哈勃太空望远镜拍摄到一颗行星
NASA于11月13日宣布，哈勃太空望远镜拍摄到一颗巨大的气体行星。
1,这是艺术家的想像绘制图。
这颗行星围绕的恒星叫Fomalhaut。位于南鱼座内，离地球25光年。这颗行星叫Fomalhaut b。
从1980年发现该恒星有一个尘盘。那时起就把它作为寻找行星的候选星。2004年哈勃望远镜拍摄了它的清晰的尘盘的照片。且尘盘内边缘很整齐。伯克利加州大学的天文学家Paul Kalas认为这是由一颗行星的引力导致的现象。他领导了一个小组来观察这颗恒星。他们比较2004年和2006年的照片，发现那个尘盘中的一个亮点正在移动。从照片上看，那个亮点几乎接近哈勃望远镜分辨率的极限。他们断定，那是一颗行星。
2，图中的尺标AU是天文单位的意思，为地球离太阳的平均距离一亿五千万公里。照片中主要的是恒星的尘盘散射光线产生的“噪声”斑点。很明显，照片中的行星的亮度几乎已接近望远镜分辨率的极限。
根据开普勒三定律，推断该行星的公转周期是872年。它离恒星的距离是100亿英里，为土星离太阳的10倍远。它的质量推测为木星的三倍多。当然，也可能是它带有一个尘盘，反射太阳光很强，因为科学家们观察它反射了三倍木星质量能反射的光度。因为它的个子巨大，又异常的明亮，所以才令人吃惊地被拍摄到。具体它的质量的准确数字有赖于未来的进一步的观测。
该恒星的年龄仅一亿年。这在恒星中就是儿童。所以，我们现在看到的就是一个太阳系刚形成时的样子。推测还应有别的行星。这是多么令人兴奋的事件。
3，该恒星在天空的位置。
这颗恒星并不在欧洲南天观测站的超级地球观测项目的候选恒星的表上。他们欲寻找的是类地行星，并且其恒星必须是自身不太活跃的。这颗恒星太年轻，自身的黑子导致的径向抖动幅度太大，会在探测器上产生强烈的假信号。从而导致科学家们产生误判。
这颗行星的发现，会加强科学家们的一个信念：恒星中相当比例的星体周围都有行星存在。
文中照片来源于NASA网站。
下面是NASA网站发布该消息的全文。
Hubble Directly Observes a Planet Orbiting Another Star
WASHINGTON -- NASA's Hubble Space Telescope has taken the first visible-light snapshot of a planet circling another star.
Estimated to be no more than three times Jupiter's mass, the planet, called Fomalhaut b, orbits the bright southern star Fomalhaut, located 25 light-years away in the constellation Piscis Australis, or the "Southern Fish."
Fomalhaut has been a candidate for planet hunting ever since an excess of dust was discovered around the star in the early 1980s by NASA's Infrared Astronomy Satellite, IRAS.
In 2004, the coronagraph in the High Resolution Camera on Hubble's Advanced Camera for Surveys produced the first-ever resolved visible-light image of the region around Fomalhaut. It clearly showed a ring of protoplanetary debris approximately 21.5 billion miles across and having a sharp inner edge.
This large debris disk is similar to the Kuiper Belt, which encircles the solar system and contains a range of icy bodies from dust grains to objects the size of dwarf planets, such as Pluto.
Hubble astronomer Paul Kalas, of the University of California at Berkeley, and team members proposed in 2005 that the ring was being gravitationally modified by a planet lying between the star and the ring's inner edge.
Circumstantial evidence came from Hubble's confirmation that the ring is offset from the center of the star. The sharp inner edge of the ring is also consistent with the presence of a planet that gravitationally "shepherds" ring particles. Independent researchers have subsequently reached similar conclusions.
Now, Hubble has actually photographed a point source of light lying 1.8 billion miles inside the ring's inner edge. The results are being reported in the November 14 issue of Science magazine.
"Our Hubble observations were incredibly demanding. Fomalhaut b is 1 billion times fainter than the star. We began this program in 2001, and our persistence finally paid off," Kalas says.
"Fomalhaut is the gift that keeps on giving. Following the unexpected discovery of its dust ring, we have now found an exoplanet at a location suggested by analysis of the dust ring's shape. The lesson for exoplanet hunters is 'follow the dust,'" said team member Mark Clampin of NASA's Goddard Space Flight Center in Greenbelt, Md.
Observations taken 21 months apart by Hubble's Advanced Camera for Surveys' coronagraph show that the object is moving along a path around the star, and is therefore gravitationally bound to it. The planet is 10.7 billion miles from the star, or about 10 times the distance of the planet Saturn from our sun.
The planet is brighter than expected for an object of three Jupiter masses. One possibility is that it has a Saturn-like ring of ice and dust reflecting starlight. The ring might eventually coalesce to form moons. The ring's estimated size is comparable to the region around Jupiter and its four largest orbiting satellites.
Kalas and his team first used Hubble to photograph Fomalhaut in 2004, and made the unexpected discovery of its debris disk, which scatters Fomalhaut's starlight. At the time they noted a few bright sources in the image as planet candidates. A follow-up image in 2006 showed that one of the objects is moving through space with Fomalhaut but changed position relative to the ring since the 2004 exposure. The amount of displacement between the two exposures corresponds to an 872-year-long orbit as calculated from Kepler's laws of planetary motion.
Future observations will attempt to see the planet in infrared light and will look for evidence of water vapor clouds in the atmosphere. This would yield clues to the evolution of a comparatively newborn 100-million-year-old planet. Astrometric measurements of the planet's orbit will provide enough precision to yield an accurate mass.
NASA's James Webb Space Telescope, scheduled to launch in 2013 will be able to make coronagraphic observations of Fomalhaut in the near- and mid-infrared. Webb will be able to hunt for other planets in the system and probe the region interior to the dust ring for structures such as an inner asteroid belt. For more information about the Hubble Space Telescope, visit:
http://www.nasa.gov/hubble
