If you've always hoped that aliens exist, you now have one more reason to be optimistic. A new study of NASA data published Monday concluded that roughly 22 percent of stars similar to the Sun are orbited by planets with temperatures conducive to liquid water and sizes similar to that of Earth. If correct, this suggests that there may be tens of billions of Earth-like planets amenable to life in the Milky Way galaxy. It’s an extremely tentative conclusion, one which astronomers at the University of California at Berkeley reached indirectly by extrapolating from mountains of data, yet it provides SETI researchers — that is, scientists searching for alien life — with loads of potential planets on which to focus their studies.
The conclusion comes from an examination of data culled from NASA’s Kepler telescope, which was launched in 2009 with the goal of estimating the percentage of stars with potentially habitable planets. The researchers focused on Kepler’s observations of 150,000 stars in the Cygnus constellation, and identified 10 “candidate” planets that are between one and two times the size of Earth and, based on their distances from the suns they orbit, are probably around Earth’s temperature. Taking into account the telescope’s technological limits and the rarity with which distant planets are actually glimpsed, the researches subsequently concluded that as many as 25 billion such planets exist in the Milky Way galaxy alone.
Geoff Marcy, an astronomer at Berkeley and co-author of the study, called the discovery “one great leap toward the possibility of life, including intelligent life, in the universe.”
Difficulties included the fact that the telescope can’t directly view the candidate planets, as the light from the stars they’re orbiting is too bright. So, the researchers inferred the planets’ existence based on periodic dimming of said light, figuring that the dimming could have resulted from a planet passing in front of the star. But even if this type of inference is correct, the planet in question would only be in Keplar’s line of site roughly one percent of the time; therefore, the researchers multiplied the number of observed planets by 100 to account for potential missed sitings. Finally, natural fluctuations in starlight could have resulted in even more false negatives, so the astronomers “inserted” fake planets into the data to see how often they were missed by the algorithms.
Ultimately, with all of these corrections — and accounting for the fact that the data they looked at only covered one small patch of sky — they determined that roughly 25 billion Earth-sized planets in our galaxy are at the right temperature to sustain liquid water.
“Earth-sized planets are not rare, so we’ll know we’ll have stuff to look at,” said Sara Seager, an astrophysicist at MIT told the Washington Post. “It's reassuring for us."