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.
Kepler’s field of view relative to the Milky Way. View larger image.
(Courtesy of NASA)
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.
Kepler focal plane array of CCD wafers, equivalent to a 95 Megapixel camera. View larger image.
(Photo courtesy of NASA)
Transit detector
Kepler’s mission is to detect minute changes in the light curve of a star as a possible indication of a transiting planet. Below is an example of a recorded light curve from the earliest detected transits.
Examples of Kepler light curves from early detected planetary transits. View larger image.
(Courtesy of NASA)
To be a candidate, at least three transits must be detected. This also gives a precise measure of the orbital period of the planet. From the star’s absolute luminosity, its mass can be inferred, and from the modulation depth of the light curve, the orbital distance and size of the planet can be estimated. The planet’s mass can be estimated from independent ground-based spectroscopic measurements of the radial motion of the host star.
Each year, as more data is analyzed, more planets, of smaller size and longer orbital periods, are detected. After three years of analyzed data, 3,538 planet candidates have been identified. These range from huge gas giants bigger than Jupiter to tiny planets much smaller than Earth. This image shows the current distribution of planet sizes and orbital periods including the latest data released on November 4, 2014.
Latest Kepler planet data. View larger image.
(Courtesy of SETI)
“There is a wide variety of systems out there,” Jason Rowe, research scientist, SETI Institute, Mountain View, Calif., said during the press conference. “If you can imagine it, there is probably a system like that out there. We are finding more and more earth like planets.”
The loss of Kepler
Sadly, as of May, 2013, the Kepler spacecraft is unable to continue its primary mission. The second reaction wheel failed. A minimum of three are required to enable the precision pointing accuracy needed for these high-precision photometric measurements. Even so, the data collection period lasted more than a year longer than the planned three and a half year mission.
The results reported on November 4 include analysis of only the first three years of observations. Though Kepler is resting in safe mode in its orbit around the sun, Earth-based scientists will continue to analyze the last year and a half of recorded data extending the list of transiting planet candidates.
If you want to participate in the hunt for new planets, join the citizen science project underway at www.PlanetHunters.org.
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