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| Kepler telescope: Earth-size planets 'number 17bn' | Kepler telescope: Earth-size planets 'number 17bn' |
| (about 1 hour later) | |
| By Jason Palmer Science and technology reporter, BBC News, California | By Jason Palmer Science and technology reporter, BBC News, California |
| Astronomers say that one in six stars hosts an Earth-sized planet in a close orbit - suggesting a total of 17 billion such planets in the galaxy. | Astronomers say that one in six stars hosts an Earth-sized planet in a close orbit - suggesting a total of 17 billion such planets in the galaxy. |
| The result comes from an analysis of planet candidates gathered by the Kepler telescope. | The result comes from an analysis of planet candidates gathered by the Kepler telescope. |
| The Kepler team also announced 461 new planet candidates, bringing the satellites' total haul to 2,740. | The Kepler team also announced 461 new planet candidates, bringing the satellites' total haul to 2,740. |
| The findings were announced at the 221st meeting of the American Astronomical Society in California. | The findings were announced at the 221st meeting of the American Astronomical Society in California. |
| Transit | Transit |
| Since its launch into orbit in 2009, Kepler has stared at a fixed part of the sky, peering at more than 150,000 stars in its field of view. | Since its launch into orbit in 2009, Kepler has stared at a fixed part of the sky, peering at more than 150,000 stars in its field of view. |
| It detects the minute dip in light coming from a star if a planet passes in front of it, in what is called a transit. | It detects the minute dip in light coming from a star if a planet passes in front of it, in what is called a transit. |
| But it is a tricky measurement to make, with the total light changing just tiny fractions of a percent, and not every dip in light is due to a planet. | |
| So Francois Fressin of the Harvard-Smithsonian Center for Astrophysics - who discovered the first Earth-sized planets set about trying to find out not only which Kepler candidates might not be planets, but also what planets might not have been visible to Kepler. | So Francois Fressin of the Harvard-Smithsonian Center for Astrophysics - who discovered the first Earth-sized planets set about trying to find out not only which Kepler candidates might not be planets, but also what planets might not have been visible to Kepler. |
| "We have to correct for two things - first [the Kepler candidate list] is incomplete," he told BBC News. | "We have to correct for two things - first [the Kepler candidate list] is incomplete," he told BBC News. |
| "We only see the planets that are transiting their host stars, stars that happen to have a planet that is well-aligned for us to see it, and [for each of those] there are dozens that do not." | "We only see the planets that are transiting their host stars, stars that happen to have a planet that is well-aligned for us to see it, and [for each of those] there are dozens that do not." |
| "The second major correction is in the list of candidates - there are some that are not true planets transiting their host star, they are other astrophysical configurations." | "The second major correction is in the list of candidates - there are some that are not true planets transiting their host star, they are other astrophysical configurations." |
| These might include for example binary stars, where one star orbits another, blocking some of the light as the stars "transit" each other. | These might include for example binary stars, where one star orbits another, blocking some of the light as the stars "transit" each other. |
| "We simulated all the possible configurations we could think of - and we found out that they could only account for 9.5% of Kepler planets, and all the rest are bona fide planets," Dr Fressin explained. | "We simulated all the possible configurations we could think of - and we found out that they could only account for 9.5% of Kepler planets, and all the rest are bona fide planets," Dr Fressin explained. |
| The results suggest that 17% of stars host a planet up to 1.25 times the size of the Earth, in close orbits lasting just 85 days or fewer - much like the planet Mercury. | The results suggest that 17% of stars host a planet up to 1.25 times the size of the Earth, in close orbits lasting just 85 days or fewer - much like the planet Mercury. |
| That means our galaxy hosts at least 17 billion Earth-sized planets. | That means our galaxy hosts at least 17 billion Earth-sized planets. |
| In the zone | In the zone |
| Even as Dr Fressin reported an analysis of the most recent Kepler catalogue, it was increased substantially by results reported by Christopher Burke of the Seti Institute. | Even as Dr Fressin reported an analysis of the most recent Kepler catalogue, it was increased substantially by results reported by Christopher Burke of the Seti Institute. |
| Dr Burke announced 461 new candidate planets, a substantial fraction of which were Earth-sized or not much larger - planets that have until now been particularly difficult to detect. | Dr Burke announced 461 new candidate planets, a substantial fraction of which were Earth-sized or not much larger - planets that have until now been particularly difficult to detect. |
| "What's particularly interesting is four new planets - less than twice the size of Earth - that are potentially in the habitable zone, the location around a star where it could potentially have liquid water to sustain life," Dr Burke told BBC News. | "What's particularly interesting is four new planets - less than twice the size of Earth - that are potentially in the habitable zone, the location around a star where it could potentially have liquid water to sustain life," Dr Burke told BBC News. |
| One of the four, dubbed KOI 172.02, is a mere 1.5 times the size of the Earth and around a star like our own Sun - perhaps as near as the current data allow to finding an "Earth 2.0". | One of the four, dubbed KOI 172.02, is a mere 1.5 times the size of the Earth and around a star like our own Sun - perhaps as near as the current data allow to finding an "Earth 2.0". |
| "It's very exciting because we're really starting to pick up the sensitivity to these things in the habitable zone - we're just really getting to the frontier of potentially life-bearing planets." | "It's very exciting because we're really starting to pick up the sensitivity to these things in the habitable zone - we're just really getting to the frontier of potentially life-bearing planets." |
| William Borucki, the driving force behind and principal investigator on the Kepler mission, said he was "delighted" by the fresh batch of results. | William Borucki, the driving force behind and principal investigator on the Kepler mission, said he was "delighted" by the fresh batch of results. |
| "The most important thing is the statistics - not to find one Earth but to find 100 Earths, that's what we'll be seeing as the years go on with the Kepler mission - because it was designed to find many Earths." | "The most important thing is the statistics - not to find one Earth but to find 100 Earths, that's what we'll be seeing as the years go on with the Kepler mission - because it was designed to find many Earths." |