天文学家利用多波段测量系外行星大气反射的偏振情形，证实热木星HD189733b在蓝光波段具有偏振现象。色彩能提供恒星温度、H-alpha分布等重要信息。虽然现在已发现了五百多颗系外行星，但在这之前，天文学家一直是使用单一滤镜研究系外行星，因而对行星颜色一无所知。天文学家利用西班牙卡纳利群岛口径2.5米的北欧光学望远镜，分别在紫外光、蓝光、绿光三个波段观测HD189733b。

[Jan 06, 2011] 

天文：天文學家首次以多波段測量系外行星偏振！

天文學家首次以多波段測量系外行星大氣反射的偏振情形，並證實了熱木星HD189733b在藍光波段具有偏振現象！

色彩提供天文學家許多寶貴的資訊，像是恆星的溫度、H-alpha的分佈，甚至是氧原子的禁線躍遷發出的綠光。現在雖然已經有五百多個系外行星被發現，但在這之前，天文學家一直是使用單一濾鏡研究系外行星，因而對系外行星的顏色一無所知。

光的特性之一是反射之後會具有偏振現象，這也是為什麼使用偏光片的太陽眼鏡，能有效防止路面或水面反射光所造成的眩光。類似的道理，照射到行星大氣所反射的光也會有偏振現象，而偏振的程度會和光的入射角、大氣層中的分子，以及觀測的波長有關。

天文學家利用西班牙卡納利群島口徑2.5米的北歐光學望遠鏡（Nordic Optical Telescope），分別在U（ultra-violet，紫外光）、B（blue，藍光）、V（visible，可見光，其實波長為綠光）三個波段觀測HD189733b這顆熱木星（距離母恆星很近的氣體巨行星）。

研究的結果發現，這顆行星在藍光的部分最為明亮，這也顯示大氣層中造成偏振的主要機制是雷立散射（Rayleigh scattering），因此這顆行星看起來會和地球天空一樣的湛藍。另外他們還發現偏振的程度在東西大距（母恆星－行星－地球三者成直角的位置）時達到最高，這說明偏振的機制是由行星大氣的散射造成，而非恆星光因巨大的星斑所產生的偏振。

這篇研究的主要貢獻，在於使用新的觀測方法來研究系外行星大氣，但HD189733b是個非常靠近母恆星的熱木星，因此受到大量的恆星光照耀。將來若要將這種技術應用到其他距離母恆星較遠的系外行星上，可是件不容易的工作呢！

參考來源：

• Universe Today: First Multi-wavelength Images of an Exoplanet
• arXiv:1101.0059v1

FirstMulti-wavelength Images of an Exoplanet

by JON VOISEY on JANUARY 4, 2011

Colors are important in astronomy. They can be used to get a quick feel for the temperature of stars, map out hydrogen alpha, or even find oxygen when it gives off a distinctive green glow from the forbidden transistion. Yet thus far, all images of exoplanets have only been taken in a single color filter leaving astronomers with a flat picture and no understanding of the color of a planet. A new paper corrects this oversight while analyzing the polarization of reflected starlight to develop an understanding of the characteristics of the planet’s atmosphere.

One of the properties of light is that it often becomes polarized upon reflection. This allows for polarized sunglasses to effectively reduce glare from road surfaces because the reflection tends to polarize the light in a preferred direction. Similarly, light striking a planet’s atmosphere will have a preferred axis of polarization. The degree of polarization will depend on many factors including, the angle of incidence (corresponding to the planetary phase), the types of molecules in the atmosphere, and the color, or wavelength, of light through which the planet is observed.

The object of interest was HD189733b and observations were taken in using the UBV filters system which uses filters in the ultra-violet, blue, and green (or “visible”) portions of the spectra. They were conduced at theNordic Optical Telescope in Spain.

To control for the variations, astronomers would need to observe the planet at several wavelengths to understand how the color was affecting the results, as well as to watch the planet for several orbits to trace how the phase impacted the observations. Presently, the authors have not gone so far as to compare various composition models against these observations as this study was largely intended to be a feasibility study at multi-wavelength polarization detection.

Results have shown that the planet is brightest in the blue portion of the spectra, a result that confirms earlier, theoretical predictions for hot Jupiters as well as tentative observational findings based on single color studies done last year. This supports the notion that the dominant mechanism of polarization is Rayleigh scattering in the atmosphere. The result of this is that the planet would likely appear to be a deep blue to the naked eye, much the same way our sky appears blue, but a much more vivid color due to the increased depth to which we would look. The observations also confirmed that polarization was greatest when the planet was near greatest elongation (as far to either side of the star as possible instead of near in front or behind when viewed from Earth) which supports that the polarization is due to scattering in the atmosphere as opposed to the starlight being initially polarized from large starspots.

Certainly, this study has demonstrated the potential for astronomers to begin exploring planetary characteristics with polarization. However, it may be some time before it becomes accepted in general use. While the findings were certainly above the background noise, there existed a significant degree of uncertainty in the measurements resulting from the faint nature of planets. Being a large, hot Jupiter, HD189733b is a strong candidate since it is close to its parent star and thus, receives a large amount of light. Using such methods for other exoplanets, more distant from their parent stars will likely prove an even more daunting task, requiring careful preparation and observations.