【媒库文选】外星生命藏在地球2.0版以外吗?

2019-07-10 11:11   参考消息网  

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

Ethan Siegel 伊桑·西格尔

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.

当我们想到远在地球边界以外的宇宙中的生命时,我们免不了会把自己的星球作为参照。地球具有若干我们认为对生命出现和蓬勃生长极其重要甚或必不可少的特征。人类世世代代幻想着地球以外的生命,极力去发现与我们相似但有着独特成功经历的另一个世界:我们自己地球的2.0版。

但是,仅仅因为生命在地球上繁衍不息未必意味着生命很可能会在类似地球的世界里繁衍不息,那只是可能而已。同样,仅仅因为在不同于地球的世界里尚未发现生命并不意味着那是不可能的。

我们有诸多理由认为,在尽可能类似太阳的恒星周边寻找尽可能类似地球的世界可能是在宇宙中其他地方寻找生命的最佳地点。系外行星学界或宇宙生物学界几乎没有人认为寻找类似所谓地球2.0版的世界是个馊主意。但是,投入我们的绝大部分资源只寻找和探索与我们自己这个生命丰富的星球具有这些相似之处的世界,那是最聪明的做法吗?

如果说科学教给了我们什么,那就是,在我们做关键实验或重大观察之前,不应该以为我们知道答案。宇宙充满惊奇,如果我们不给自己这个让宇宙惊到我们的机会,我们就会得出片面——因此根本不科学——的结论。

尽管我们有充分理由认为,生命在与地球非常相似的世界里可能无处不在,或者至少有机会存在,但在与我们自身不同的世界里,生命可能更丰富,这似乎也是有道理的。

或许,围绕大型行星运动的系外卫星甚至比地球这样的世界更适合生命发源。

或许,行星本身上面的液态水不是生命的必要条件,因为或许合适类型的细胞壁或细胞膜可以让水以溶液状态存在。

或许,放射性同位素衰变、地热源甚或化学能源能够以外部源头为生命提供其所需;或许,根本没有母恒星的流浪行星可能是外星生命的家园。

或许,就连超级类地行星在合适条件下也许都是可能适宜居住的。要在一颗行星上查找生命的迹象,我们可以从多条不同的调查线索解开这个谜题。我们可以:

·等待行星凌日,试着对被吸收的光做分光镜勘测,探查外大气层的成分;

·我们可以利用直接成像技术试着解析世界本身,探寻季节变化和诸如世界定期变绿等迹象;

·或者,我们可以探寻可能表明存在一颗被智能或非智能居民控制的行星的核、中微子或技术痕迹。

也许宇宙中的生命很稀有,如果是这样,就需要我们去审视许多候选行星,以获得一次成功的发现。但是,如果我们只搜索与地球有着类似特点的行星,如果我们局限于审视与我们自身相似的母恒星和太阳系,那我们注定会对外空间的状况做出片面陈述。

你可能认为,在搜寻地外生命时,越多就越好,寻找地球以外生命的最佳方式是审视更多可能是我们梦寐已久的地球2.0版的候选行星。但是,与地球不同的行星可能是我们从未考虑过的生命的家园,我们不找一找是不会知晓的。越多就越好,但“越不同”也越好。作为科学家,我们必须当心,不要在我们还没有真正开始找寻之前就让我们的发现带有片面性。(葛雪蕾译自美国《福布斯》双周刊网站6月14日文章)

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