means that the galaxy is at least twice as fertile as estimated in one of the first analyses of Kepler data, in 2013.”

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