太空望遠鏡 發現兩神秘新星體 體積小溫度高 非恆星行星

【明報專訊】去年3月發射升空的美國開普勒（Kepler）太空望遠鏡，發現兩個前所未見的神秘新型星體，它們的溫度逾1萬℃，遠比一般行星熱，但卻比一般恆星小，故既非行星也非恆星，不符合目前所知天體的定義，故很可能是一種新等級的天體。
今次是開普勒望遠鏡首次發現新星體，這兩個星體各自環繞自己的恆星運轉，但卻比恆星的溫度還高逾數千℃，約達14400℃，足以融化鉛或鐵，這顯示它們不是一般的行星，其體積也比太陽系中任何一顆行星大。NASA天體物理學家莫爾斯（Jon Morse）說：「宇宙在不停在製造一些超乎我們想像的物質。」由於這兩顆星體的特性都不完全符合目前所知天體定義，因此尚難將它們歸類，科學家暫稱這它們為「熱伙伴」（Hot Companions）。
專家料屬新生行星衰亡白矮星
NASA研究員羅爾（Jason Rowe）推測它們可能是新生的行星，因為新生的行星通常溫度相當高，估計它們大概只形成約2億年左右。另一所太空望遠鏡科學機構的學者吉利蘭（Ronald Gilliland）則認為，它們可能是正在死亡的白矮星，也就是處於演化末期正逐漸縮小的恆星。
另發現5太陽系外行星
除了這兩種星體，開普勒望遠鏡亦找到了5顆太陽系外行星。開普勒計劃首席科學家博魯茨基（William Borucki）指出，發現為人類對「行星系統如何從氣體和塵埃形成的認知」作出了重要貢獻。新發現的行星跟許多系外行星一樣，都是所謂的「熱木星」。它們的質量與木星接近，這5顆行星由於非常接近其所屬恆星，公轉周期只介乎3.3個地球日至4.9個地球日之間，表面溫度介乎1204℃至1649℃之間，比岩漿更熱，可以將金熔化，並不適合任何已知生物生存。其中最小的一顆與太陽系中體積排行第4的行星海王星相當，前者比後者質量更大，最大的則與太陽系中最大的行星木星相當。這5顆行星分別被命名為「開普勒4b」、「開普勒5b」、「開普勒6b」、「開普勒7b」、「開普勒8b」。
望遠鏡去年升空 尋似地球行星
開普勒去年3月升空，是全球首個專門用於尋找與地球相似的「類地行星」，它的視野比任何太空望遠鏡都要廣闊。開普勒也能夠監測行星從恆星前方經過的「凌日」現象，並觀測到凌日深度（恆星亮度減弱的程度），據此計算出行星的大小。此外，再綜合其他數據，科學家便可推測一顆行星是否適合生物生存。NASA估計，找到像地球這樣大的行星大概需時最少3年。

NASA’s Kepler Space Telescope Discovers its First Five Exoplanets 01.04.10

This artist's concept shows a cloudy Jupiter-like planet that orbits very close to its fiery hot star.NASA/JPL-Caltech/T. Pyle (SSC) Click image for full-resolution. NASA's Kepler space telescope, designed to find Earth-size planets in the habitable zone of sun-like stars, has discovered its first five new exoplanets, or planets beyond our solar system. Kepler's high sensitivity to both small and large planets enabled the discovery of the exoplanets, named Kepler 4b, 5b, 6b, 7b and 8b. The discoveries were announced Monday, Jan. 4, by the members of the Kepler science team during a news briefing at the American Astronomical Society meeting in Washington. "These observations contribute to our understanding of how planetary systems form and evolve from the gas and dust disks that give rise to both the stars and their planets," said William Borucki of NASA's Ames Research Center in Moffett Field, Calif. Borucki is the mission's science principal investigator. "The discoveries also show that our science instrument is working well. Indications are that Kepler will meet all its science goals." Known as "hot Jupiters" because of their high masses and extreme temperatures, the new exoplanets range in size from similar to Neptune to larger than Jupiter. They have orbits ranging from 3.3 to 4.9 days. Estimated temperatures of the planets range from 2,200 to 3,000 degrees Fahrenheit, hotter than molten lava and much too hot for life as we know it. All five of the exoplanets orbit stars hotter and larger than Earth's sun. "It's gratifying to see the first Kepler discoveries rolling off the assembly line," said Jon Morse, director of the Astrophysics Division at NASA Headquarters in Washington. "We expected Jupiter-size planets in short orbits to be the first planets Kepler could detect. It's only a matter of time before more Kepler observations lead to smaller planets with longer period orbits, coming closer and closer to the discovery of the first Earth analog." Launched on March 6, 2009, from Cape Canaveral Air Force Station in Florida, the Kepler mission continuously and simultaneously observes more than 150,000 stars. Kepler's science instrument, or photometer, already has measured hundreds of possible planet signatures that are being analyzed. While many of these signatures are likely to be something other than a planet, such as small stars orbiting larger stars, ground-based observatories have confirmed the existence of the five exoplanets. The discoveries are based on approximately six weeks' worth of data collected since science operations began on May 12, 2009. Kepler looks for the signatures of planets by measuring dips in the brightness of stars. When planets cross in front of, or transit, their stars as seen from Earth, they periodically block the starlight. The size of the planet can be derived from the size of the dip. The temperature can be estimated from the characteristics of the star it orbits and the planet's orbital period. Kepler will continue science operations until at least November 2012. It will search for planets as small as Earth, including those that orbit stars in a warm habitable zone where liquid water could exist on the surface of the planet. Since transits of planets in the habitable zone of solar-like stars occur about once a year and require three transits for verification, it is expected to take at least three years to locate and verify an Earth-size planet. According to Borucki, Kepler's continuous and long-duration search should greatly improve scientists' ability to determine the distributions of planet size and orbital period in the future. "Today's discoveries are a significant contribution to that goal," Borucki said. "The Kepler observations will tell us whether there are many stars with planets that could harbor life, or whether we might be alone in our galaxy." Kepler is NASA's 10th Discovery mission. Ames is responsible for the ground system development, mission operations and science data analysis. NASA's Jet Propulsion Laboratory in Pasadena, Calif., managed the Kepler mission development. Ball Aerospace & Technologies Corp. of Boulder, Colo., was responsible for developing the Kepler flight system. Ball and the Laboratory for Atmospheric and Space Physics at the University of Colorado in Boulder are supporting mission operations. Ground observations necessary to confirm the discoveries were conducted with ground-based telescopes the Keck I in Hawaii; Hobby-Ebberly and Harlan J. Smith 2.7m in Texas; Hale and Shane in California; WIYN, MMT and Tillinghast in Arizona; and Nordic Optical in the Canary Islands, Spain.