【媒庫文選】外星生命藏在地球2.0版以外嗎?
2019年07月10日11:11

原標題:【媒庫文選】外星生命藏在地球2.0版以外嗎?

Is Alien Life Hiding Beyond Earth 2.0? 外星生命藏在地球2.0版以外嗎?

Ethan Siegel 伊桑·西格爾

  When we think about life out there in the Universe, far beyond the limits of Earth, we can‘t help but look to our own planet as a guide. Earth has a number of features that we think are extremely important — perhaps even essential — for enabling life to arise and thrive. For generations, humans have dreamed of life beyond Earth, striving to find another world similar to our own but with its own unique success story: our own Earth 2.0.

  But just because life succeeded here on Earth doesn‘t necessarily mean that life is likely to succeed on Earth-like worlds, only that it‘s possible. Similarly, just because life hasn‘t been found on non-Earth-like worlds doesn‘t mean that it isn‘t possible.

  There are lots of reasons to believe that looking for a world as Earth-like as possible, around a star as Sun-like as possible, might be the best place to look for life elsewhere in the Universe. Practically no one in the exoplanet or astrobiology communities thinks that looking for worlds similar to a proverbial Earth 2.0 is a bad idea. But is it the smartest course of action to invest the overwhelming majority of our resources in solely looking for and investigating worlds that have these similarities to our own, life-rich planet?

  If science has taught us anything, it‘s that we shouldn‘t assume we know the answer before doing the key experiments or making the critical observations. The Universe is full of surprises, and if we don‘t give ourselves the opportunity to allow the Universe to surprise us, we‘re going to draw biased — and therefore, fundamentally unscientific — conclusions.

  While we have every reason to believe that life might be ubiquitous — or at least have a chance — on worlds that are very similar to Earth, it‘s also very plausible that life may be more plentiful on worlds that aren‘t like our own.

  Perhaps exomoons orbiting large planets are even more conducive to life originating than a world like Earth is.

  Perhaps liquid water on the planet itself isn‘t a requirement for life, as perhaps the right kind of cell wall or membrane can enable water to exist in an aqueous state.

  Perhaps radioisotope decay, geothermal sources, or even chemical sources of energy could provide life with the external source it needs; perhaps rogue planets — without parent stars at all — might be home to alien life.

  Perhaps even super-Earths might be potentially habitable under the right circumstances. To examine a planet for hints of life, we can approach this puzzle with many different lines of inquiry. We can:

  ·wait for a planetary transit and try to perform spectroscopy on the absorbed light, probing the contents of an exo-atmosphere,

  ·we can try and resolve the world itself with direct imaging, looking for seasonal variations and signs such as the periodic greening of the world,

  ·or we can look for nuclear, neutrino, or techno-signatures that might indicate the presence of a planet being manipulated by its inhabitants, whether they are intelligent or not.

  It may be the case that life is rare in the Universe, in which case it will require us to look at a lot of candidate planets in order to reveal a successful detection. But if we search exclusively for planets that have similar properties to Earth, and we restrict ourselves to looking at parent stars and solar systems that are similar to our own, we are doomed to get a biased representation of what‘s out there.

  You might think, in the search for extraterrestrial life, that more is more, and that the best way to find life beyond Earth is to look at greater numbers of candidate planets that might be the Earth 2.0 we‘ve been dreaming of for so long. But non-Earth-like planets could be home to life that we‘ve never considered, and we won‘t know unless we look. More is more, but “different” is also more. We must be careful, as scientists, not to bias our findings before we‘ve even truly started looking.

  當我們想到遠在地球邊界以外的宇宙中的生命時,我們免不了會把自己的星球作為參照。地球具有若幹我們認為對生命出現和蓬勃生長極其重要甚或必不可少的特徵。人類世世代代幻想著地球以外的生命,極力去發現與我們相似但有著獨特成功經曆的另一個世界:我們自己地球的2.0版。

  但是,僅僅因為生命在地球上繁衍不息未必意味著生命很可能會在類似地球的世界里繁衍不息,那隻是可能而已。同樣,僅僅因為在不同於地球的世界里尚未發現生命並不意味著那是不可能的。

  我們有諸多理由認為,在儘可能類似太陽的恒星周邊尋找儘可能類似地球的世界可能是在宇宙中其他地方尋找生命的最佳地點。系外行星學界或宇宙生物學界幾乎沒有人認為尋找類似所謂地球2.0版的世界是個餿主意。但是,投入我們的絕大部分資源只尋找和探索與我們自己這個生命豐富的星球具有這些相似之處的世界,那是最聰明的做法嗎?

  如果說科學教給了我們什麼,那就是,在我們做關鍵實驗或重大觀察之前,不應該以為我們知道答案。宇宙充滿驚奇,如果我們不給自己這個讓宇宙驚到我們的機會,我們就會得出片面——因此根本不科學——的結論。

  儘管我們有充分理由認為,生命在與地球非常相似的世界里可能無處不在,或者至少有機會存在,但在與我們自身不同的世界里,生命可能更豐富,這似乎也是有道理的。

  或許,圍繞大型行星運動的系外衛星甚至比地球這樣的世界更適合生命發源。

  或許,行星本身上面的液態水不是生命的必要條件,因為或許合適類型的細胞壁或細胞膜可以讓水以溶液狀態存在。

  或許,放射性同位素衰變、地熱源甚或化學能源能夠以外部源頭為生命提供其所需;或許,根本沒有母恒星的流浪行星可能是外星生命的家園。

  或許,就連超級類地行星在合適條件下也許都是可能適宜居住的。要在一顆行星上查找生命的跡象,我們可以從多條不同的調查線索解開這個謎題。我們可以:

  ·等待行星淩日,試著對被吸收的光做分光鏡勘測,探查外大氣層的成分;

  ·我們可以利用直接成像技術試著解析世界本身,探尋季節變化和諸如世界定期變綠等跡象;

  ·或者,我們可以探尋可能表明存在一顆被智能或非智能居民控製的行星的核、中微子或技術痕跡。

  也許宇宙中的生命很稀有,如果是這樣,就需要我們去審視許多候選行星,以獲得一次成功的發現。但是,如果我們只搜索與地球有著類似特點的行星,如果我們局限於審視與我們自身相似的母恒星和太陽系,那我們註定會對外空間的狀況做出片面陳述。

  你可能認為,在搜尋地外生命時,越多就越好,尋找地球以外生命的最佳方式是審視更多可能是我們夢寐已久的地球2.0版的候選行星。但是,與地球不同的行星可能是我們從未考慮過的生命的家園,我們不找一找是不會知曉的。越多就越好,但“越不同”也越好。作為科學家,我們必須當心,不要在我們還沒有真正開始找尋之前就讓我們的發現帶有片面性。(葛雪蕾譯自美國《福布斯》雙週刊網站6月14日文章)

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