NASA scientists reported Nov 4, 2014, on the summary of results from the third year of the Kepler Space Telescope observations. Based on the results from measurements on 40,000 Sun-like stars, Erik Petigura, PhD student at the University of California, Berkeley, Calif. reported that about 22 percent of all sun-like stars should host a roughly earth-size planet in the habitable zone.
The percentage of stars with planets of any size is much larger. Based on the total number of identified stars with planets, William Borucki, Kepler science principal investigator, estimated roughly 70 percent of all sun-like stars in our galaxy may host planets. At the November 4 press conference, he said, "Planets around stars are the norm rather than the exception." He went on to add, "If we ever got star travel, you'd see a lot of traffic jams."
"Since there are about 200 billion stars in our galaxy with 40 billion of them like our Sun," Geoff Marcy, professor of astronomy at UC Berkeley and noted planet hunter said, "that gives us about 8.8 billion Earth-size planets in the Milky Way."
"What this means is, when you look up at the thousands of stars in the night sky," Petigura added, "the nearest sun-like star with an Earth-size planet in the habitable zone is probably only about 12 light years away and can be seen with the naked eye."
The habitable zone is the range of orbital distances from a star that would support liquid water on the planet. For a sun-like star, this is roughly 0.7 to 3 times the Earth's orbit. The habitable zone is sometimes referred to as the "Goldilocks" zone, not too hot, and not too cold.
Since its launch in March, 2009, the Kepler satellite, in a trailing orbit around the sun, has fixed its gaze on a patch of the sky about a hands breath in size, held at arms length. In the field of view of the 95 megapixel camera are over 100,000 stars which Kepler reads every six seconds. The image below shows the area of the sky under Kepler's fixed gaze.
About 5 percent of the pixels record the light intensity of a star and the data is sent back to earth every 30 minutes. With integration times as long as 6.5 hours, the noise floor is about 10 ppm of intensity variability for a 12th magnitude star. The array of 21 active CCD wafers in the focal plane of the telescope is shown in the image below.
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Kepler focal plane array of CCD wafers, equivalent to a 95 Megapixel camera. View larger image.
(Photo courtesy of NASA)