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天文学家发现首颗疑似系外“月球”

2014-04-15    来源:新华网    【      美国外教 在线口语培训

Astronomers spot first possible "exomoon"

An international team of astronomers said Thursday they may have spotted the first "exomoon, " or a moon orbiting a planet that lies outside the solar system.

But the object could also be a huge planet orbiting a small, faint star, the team reported in the Astrophysical Journal.

"We won't have a chance to observe the exomoon candidate again, " lead author David Bennett of the University of Notre Dame said in a statement released by the U.S. space agency NASA. "But we can expect more unexpected finds like this."

The possible exomoon was discovered during a study led by the joint Japan-New Zealand-American Microlensing Observations in Astrophysics (MOA) and the Probing Lensing Anomalies NETwork ( PLANET) programs, using telescopes in New Zealand and Australia.

The astronomers used a technique called gravitational microlensing, in which one star passes in front of another as seen from Earth. The closer star can act like a magnifying glass to focus and brighten the light of the more distant one for a period of days to weeks during the event.

If the foreground star has a planet circling around it, the planet will act as a second lens to brighten or dim the light even more. By carefully scrutinizing these brightening events, astronomers can figure out the mass of the foreground star relative to its planet.

In some cases, however, the foreground object could be a free- floating planet, not a star. Researchers might then be able to measure the mass of the planet relative to its orbiting companion.

In the new study funded by NASA, the astronomers found the ratio of the larger body to its smaller companion is 2,000 to 1, although the nature of the foreground, lensing object is not clear.

That means the pair could be either a small, faint star circled by a planet about 18 times the mass of Earth, or a planet more massive than Jupiter coupled with a moon weighing less than Earth, the astronomers said.

But they cannot tell which of these two scenarios is correct, since the encounter can be witnessed only once.

"One possibility is for the lensing system to be a planet and its moon, which if true, would be a spectacular discovery of a totally new type of system," said Wes Traub, chief scientist for NASA's Exoplanet Exploration Program office at NASA's Jet Propulsion Laboratory, who was not involved in the study.

"The researchers' models point to the moon solution, but if you simply look at what scenario is more likely in nature, the star solution wins," Traub added.

While astronomers are actively looking for exomoons, for example, using data from NASA's Kepler mission, so far they have not found any.

相关新闻:一个由多国天文学家组成的科研小组10日说,他们可能已找到太阳系外第一颗围绕行星运行的卫星。但由于观测时机已过,无法进一步观测确认这一发现,因此“这个系外卫星及其伴侣的真正身份将永远无法弄清”。

人类已经发现了约1700颗太阳系外行星,但迄今没有确认发现一颗系外卫星。在新研究中,上述科研小组利用设在新西兰和澳大利亚的望远镜,发现了一个叫做MOA-2011-BLG-262的天体系统,其中那个较小的天体很有可能是一颗天然卫星。

这一成果借助了微引力透镜效应,即从地球上看去,一颗遥远天体发出的光,会在引力的作用下被中间的某颗恒星或“漫游”行星聚焦,从而变得更亮,就像透镜一样。分析这一亮度,可以了解中间恒星或行星的许多信息,包括它有没有绕转星球,如果有,它们之间的质量比是多少等。

美国圣母大学戴维·贝内特等人发现,尽管此次观测到的中间天体的身份不清楚,但其质量是绕它运行的小星球的2000倍。这意味着有两种可能:要么是一颗暗淡的小型恒星被一颗质量为地球18倍的行星绕转;要么是一颗质量与木星相当的行星,被一颗质量不及地球的卫星绕转。

但从地球上观测时,遥远天体刚好和中间作为透镜的天体在视线方向对齐的机会只有一次,错过了就无法再次观测,研究人员也不清楚到底哪种可能性更大。美国航天局喷气推进实验室行星科学家韦斯·特劳布说,此研究的模型指向了卫星的答案,如果这是正确的,那么将是一个“令人惊叹的发现”。不过,特劳布也不能排除是行星的可能性。

研究人员指出,这一谜团的答案依赖于透镜天体与地球的距离。如果距离较近,则答案是卫星;如果距离较远,则答案是行星。因此,错过这颗疑似系外“月球”之后,他们只能寄希望于今后再意外发现其他的系外卫星进行对比分析。相关研究论文发表在新一期美国《天体物理学杂志》上。(新华网)



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