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          There could be 300 million potentially habitable planets in our galaxy

          When we look up at the night sky, we see thousands of stars, which is just a tiny fraction of the billions of stars in our galaxy. And we can imagine that many of these stars could host exoplanets, meaning that the number of potential planets out there is enormous. But how many of these planets might be habitable? A new study has come up with an estimate.

          Researchers from NASA, the SETI Institute, and others, found that there may be as many as 300 million potentially habitable planets in our galaxy alone. And some of these might even be close by, within 30 light-years of our sun.

          To come up with the estimate, researchers used data from both the Kepler Space Telescope and the Gaia space observatory to build up a picture of the stars and exoplanets in our galaxy. Previous estimates of the number of habitable worlds were based on how far planets were from their star, but the new figure also takes into account factors like how much light a planet receives, which is relevant to whether it could have liquid water on its surface.

          This illustration depicts Kepler-186f, the first validated Earth-size planet to orbit a distant star in the habitable zone.
          This illustration depicts Kepler-186f, the first validated Earth-size planet to orbit a distant star in the habitable zone. NASA Ames/JPL-Caltech/T. Pyle

          This information can help point exoplanet-hunting tools like NASA’s Transiting Exoplanet Survey Satellite (TESS) in the right direction. “Knowing how common different kinds of planets are is extremely valuable for the design of upcoming exoplanet-finding missions,” said co-author Michelle Kunimoto, who works on the TESS team at the Massachusetts Institute of Technology. “Surveys aimed at small, potentially habitable planets around Sun-like stars will depend on results like these to maximize their chance of success.”

          Future research will need to focus on whether exoplanets have an atmosphere and what it is composed of, which is another key component of habitability. This is hard to assess with current instruments, but next-generation telescopes like the James Webb Space Telescope will be able to investigate exoplanet atmospheres more closely.

          For now, the estimate of potentially habitable planets has implications on the biggest of questions — whether we are alone in the universe. “This is the first time that all of the pieces have been put together to provide a reliable measurement of the number of potentially habitable planets in the galaxy,” said co-author Jeff Coughlin, an exoplanet researcher at the SETI Institute and Director of Kepler’s Science Office. “This is a key term of the Drake Equation, used to estimate the number of communicable civilizations — we’re one step closer on the long road to finding out if we’re alone in the cosmos.”

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