哈勃太空望遠鏡拍攝到一顆行星

哈勃太空望遠鏡拍攝到一顆行星


NASA於11月13日宣布，哈勃太空望遠鏡拍攝到一顆巨大的氣體行星。


1,這是藝術家的想像繪製圖。

這顆行星圍繞的恒星叫Fomalhaut。位於南魚座內，離地球25光年。這顆行星叫Fomalhaut b。

從1980年發現該恒星有一個塵盤。那時起就把它作為尋找行星的候選星。2004年哈勃望遠鏡拍攝了它的清晰的塵盤的照片。且塵盤內邊緣很整齊。伯克利加州大學的天文學家Paul Kalas認為這是由一顆行星的引力導致的現象。他領導了一個小組來觀察這顆恒星。他們比較2004年和2006年的照片，發現那個塵盤中的一個亮點正在移動。從照片上看，那個亮點幾乎接近哈勃望遠鏡分辨率的極限。他們斷定，那是一顆行星。


2，圖中的尺標AU是天文單位的意思，為地球離太陽的平均距離一億五千萬公裏。照片中主要的是恒星的塵盤散射光線產生的“噪聲”斑點。很明顯，照片中的行星的亮度幾乎已接近望遠鏡分辨率的極限。

根據開普勒三定律，推斷該行星的公轉周期是872年。它離恒星的距離是100億英裏，為土星離太陽的10倍遠。它的質量推測為木星的三倍多。當然，也可能是它帶有一個塵盤，反射太陽光很強，因為科學家們觀察它反射了三倍木星質量能反射的光度。因為它的個子巨大，又異常的明亮，所以才令人吃驚地被拍攝到。具體它的質量的準確數字有賴於未來的進一步的觀測。

該恒星的年齡僅一億年。這在恒星中就是兒童。所以，我們現在看到的就是一個太陽係剛形成時的樣子。推測還應有別的行星。這是多麽令人興奮的事件。


3，該恒星在天空的位置。

這顆恒星並不在歐洲南天觀測站的超級地球觀測項目的候選恒星的表上。他們欲尋找的是類地行星，並且其恒星必須是自身不太活躍的。這顆恒星太年輕，自身的黑子導致的徑向抖動幅度太大，會在探測器上產生強烈的假信號。從而導致科學家們產生誤判。

這顆行星的發現，會加強科學家們的一個信念：恒星中相當比例的星體周圍都有行星存在。

文中照片來源於NASA網站。

下麵是NASA網站發布該消息的全文。



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