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(gè)新地球

地球之外，還有人嗎？自人類(lèi)誕生之初，天文學(xué)家和夢(mèng)想家就一直在問(wèn)這個(gè)問(wèn)題——然而，這個(gè)謎團(tuán)至今仍未解開(kāi)。我們是宇宙中唯一的生命存在？還是說(shuō)像我們?nèi)祟?lèi)一樣的鄰居就躲在太陽(yáng)系之外？美國(guó)宇航局為了找到真相，制定了一個(gè)任務(wù)。2009年3月，美國(guó)宇航局推出了“開(kāi)普勒太空望遠(yuǎn)鏡任務(wù)”，這是其發(fā)現(xiàn)計(jì)劃中的一項(xiàng)任務(wù)，該任務(wù)旨在尋找可能維系生命的行星。在最初運(yùn)行僅16個(gè)月內(nèi)，開(kāi)普勒太空望遠(yuǎn)鏡就找到2326顆潛在行星。一旦得到后續(xù)研究的證實(shí)，這些潛在行星就會(huì)顯著提高系外行星的數(shù)目，大大超過(guò)目前（編注：英文原文發(fā)表于2012年7月）已知的系外行星的總數(shù)（702顆）。

開(kāi)普勒-22b是開(kāi)普勒太空望遠(yuǎn)鏡發(fā)現(xiàn)的第一顆運(yùn)行軌道位于恒星宜居帶的系外行星。這意味著開(kāi)普勒-22b可能像地球一樣存在液態(tài)水，由此成為最接近地球的姊妹行星。

太陽(yáng)落兩次山的行星

為什么我們要尋找開(kāi)普勒-22b這樣的系外行星？專(zhuān)家表示，這也許關(guān)系著人類(lèi)的未來(lái)。找到一個(gè)可以維系生命的行星或許是我們?nèi)祟?lèi)謀求生存的關(guān)鍵，因?yàn)槲覀兊牡厍蚩赡軙?huì)遭到小行星撞擊、太陽(yáng)耀斑侵襲或人類(lèi)自己愚昧無(wú)知的糟蹋。然而，美國(guó)宇航局發(fā)現(xiàn)的行星并非都適合人類(lèi)居住。開(kāi)普勒-16b （編注：見(jiàn)圖②）是目前發(fā)現(xiàn)的最“像塔圖因的”行星。美國(guó)宇航局之所以這么說(shuō)，是因?yàn)檫@顆行星擁有兩個(gè)恒星，就像電影《星球大戰(zhàn)》里天行者盧克的母星塔圖因那樣。盡管開(kāi)普勒-16b圍繞著兩顆恒星運(yùn)行，但它卻被認(rèn)為是異常寒冷且表面是氣態(tài)的行星。

去何方搜尋？

考慮到我們的銀河系浩瀚無(wú)邊，你或許以為開(kāi)普勒太空望遠(yuǎn)鏡會(huì)巡視太空的每一寸角落。然而，由于任務(wù)的局限性，開(kāi)普勒太空望遠(yuǎn)鏡只聚焦于涵蓋天鵝座和天琴座的這一大片區(qū)域。由于地球自身原因，我們難以常年觀察到太空的所有區(qū)域，因此開(kāi)普勒太空探測(cè)器被安置在黃道面上方。開(kāi)普勒太空望遠(yuǎn)鏡可以同時(shí)觀測(cè)十萬(wàn)顆恒星。之所以選中天鵝座和天琴座所在的區(qū)域，是因?yàn)檫@片區(qū)域分布著大量類(lèi)似太陽(yáng)的恒星。那我們最終的目標(biāo)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(kāi)普勒太空望遠(yuǎn)鏡一個(gè)。此處為熾熱氣態(tài)行星HD 209458 b的構(gòu)想圖（編注：見(jiàn)圖③），由一位藝術(shù)家根據(jù)哈勃太空望遠(yuǎn)鏡和斯皮策太空望遠(yuǎn)鏡收集的信息描繪而成。這兩個(gè)太空望遠(yuǎn)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(lèi)似太陽(yáng)的恒星運(yùn)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(zhuān)家表示，該行星存在孕育生命的分子，這表明與其類(lèi)似的行星可能存在生命，不過(guò)這類(lèi)行星還要有巖石表面才行。

另類(lèi)行星

可曾想過(guò)，再過(guò)50億年左右太陽(yáng)壽終正寢之后，我們的太陽(yáng)系會(huì)有什么樣的命運(yùn)？此處為一位藝術(shù)家構(gòu)想的脈沖星行星系統(tǒng)圖（編注：見(jiàn)圖④）。這顆名為PSR B1257+12的脈沖星于1992年被天文學(xué)家亞歷山大·沃爾茲森發(fā)現(xiàn)。至少有兩顆類(lèi)似地球大小的行星繞著它運(yùn)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(shí)發(fā)生大爆炸而形成的第二代行星。

開(kāi)普勒-10b行星的懸崖

2011年是開(kāi)普勒太空望遠(yuǎn)鏡任務(wù)的“豐收年”，成千上萬(wàn)的潛在系外行星在這一年被發(fā)現(xiàn)。2011年年初，美國(guó)宇航局宣布發(fā)現(xiàn)開(kāi)普勒-10b行星。這是目前發(fā)現(xiàn)的體積最小、擁有巖石最多的行星。盡管開(kāi)普勒-10b溫度太高，不能維系生命，但這一發(fā)現(xiàn)表明，開(kāi)普勒太空望遠(yuǎn)鏡任務(wù)能夠找到更接近理想狀態(tài)的行星——一個(gè)可以像地球一樣承載生命的星球。這張照片（編注：見(jiàn)圖⑤）是一位藝術(shù)家描繪的開(kāi)普勒-10b的熔巖懸崖景象，該懸崖的溫度被認(rèn)為高達(dá)2500華氏度（編注：約1371度）。這也就是說(shuō)，開(kāi)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(gè)新世界

一個(gè)星球是否必須得跟地球一模一樣才能孕育生命？美國(guó)宇航局對(duì)此沒(méi)有確切答案。我們這里看到的是一位藝術(shù)家構(gòu)想的一顆行星（編注：見(jiàn)圖⑥），它圍繞著一顆比我們太陽(yáng)的溫度還要低的恒星（這類(lèi)恒星被稱(chēng)為M型紅矮星和褐矮星）運(yùn)行。現(xiàn)在依然不清楚溫度較低的恒星能否有助于年輕行星產(chǎn)生那種能形成生命的相同化學(xué)混合物——正是這種化學(xué)混合物給地球帶來(lái)了生命。因?yàn)橐活w行星要想維系生命，必須擁有溫度處于沸點(diǎn)以下但高于冰點(diǎn)的水。此外，它還得有充足的空氣——但又不能過(guò)多。這一微妙的平衡取決于恒星距離行星有多近。

未來(lái)會(huì)有更多發(fā)現(xiàn)

這是一位藝術(shù)家構(gòu)想的一張HD 113766雙恒星系統(tǒng)圖（編注：見(jiàn)圖⑦）。這個(gè)雙恒星系統(tǒng)的年齡被認(rèn)為在1000～1600萬(wàn)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(lèi)地行星搜尋工作的不斷展開(kāi)，專(zhuān)家們對(duì)此表示樂(lè)觀。納塔莉·巴塔利亞是開(kāi)普勒太空望遠(yuǎn)鏡任務(wù)科學(xué)組的副組長(zhǎng)。她說(shuō)：“我們正在鎖定真正跟地球大小一樣、適合人類(lèi)居住的行星?！?/p>