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講座大師 - 第十九屆 |
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| Prof. William Borucki |
2015年邵逸夫天文獎得主
美國航空太空總署太空科學家
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講題1：Introduction to the KEPLER Mission: development and science results
簡介克卜勒任務：發展和科學成果
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The Kepler Mission is a space observatory launched in 2009 by NASA to monitor 170,000 stars for four years to determine if Earth-size and larger planets in and near the habitable zone of solar-like stars are common or rare in our galaxy. At the time of the first Kepler proposal in 1992, no exoplanets had been discovered. At the conclusion of the Mission in 2013, over 4600 planetary candidates had been discovered and a new era of astronomy had opened.
An explanation of the photometric method and the reasons for its use are presented. Before the Kepler Mission could be chosen for flight development, many obstacles had to be overcome because of its requirement for extremely high photometric precision. No spacecraft mission had ever been flown that could simultaneously do automated photometry of 170,000 stars with the 10 ppm photometric precision necessary to discover the thousands of planets needed to obtain reliable statistics of exoplanet distributions. It took seventeen years of effort before NASA agreed to fund the Kepler Mission concept as space mission. The timeline and accomplishments necessary to get the Mission accepted are discussed.
A paradigm for the formation of the Solar System is presented for comparison between what was expected and what was discovered. Over 4600 planetary candidates have been found with over 2000 already confirmed as planets. The planets display an enormous range of sizes, temperatures, and types of stellar hosts. In particular, exoplanets near the size of Earth’s moon to those larger than Jupiter have been found with orbital periods that range from 8.5 hours to over 1000 days. Dozens of planetary candidates in the habitable zone have been found. Planets and planetary systems completely different from our Solar System are common. Observed planetary structures are quite unlike that in our Solar System; giant planets are often found much closer to the host star than expected from the model for the formation of our Solar System. The discovery of a planetary system 6 billion years older than ours implies that planetary formation began at the time when our galaxy was in the process of formation. Planets are found in binary and trinary star systems. Illustrations of these results are presented. Calculations based on the Kepler Mission discoveries show that the average number of planets per star exceeds one; planets are more common than stars. Implications of this discovery are examined.
Brief discussions of future missions and their expected results are included.
克卜勒任務是指2009年由美國航太總署發射的太空望遠鏡，計畫以四年時間觀測17萬顆恆星，用於確認在我們的銀河系內，類似太陽的恆星在其適居帶內和鄰近區域，是否普遍存在有約如地球大小和更大的行星，抑或少有？當克卜勒計畫在1992年首次提出時，尚未發現有太陽系外的行星。但克卜勒任務在2013年總結時，數逾4600顆候選行星已經被發現，開啟了天文學的新紀元。
我將解釋測量光度的方法和應用該法的理由。在克卜勒任務被選中，進行太空飛行之前，由於需要極高的測光精度，因此有許多障礙必須克服。在過去的太空飛行任務中，未曾有過能同時自動記錄17萬顆恆星，且測光精度高達10 ppm者。這種技術是必須的，用以發現數以千計的行星，並藉以得到這些系外行星的可靠統計分布。我們努力工作了十七年，才使得航太總署同意撥款推動克卜勒構想，成為太空飛行任務。我們為贏取該任務被認可，所經歷的時程和所取得的成就，將會在本演講中論及。
我將以太陽系的形成作為一個範例，來對照事前的預測和其後的發現。在我們所找到的超過4600顆的候選行星之中，其中有2000顆早已被確認為行星。這些行星的大小、溫度、以及主恆星的型式，呈現出很大的分布範圍。特別的是，從大小接近月亮，以至大於木星的系外行星，它們的公轉週期從8.5小時，延伸至超過1000天。位於適居帶的數十個候選行星已經被發現。行星和行星系統迥異於我們的太陽系是常見的現象。我們觀察到的行星結構非常不同於我們太陽系的行星：巨大的行星相距其環繞的恆星，常比太陽系的形成模型所預期者，要近得多。有一個行星系統被發現形成於六十億年前，比我們的太陽系還老，這意謂該行星系統的形成，起始於當我們的銀河系正在形成的時期。行星也被發現存在於雙星和三星系統中。我將舉例說明這些成果。以克卜勒任務的發現為基礎，進行的計算顯示每一個恆星平均擁有超過一個以上的行星，亦即相對於恆星而言，行星較為常見。我會仔細說明這個發現的意義。
在本演講中，我會簡介將來的太空飛行任務和預期的結果。
藉由此次的演講，我將替各位闡述有哪些因素造就了這些諾貝爾獎得主;是甚麼原因使得LMB能夠如此成功，以及在什麼樣的研究環境下才能夠產生出如此卓越的創造力。
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講題2：Comprehensive discussions of Kepler Mission development and the exoplanet and astrophysical results
綜合討論克卜勒任務的發展和系外行星以及天文物理的研究成果
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At the time of the first Kepler proposal in 1992, no exoplanets had been discovered. At the conclusion of the Mission in 2013, over 4600 planetary candidates had been discovered and a new era of astronomy had opened.
Before the Kepler Mission could be chosen for flight development, many obstacles had to be overcome because of its requirement for extremely high photometric precision. No spacecraft mission had ever been flown that could simultaneously do automated photometry of 170,000 stars with the 10 ppm photometric precision necessary to discover the thousands of planets needed to obtain reliable statistics of exoplanet distributions. It took seventeen years of effort before NASA agreed to fund the Kepler Mission concept for development. To gain acceptance, it was necessary to accomplish the following tasks:
(1) demonstrate that an appropriate combination of detectors and data analysis techniques were available that had the precision necessary to detect transits of Earth-size planets,
(2) build an observatory to demonstrate that automated observations of thousands of stars simultaneously and demonstrate that the automated analysis of the observations were practical, and
(3) develop a laboratory test facility and prototype instrument to demonstrate the 10 ppm photometric precision needed to find Earth-sized planets in the presence of the noise expected from on-orbit operation.
Specific examples of the methods and equipment used to satisfy these requirements are presented.
Planets have been found with average densities as low as Styrofoam (0.05 gr/cc) and as high as that of tungsten (17 gr/cc), and with sizes between that of the Moon to double that of Jupiter. A large fraction of exoplanets do not have an analog in our Solar System; they have sizes between that of Earth and Neptune. Analysis indicates that planets smaller than 1.6 times the radius of Earth are likely to rocky planets whereas planets substantially larger are likely to have substantial hydrogen/helium atmospheres. It is possible that some of them represent ocean planets. Giant planets are found orbiting much closer to their star than Mercury does to the Sun, but there is a paucity of Neptune-size and larger planets that have very short period orbits. The high frequency of 4- and 5-planet systems indicates that flat systems are common. However, in about 20% of planetary systems, a planet is found orbiting at a large angle relative to the stellar equatorial plane. Some planets orbit in retrograde (relative to the stellar spin direction) orbits. Many compact systems are seen and many of these planets are in near-resonant orbits, but seldom in resonant orbits. Observed planetary structures are quite unlike that in our Solar System; some of the planets have very high eccentricities. The discovery of a planetary system 6 billion years older than ours implies that planetary formation began at the time when our galaxy was in the process of formation. Planets are found in binary and trinary star systems. Although over a dozen Earth-size and superEarth-size planets have been detected in the habitable zone; none represent an exact Earth-Sun analog. The Kepler results require that simple models of the formation of the Solar System be revised to account for the observed complexities of planetary system structures.
A combination of several observational and modeling techniques is used to deduce the characteristics of the planets and the host stars. Observational techniques include; high-precision radial velocity measurements to deduce planetary masses, spectroscopic observations to characterize host stars, adaptive optics, speckle, and HST to search for background stars that could account for the transit signal or dilute it, and Spitzer IR observations to check that the transits are achromatic. The planetary masses determined by variations of the epochs of transits caused by the gravitational interaction among planets complement those from radial-velocity measurements. Astrophysical results from the observations of 170,000 stars for a period of four years are presented. These results include unusual stars and planets, and asteroseismic measurements of age, size, and interior structure of red giant stars.
An in-depth discussion of the Mission development and a comprehensive overview of the exoplanet and astrophysical results are presented as well as a description of the methods used to obtain the results.
當克卜勒計畫在1992年首次提出時，尚未發現有太陽系外的行星。但克卜勒任務在2013年總結時，數逾4600顆候選行星已經被發現，開啟了天文學的新紀元。
在克卜勒任務被選中，進行太空飛行之前，由於需要極高的測光精度，因此有許多障礙必須克服。在過去的太空飛行任務中，未曾有過能同時自動記錄17萬顆恆星，且測光精度高達10 ppm者。這種技術是必須的，用以發現數以千計的行星，並藉以得到這些系外行星的可靠統計分布。我們努力工作了十七年，才使得航太總署同意撥款，推動克卜勒任務的構想。為了贏取認可，我們必須完成下列課題：
(1) 以實例論證一種適當組合的偵測器和數據分析技術，其精確度可得以偵測如地球大小的行星的運行。
(2) 建立一座觀測台，展示同時自動觀測數以千計的恆星和自動分析這些觀測數據，是實際可行的操作。
(3) 發展出一套實驗室的測試設備和原型儀器，展示具有10 ppm的光度測量精度。考慮行星在軌道運行時發出的雜訊干擾，要想找出如地球大小的行星，此等精度是必要的。
我將介紹幾個例子，用以滿足上述要求的方法和裝備。
我們所發現的系外行星，其平均密度有些甚低，如同保麗龍(0.05 g/cm 3 )，有些則甚高，如同鎢(17 g/cm 3 )；其大小介於月亮和木星兩倍大之間。有甚高比例的系外行星不同於我們的太陽系，它們的大小介於地球和海王星之間。從數據的分析顯示行星小於地球半徑的1.6倍者，可能是岩石行星；而體積顯著大得多的行星，可能擁有主要由氫和氦構成的大氣層，其中可能有些行星屬於海洋行星。以水星至太陽的距離相比，我們發現巨大的行星以近得多的距離，環繞其主體恆星公轉，但是大小如海王星和大一些的行星，具有很短的公轉週期，卻很少見。4-和5-行星系統的高顯現頻率，說明了平面的行星系統是普遍常見的。無論如何，在大約20%的行星系統中，行星以大角度相對於恆星的赤道面，環繞恆星運轉。有一些行星在公轉軌道上以逆行方向繞行(相對於恆星的自轉方向)。我們觀測到許多小型的行星系統，其中有甚多行星的公轉軌道接近共振，但是卻很少看到有處於共振軌道者。這些被觀察到的行星軌道形狀和我們的太陽系很不一樣：有些行星軌道的偏心率很大。我們發現有一個行星系統有六十億歲的年齡，比我們的太陽系還老，這意謂該行星系統的形成，起始於當我們的銀河系正在形成的時期。在雙恆星和參恆星的系統中，同樣找到有環繞的行星。雖然我們已經發現有數以十計的如同地球大小和超地球大小的行星，位在適居帶內，但是沒有一顆能完全類同於地球對太陽。克卜勒任務所獲致的結果，需要修改太陽系形成的簡單模型，才能用以解釋所觀察到的行星系統結構的複雜性。
數種觀察和模型化技術的結合，被用於推論行星和主體恆星系統的特性。觀測技術包括：高精度徑向速度的測量，用於推算行星的質量；光譜測量用於定出主體恆星的特徵；自調光學；散斑；哈伯太空望遠鏡用於探查背景恆星，用於修正行星凌星時減弱的光度訊號；史匹哲紅外線太空望遠鏡用於檢驗行星凌星時的光度訊號是消色差的。由於行星之間的重力交互作用，會導致行星凌星時間的變動，藉此法決定的行星質量可用於補正利用徑向速度測量法所得的質量。在本演講中，我會展示在四年期間觀察17萬顆行星的天文觀測結果。這些結果包括不尋常的恆星和行星，利用星震測得的紅巨星的年齡、大小、和內部結構。
我將深入討論克卜勒任務的發展，綜合概述系外行星的天文觀測結果，以及說明獲取這些結果所使用的方法。
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