在一個星團的類太陽的恆星周圍發現行星

天文學家在一個叫Beehive star cluster 的兩個和太陽類似的恆星的周圍各發現一個行星。

這兩顆行星都是hot Jupiters，它們是像木星一樣的巨大氣體行星，並且是圍繞其恆星的軌道很近，因而其表面的溫度很高。

1，藝術家對這兩顆行星的想像圖。

這個叫蜂巢的星團是離太陽系最近的星團。它距太陽系大約是577光年。星團的中心明亮的直徑大約22.8光年。在這樣小的空間中，密布了大約1000顆恆星。在天文上，這樣的恆星分布是非常密集的，所以把它稱為“蜂巢”。比較一下，距我們最近的恆星大約是4光年多的距離。

在這樣狹小的空間上，分布着如此眾多的恆星，它們彼此間引力作用，綁縛住一起。這麼多的恆星是一個巨大的星雲凝聚而成的，它們是同時誕生的，它們的化學組成幾乎是一樣的。這些恆星的年齡大約是幾億年的數量級，這是很年輕的恆星。

這次發現的一個意義是：在一個恆星間引力互相作用的環境下，在恆星的周圍像其它孤立的恆星一樣會形成行星系統。這次發現為此類的理論提供了有力的證據。

2，這兩顆恆星在天空中的相對位置。版權為Stuart Heggie 擁有。

另一個意義是：灼熱的類木星的行星圍繞着宿主恆星在很近的軌道上旋轉，其公轉周期僅有幾天。這對行星的形成的各種理論假設提供了新證據。按照理論推測，如此巨大的氣體行星現在運行的軌道一定不是它誕生的地方。它必定是在更遠的軌道上誕生的。由於引力的相互作用，這些行星會不斷向更低的軌道遷移。因為這些恆星都是很年青的，目前的觀察數據說明巨大的氣體行星的軌道遷移已“完成”了，這表明了軌道遷移的速率是多麼快。這為近一步了解太陽系的形成提供了可觀測的“樣本”。

我們推斷，許多年齡較長的恆星周圍的巨型氣體行星大概被宿主恆星吞沒了。

最後是兩個圖像的外鏈地址。

下面是NASA網站關於這個發現的報道原文。

PASADENA, Calif. -- NASA-funded astronomers have, for the first time, spotted planets orbiting sun-like stars in a crowded cluster of stars. The findings offer the best evidence yet that planets can sprout up in dense stellar environments. Although the newfound planets are not habitable, their skies would be starrier than what we see from Earth.

The starry-skied planets are two so-called hot Jupiters, which are massive, gaseous orbs that are boiling hot because they orbit tightly around their parent stars. Each hot Jupiter circles a different sun-like star in the Beehive Cluster, also called the Praesepe, a collection of roughly 1,000 stars that appear to be swarming around a common center.

The Beehive is an open cluster, or a grouping of stars born at about the same time and out of the same giant cloud of material. The stars therefore share a similar chemical composition. Unlike the majority of stars, which spread out shortly after birth, these young stars remain loosely bound together by mutual gravitational attraction.

"We are detecting more and more planets that can thrive in diverse and extreme environments like these nearby clusters," said Mario R. Perez, the NASA astrophysics program scientist in the Origins of Solar Systems Program. "Our galaxy contains more than 1,000 of these open clusters, which potentially can present the physical conditions for harboring many more of these giant planets."

The two new Beehive planets are called Pr0201b and Pr0211b. The star's name followed by a "b" is the standard naming convention for planets.

"These are the first 'b's' in the Beehive," said Sam Quinn, a graduate student in astronomy at Georgia State University in Atlanta and the lead author of the paper describing the results, which was published in the Astrophysical Journal Letters.

Quinn and his team, in collaboration with David Latham at the Harvard-Smithsonian Center for Astrophysics, discovered the planets by using the 1.5-meter Tillinghast telescope at the Smithsonian Astrophysical Observatory's Fred Lawrence Whipple Observatory near Amado, Arizona to measure the slight gravitational wobble the orbiting planets induce upon their host stars. Previous searches of clusters had turned up two planets around massive stars but none had been found around stars like our sun until now.

"This has been a big puzzle for planet hunters," Quinn said. "We know that most stars form in clustered environments like the Orion nebula, so unless this dense environment inhibits planet formation, at least some sun-like stars in open clusters should have planets. Now, we finally know they are indeed there."

The results also are of interest to theorists who are trying to understand how hot Jupiters wind up so close to their stars. Most theories contend these blistering worlds start out much cooler and farther from their stars before migrating inward.

"The relatively young age of the Beehive cluster makes these planets among the youngest known," said Russel White, the principal investigator on the NASA Origins of Solar Systems grant that funded this study. "And that's important because it sets a constraint on how quickly giant planets migrate inward -- and knowing how quickly they migrate is the first step to figuring out how they migrate."

The research team suspects planets were turned up in the Beehive cluster because it is rich in metals. Stars in the Beehive have more heavy elements such as iron than the sun has.

According to White, "Searches for planets around nearby stars suggest that these metals act like a 'planet fertilizer,' leading to an abundant crop of gas giant planets. Our results suggest this may be true in clusters as well."

NASA's Jet Propulsion Laboratory, Pasadena, Calif., manages NASA's Exoplanet Exploration Program office. More information about exoplanets and NASA's planet-finding program is available at: http://planetquest.jpl.nasa.gov .

http://www.nasa.gov/images/content/687507main_pia15802-43_800-600.jpg 

藝術家的想像圖地址。

http://www.nasa.gov/images/content/687314main_pia15801-43_946-710.jpg

恆星在天空中的位置地址。

版權為Stuart Heggie 擁有。