Hot on the astronomical news that there are at least 100 billion planets in the Milky Way galaxy is the the news that 17 billion of them are roughly the same size as the Earth. That's one out of every six worlds, which is very good news for those searching for an Earth Two or twin of our home world.
Harvard-Smithsonian Center for Astrophysics astronomer Francois Fressin presented the findings at a press conference at a gathering of the American Astronomical Society in Long Beach, Calif., Monday, Jan. 7. The study, which has been accepted for publication in The Astronomical Journal, takes the data derived from 16 months of surveying done via the Kepler Space Telescope.
In the months-long survey, Kepler identified 2,400 planet candidates. According to Physorg.com, the astronomers then asked: How many of the signals were real and how many planets did Kepler miss?
By extrapolating data from the survey, Fressin and his colleagues were able to ascertain a more accurate planet count and to also determine the size of the planets.
The scientists discovered that a full 50 percent of stars had a planet roughly the size of the Earth or larger in close orbit around a parent star. If larger planets were added in, worlds that orbited as far out as the our Earth, that number increased to 70 percent. Extrapolation told the astronomers that nearly every Sun-like star had at least one Earth-sized planet.
Larger planets were found to be rare, with Neptune-sized planets and gas giants only making up 8 percent of all planets (3 and 5 percent, respectively).
But of them all, only gas giants seemed to prefer a particular type of star. Neptune-like worlds and smaller could be found as frequently circling red dwarfs as Sun-like stars. The findings contradicted earlier studies.
Co-author of the study Guillermo Torres notes that: "Earths and super-Earths aren't picky. We're finding them in all kinds of neighborhoods."
If Fressin's name sounds familiar, it should. He led the study that discovered the first ever Earth-sized planets. Back in December 2011, Fressin and a team announced that not only had they found the first rocky type world with a size similar to Earth's, but they had found two circling the star Kepler-20.
Still, 17 billion Earth-sized planets does not mean 17 billion Earths. Far from it. The planets still have to meet the habitable zone requirements around their particular stars, parameters that will narrow the candidates down considerably. But the numbers are promising...
The latest news of Earth-sized planets follows the announcement from CalTech scientist Jonathan Swift and his colleagues that the Milky Way galaxy harbored as many as 100 billion planets. Swift et. al. also used data derived from the Kepler Space Telescope, targeting red dwarf stars in particular because they make up approximately 75 percent of the stars in the galaxy.
More good news for planet hunters, especially those looking for an Earth twin: Swift also said that the estimates were necessarily conservative due to using data from compact planetary systems.
17 billion Earth-sized planets, then, might well be a conservative approximation as well.