漫漫“追星”路

A New Earth

Is there anybody out there？ Astronomers and dreamers have been asking this question since the dawn of humanity—and yet， the mystery remains. Are we the only life forms in the universe， or are neighbors like us lurking1） just beyond the reaches of our solar system？ NASA has made it a mission to discover the truth. In March 2009， the space agency launched the Kepler Mission， a NASA Discovery program designed to look for possible life-supporting planets. In just the first 16 months of operation， Kepler has discovered 2，326 potential planets. Once confirmed by follow-up studies， these potential discoveries could significantly raise the tally2） of exoplanets3） well over its current count of 702.

Kepler-22b was the first exoplanet discovered by Kepler to orbit in a star's habitable4） zone. This means that Kepler-22b may have liquid water on it like Earth， making it our closest possible sister planet.

Where the Sun Sets Twice

Why are we looking for exoplanets such as Kepler-22b？ Experts say the future of humanity may depend on it. Finding a planet that can sustain life may be the key to the survival of our people， as our planet could fall victim to5） an asteroid6）， solar flare or our own obtuse7） treatment of it. However， not all planets NASA has discovered are livable. Kepler-16b is the most \"Tatooine8）-like\" planet yet found. NASA refers to its two suns， which resemble Luke Skywalker's home planet in Star Wars. Even though it circles two stars， the planet is thought to be extremely cold and have a gaseous9） surface.

Where Are We Looking？

Given the vastness of our galaxy， you would think the Kepler spacecraft would be sweeping every inch of the skies. However， due to mission constraints， Kepler is focused on one large area that includes the constellations10） Cygnus and Lyra. The Earth makes it difficult to observe all parts of the sky all year round， so the Kepler spacecraft is positioned above the ecliptic plane11）. Kepler can watch 100，000 stars simultaneously. The Cygnus and Lyra region was chosen because of its abundance of stars similar to our sun. The end goal？ That we will find planets like Earth.

Exotic12） Atmospheres

Kepler is not the only space instrument trained13） on exoplanets. Here is an artist's view of the hot gas planet， HD 209458 b， as depicted from information obtained by the Hubble and Spitzer space telescopes. The two instruments revealed that there are molecules of methane14）， water vapor and carbon dioxide in the planet's atmosphere. HD 209458 b orbits a sun-like star 150 light years away in the constellation Pegasus. This planet is not habitable， but experts say that the presence of life-bearing molecules could indicate life on similar planets， but with a rocky surface.

Extreme Planets

Ever wonder what happens to our solar system after the sun runs its course15） in 5 or so billion years？ Here is an artist's conception of a pulsar16） planet system. In 1992， astronomer Aleksander Wolszczan discovered this pulsar， which is named PSR B1257+12. There are at least two Earth-like sized planets rotating it. Radiation from the pulsar is likely \"raining\" down on the planets， causing magnificent auroras17） all through their atmospheres. These planets may be part of a second generation of planets that formed after the blast of the dying star.

The Cliffs of Kepler-10b

2011 was a \"banner year18）\" for the Kepler Mission with the discovery of thousands of potential exoplanets. Early in 2011， NASA announced the discovery of Kepler- 10b， the smallest planet yet found and by far the rockiest. While Kepler-10b was too hot to support life， it showed that the Kepler Mission was capable of finding planets closer to the grand prize—one that could sustain life as Earth does. This photo is an artist's depiction of the molten19） cliffs of Kepler-10b， which is thought to sustain temperatures up to 2，500 degrees Fahrenheit20）. This means that the surface of Kepler-10b is hotter than any lava flow21） on Earth—and hot enough to melt iron.

A New World

Does a planet have to be just like Earth to support life？ NASA isn't sure. Here we see an artist's conception of a planet around a star that is cooler than our sun—these are called M-dwarfs and brown dwarfs. It is still unclear if cooler stars can support young planets with the same life-forming chemical mix that began life on Earth. For a planet to sustain life， it must be able to carry water below boiling point yet above freezing. In addition， it must have enough air—but not too much. This delicate balance depends on the closeness of the star to the planet.

More Discoveries to Come

Here is an artist's depiction of a two-star system called HD 113766. This two-star system is thought to be 10 to 16 million years old， the right age for planets to come into formation. But could this be a planet that supports life？ As the quest for an Earth-like planet continues， experts are optimistic. Natalie Batalha is Kepler's deputy science team lead. She said， \"We are homing in on22） the truly Earth-size， habitable planets.\"

一個新地球

地球之外，還有人嗎？自人類誕生之初，天文學家和夢想家就一直在問這個問題——然而，這個謎團至今仍未解開。我們是宇宙中唯一的生命存在？還是說像我們人類一樣的鄰居就躲在太陽系之外？美國宇航局為了找到真相，制定了一個任務。2009年3月，美國宇航局推出了“開普勒太空望遠鏡任務”，這是其發現計劃中的一項任務，該任務旨在尋找可能維系生命的行星。在最初運行僅16個月內，開普勒太空望遠鏡就找到2326顆潛在行星。一旦得到后續研究的證實，這些潛在行星就會顯著提高系外行星的數目，大大超過目前（編注：英文原文發表于2012年7月）已知的系外行星的總數（702顆）。

開普勒-22b是開普勒太空望遠鏡發現的第一顆運行軌道位于恒星宜居帶的系外行星。這意味著開普勒-22b可能像地球一樣存在液態水，由此成為最接近地球的姊妹行星。

太陽落兩次山的行星

為什么我們要尋找開普勒-22b這樣的系外行星？專家表示，這也許關系著人類的未來。找到一個可以維系生命的行星或許是我們人類謀求生存的關鍵，因為我們的地球可能會遭到小行星撞擊、太陽耀斑侵襲或人類自己愚昧無知的糟蹋。然而，美國宇航局發現的行星并非都適合人類居住。開普勒-16b （編注：見圖②）是目前發現的最“像塔圖因的”行星。美國宇航局之所以這么說，是因為這顆行星擁有兩個恒星，就像電影《星球大戰》里天行者盧克的母星塔圖因那樣。盡管開普勒-16b圍繞著兩顆恒星運行，但它卻被認為是異常寒冷且表面是氣態的行星。

去何方搜尋？

考慮到我們的銀河系浩瀚無邊，你或許以為開普勒太空望遠鏡會巡視太空的每一寸角落。然而，由于任務的局限性，開普勒太空望遠鏡只聚焦于涵蓋天鵝座和天琴座的這一大片區域。由于地球自身原因，我們難以常年觀察到太空的所有區域，因此開普勒太空探測器被安置在黃道面上方。開普勒太空望遠鏡可以同時觀測十萬顆恒星。之所以選中天鵝座和天琴座所在的區域，是因為這片區域分布著大量類似太陽的恒星。那我們最終的目標呢？就是找到像地球一樣的行星。

別樣的大氣層

聚焦系外行星的太空儀器不只開普勒太空望遠鏡一個。此處為熾熱氣態行星HD 209458 b的構想圖（編注：見圖③），由一位藝術家根據哈勃太空望遠鏡和斯皮策太空望遠鏡收集的信息描繪而成。這兩個太空望遠鏡顯示，HD 209458 b的大氣中含有甲烷、水蒸氣和二氧化碳分子。它圍繞著一顆類似太陽的恒星運行，該恒星位于飛馬座，距離我們150光年。雖然HD 209458 b并不宜居，但專家表示，該行星存在孕育生命的分子，這表明與其類似的行星可能存在生命，不過這類行星還要有巖石表面才行。

另類行星

可曾想過，再過50億年左右太陽壽終正寢之后，我們的太陽系會有什么樣的命運？此處為一位藝術家構想的脈沖星行星系統圖（編注：見圖④）。這顆名為PSR B1257+12的脈沖星于1992年被天文學家亞歷山大·沃爾茲森發現。至少有兩顆類似地球大小的行星繞著它運行。這顆脈沖星的輻射可能“灑”向這些行星，使它們的大氣層遍布宏偉壯麗的極光。這些行星可能屬于恒星死亡時發生大爆炸而形成的第二代行星。

開普勒-10b行星的懸崖

2011年是開普勒太空望遠鏡任務的“豐收年”，成千上萬的潛在系外行星在這一年被發現。2011年年初，美國宇航局宣布發現開普勒-10b行星。這是目前發現的體積最小、擁有巖石最多的行星。盡管開普勒-10b溫度太高，不能維系生命，但這一發現表明，開普勒太空望遠鏡任務能夠找到更接近理想狀態的行星——一個可以像地球一樣承載生命的星球。這張照片（編注：見圖⑤）是一位藝術家描繪的開普勒-10b的熔巖懸崖景象，該懸崖的溫度被認為高達2500華氏度（編注：約1371度）。這也就是說，開普勒-10b的表面溫度比地球上任何熔巖流的溫度還要高，足以熔化鐵。

一個新世界

一個星球是否必須得跟地球一模一樣才能孕育生命？美國宇航局對此沒有確切答案。我們這里看到的是一位藝術家構想的一顆行星（編注：見圖⑥），它圍繞著一顆比我們太陽的溫度還要低的恒星（這類恒星被稱為M型紅矮星和褐矮星）運行。現在依然不清楚溫度較低的恒星能否有助于年輕行星產生那種能形成生命的相同化學混合物——正是這種化學混合物給地球帶來了生命。因為一顆行星要想維系生命，必須擁有溫度處于沸點以下但高于冰點的水。此外，它還得有充足的空氣——但又不能過多。這一微妙的平衡取決于恒星距離行星有多近。

未來會有更多發現

這是一位藝術家構想的一張HD 113766雙恒星系統圖（編注：見圖⑦）。這個雙恒星系統的年齡被認為在1000～1600萬年，正是適合行星形成的年齡。但是，形成的新行星能否維系生命？隨著類地行星搜尋工作的不斷展開，專家們對此表示樂觀。納塔莉·巴塔利亞是開普勒太空望遠鏡任務科學組的副組長。她說：“我們正在鎖定真正跟地球大小一樣、適合人類居住的行星。”