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http://www.cnbeta.com/articles/166482.htm
美国太空总署(NASA)自本月初发现凯普勒22b行星(Kepler-22b)后又传捷报，昨(20)日英国科学杂志《NATURE》公布消息，在太阳系外发现两颗约地球一般大小的行星，分别命名为凯普勒20e(Kepler-20e)和凯普勒20f(Kepler-20f)，成为天文学上一座重要的里程碑。
凯普勒20e的直径为11,000公里，约为地球的0.87倍，比金星略小；凯普勒20f的直径则为13,190公里，约为地球的1.03倍。两者皆位于距离地球约950光年的凯普勒20星系(Kepler-20 )中。
凯普勒20星系为一类似太阳系的恒星系，除了本次发现的2颗行星外，还包含另外3颗行星。NASA推测此2行星皆为岩石所组成，但因距离恒星较近，地表温度高，无法存在液态水，被认为不适合生命居住。
NASA凯普勒任务 (Kepler mission)的首要目标为在适居带内，找到类似地球的行星，自任务开始总共发现了2300个以上的天体，其中有48个推断可能存在有生命不可或缺的物质「水」。
本次所发现的两颗行星，为凯普勒任务迄今发现在类太阳系行星系中的最小行星，也是与地球大小最为接近的行星，虽非位于适居带内，但两者都为凯普勒任务建立了新的里程碑。
▼凯普勒20e和凯普勒20f与地球、金星的大小对照图。
〔图／翻摄自NASA〕
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Artist's Concept of Kepler-20eNASA's Kepler mission has discovered the first Earth-size planets orbiting a sun-like star outside our solar system. The planets, called Kepler-20e and Kepler-20f, are too close to their star to be in the so-called habitable zone where liquid water could exist on a planet's surface, but they are the smallest exoplanets ever confirmed around a star like our sun.
Kepler-20e is the first planet smaller than the Earth discovered to orbit a star other than the sun. A year on Kepler-20e only lasts 6 days, as it is much closer to its host star than the Earth is to the sun. The temperature at the surface of the planet, around 1400 degrees Fahrenheit, is much to hot to support life, as we know it.
Kepler-20e is likely to be entirely rocky and without an atmosphere. The planet is tidally locked, always showing the same side to its host star, as the moon to the Earth, and could have large temperature differences between its permanent night and day sides.
Astronomers think that the planet is likely to be geologically active, due to its own formation process and the strong gravitational interactions with its host star. In this artistic depiction, the planet is represented with active volcanoes on both the night and day sides.
For more information about the Kepler mission and to view the digital press kit, visit: http://www.nasa.gov/kepler
Image credit: NASA/Ames/JPL-Caltech
NASA Discovers First Earth-size Planets Beyond Our Solar System
12.20.11
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This artist's conception illustrates Kepler-20e. Image credit: NASA/Ames/JPL-Caltech
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This artist's conception illustrates Kepler-20f. Image credit: NASA/Ames/JPL-Caltech
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This chart compares artist's concept images of the first Earth-size planets found around a sun-like star to planets in our own solar system, Earth and Venus. Image credit: NASA/Ames/JPL-Caltech
MOFFET FIELD, Calif. -- NASA's Kepler mission has discovered the first Earth-size planets orbiting a sun-like star outside our solar system. The planets, called Kepler-20e and Kepler-20f, are too close to their star to be in the so-called habitable zone where liquid water could exist on a planet's surface, but they are the smallest exoplanets ever confirmed around a star like our sun.
The discovery marks the next important milestone in the ultimate search for planets like Earth. The new planets are thought to be rocky. Kepler-20e is slightly smaller than Venus, measuring 0.87 times the radius of Earth. Kepler-20f is a bit larger than Earth, measuring 1.03 times its radius. Both planets reside in a five-planet system called Kepler-20, approximately 1,000 light-years away in the constellation Lyra.
Kepler-20e orbits its parent star every 6.1 days and Kepler-20f every 19.6 days. These short orbital periods mean very hot, inhospitable worlds. Kepler-20f, at 800 degrees Fahrenheit, is similar to an average day on the planet Mercury. The surface temperature of Kepler-20e, at more than 1,400 degrees Fahrenheit, would melt glass.
“The primary goal of the Kepler mission is to find Earth-sized planets in the habitable zone," said Francois Fressin of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., lead author of a new study published in the journal Nature. "This discovery demonstrates for the first time that Earth-size planets exist around other stars, and that we are able to detect them.”
The Kepler-20 system includes three other planets that are larger than Earth but smaller than Neptune. Kepler-20b, the closest planet, Kepler-20c, the third planet, and Kepler-20d, the fifth planet, orbit their star every 3.7, 10.9 and 77.6 days. All five planets have orbits lying roughly within Mercury's orbit in our solar system. The host star belongs to the same G-type class as our sun, although it is slightly smaller and cooler.
The system has an unexpected arrangement. In our solar system, small, rocky worlds orbit close to the sun and large, gaseous worlds orbit farther out. In comparison, the planets of Kepler-20 are organized in alternating size: large, small, large, small and large.
"The Kepler data are showing us some planetary systems have arrangements of planets very different from that seen in our solar system," said Jack Lissauer, planetary scientist and Kepler science team member at NASA's Ames Research Center in Moffett Field, Calif. "The analysis of Kepler data continue to reveal new insights about the diversity of planets and planetary systems within our galaxy."
Scientists are not certain how the system evolved but they do not think the planets formed in their existing locations. They theorize the planets formed farther from their star and then migrated inward, likely through interactions with the disk of material from which they originated. This allowed the worlds to maintain their regular spacing despite alternating sizes.
The Kepler space telescope detects planets and planet candidates by measuring dips in the brightness of more than 150,000 stars to search for planets crossing in front, or transiting, their stars. The Kepler science team requires at least three transits to verify a signal as a planet.
The Kepler science team uses ground-based telescopes and the Spitzer Space Telescope to review observations on planet candidates the spacecraft finds. The star field Kepler observes in the constellations Cygnus and Lyra can be seen only from ground-based observatories in spring through early fall. The data from these other observations help determine which candidates can be validated as planets.
To validate Kepler-20e and Kepler-20f, astronomers used a computer program called Blender, which runs simulations to help rule out other astrophysical phenomena masquerading as a planet.
On Dec. 5 the team announced the discovery of Kepler-22b in the habitable zone of its parent star. It is likely to be too large to have a rocky surface. While Kepler-20e and Kepler-20f are Earth-size, they are too close to their parent star to have liquid water on the surface.
"In the cosmic game of hide and seek, finding planets with just the right size and just the right temperature seems only a matter of time," said Natalie Batalha, Kepler deputy science team lead and professor of astronomy and physics at San Jose State University. "We are on the edge of our seats knowing that Kepler's most anticipated discoveries are still to come."
NASA's Ames Research Center in Moffett Field, Calif., manages Kepler's ground system development, mission operations and science data analysis. JPL managed the Kepler mission's development.
Ball Aerospace and Technologies Corp. in Boulder, Colo., developed the Kepler flight system and supports mission operations with the Laboratory for Atmospheric and Space Physics at the University of Colorado in Boulder.
The Space Telescope Science Institute in Baltimore archives, hosts and distributes Kepler science data. Kepler is NASA's 10th Discovery Mission and is funded by NASA's Science Mission Directorate at the agency's headquarters in Washington.
For more information about the Kepler mission and to view the digital press kit, visit: http://www.nasa.gov/kepler
This artist's animation flies through the Kepler-20 star system, where NASA's Kepler mission discovered the first Earth-size planets around a star beyond our own. Animation credit: NASA/Ames/JPL-Caltech
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,当Kp指数达到6或以上时，我国境内有可能观察到极光；当前极光活动水平
，极光为9时漠河可能见极光，
级，当地磁暴级别达到G2或以上时，我国境内有可能见极光
置顶卡
变色卡
千斤顶
显身卡
