Capturing even a faint, blurry image of a distant world that looks something like our own would mark a profound shift in history – and our place in the universe.
The hunt for such a world has been described as a search for “Earth 2.0,” an “Earth-like” planet, even an “Earth twin.” But each of these terms implies its own raft of assumptions. We might, or might not, catch a glimpse of an exoplanet – a planet orbiting another star – that looks like present-day Earth. A blue, water-covered world marbled with clouds of white.
Yet even our own planet probably looked very different in the deep past.
The term “Earth-like” also carries some burdensome baggage. That begins with how we define it. Which characteristics, exactly, make Earth Earth-like? How would we recognize these qualities on a planet hundreds or thousands of light-years away?
“The kinds of planets that could be [considered] Earth-like may be very different from modern Earth,” said Giada Arney, an astronomer and astrobiologist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.
And suppose life elsewhere is not “life as we know it.” Would we recognize life as we don’t know it?
This all comes down to the one big question we’ve been asking ourselves through the ages: Are we alone?
We’ve taken a journey through NASA’s efforts to answer this question:
How life on Earth might have looked from a vast distance over billions of years, and how it might look somewhere else
As we search the heavens for a possible twin, we’ll have to take into account the stages of growth – infancy to maturity. A rocky planet with the beginnings of life could be shrouded in an orange haze, as Earth once might have been.
Timing is everything. Our own Earth was uninhabitable for millions of years, and we might find a planet at a similar stage – perhaps an unimaginably hot or cold surface that had a liquid water ocean in the past, or might develop one in the future.
“We tend to talk about Earth-like planets as planets like ours is today,” said Douglas Hudgins, program scientist for NASA’s Exoplanet Exploration Program at NASA Headquarters in Washington. “But our planet has been radically different throughout its history, while still supporting an abundance of life.”
Where we might find signs of life in our own solar system
Mars, once warm and wet and now cold, dry and forbidding, might reveal evidence of past life. Even present life can’t be ruled out.
A life-bearing world also might be covered in a crust of ice. Jupiter’s moon, Europa, and Saturn’s Enceladus, both hide subsurface oceans sealed inside icy shells.
Titan, another moon of Saturn, also is the only other solar system body with rain, rivers and lakes. The liquid in this case, however, is composed of methane and ethane instead of water, but could be home to “weird life.” Titan also might have a subterranean ocean. If other planets and moons in our solar system harbor life forms, it isn’t obvious – but the possibility is being actively investigated by NASA and other space agencies.
“The more we study our own cosmic backyard, the more surprises we find,” said Morgan Cable, a researcher with the Astrobiology and Ocean Worlds Group at NASA’s Jet Propulsion Laboratory in Southern California. “And I’m excited. We’ll be surprised more and more as we continue to extend our senses to the outer solar system and beyond.”
The sky full of exoplanets found so far, and how they compare to our own world
Scientists have confirmed more than 4,000 exoplanets in our galaxy, many of them likely rocky worlds in Earth’s size-range. Thousands more are expected to be confirmed in the years ahead. But one very special type of planet still eludes us: a world in Earth’s size range orbiting a Sun-like star, at a distance that would give it a year comparable to our own.
While it’s possible that such planets will turn out to be rare, another factor better explains the mystery. Present-day technology used in the hunt for exoplanets, primarily space telescopes and the instruments they carry, has a tough time resolving such systems.
即使捕捉到一个遥远世界的模糊图像，看起来也像我们自己的世界，这将标志着历史的深刻转变——以及我们在宇宙中的地位。
寻找这样一个世界被描述为寻找“地球2.0”，一个“类似地球”的行星，甚至是一个“地球双胞胎”但这些术语中的每一个都隐含着自己的大量假设。我们可能会，也可能不会，瞥见一颗系外行星——一颗围绕另一颗恒星运行的行星——看起来像今天的地球。一个蓝色的、被水覆盖的世界，布满了白色的云彩。
然而，即使是我们自己的星球在过去也可能看起来非常不同。
“类地球”一词也带来了一些负担。这从我们如何定义它开始。到底是什么特征使地球像地球？在数百或数千光年之外的星球上，我们如何识别这些品质？
位于马里兰州绿带的NASA戈达德太空飞行中心的天文学家和天体生物学家贾达·阿尼说：“可以被认为是类似地球的行星可能与现代地球有很大不同。”。
假设其他地方的生活不是“我们所知道的生活”我们会像不认识生命一样认识它吗？
这一切都归结为我们多年来一直在问自己的一个大问题：我们是孤独的吗？
我们通过NASA的努力来回答这个问题：
数十亿年来，地球上的生命从遥远的地方看起来是什么样子，以及它在其他地方可能是什么样子
当我们在天堂寻找一对可能的双胞胎时，我们必须考虑到成长的各个阶段——从婴儿期到成熟期。一个有生命起源的岩石行星可能会像地球曾经那样被橙色的薄雾笼罩。
时间就是一切。我们自己的地球在数百万年内都不适合居住，我们可能会发现一颗处于类似阶段的行星——可能是一个无法想象的热或冷的表面，过去有一个液态海洋，或者未来可能会形成一个液态海洋。
华盛顿NASA总部NASA系外行星探索项目的项目科学家道格拉斯·哈金斯（Douglas Hudgins）说：“我们倾向于谈论类地行星，就像今天的类地行星一样。”。“但我们的星球在其整个历史上都有着根本的不同，同时仍然维持着丰富的生命。”
在那里我们可以在我们自己的太阳系中找到生命的迹象
火星曾经温暖潮湿，现在又冷又干，令人望而生畏，它可能会揭示过去生命的证据。即使是现在的生活也不能排除。
一个有生命的世界也可能被冰层覆盖。木星的卫星欧罗巴和土星的土卫二都隐藏着密封在冰壳中的地下海洋。
土卫六是土星的另一颗卫星，也是太阳系中唯一有雨水、河流和湖泊的天体。然而，这种液体由甲烷和乙烷组成，而不是水，但可能是“奇怪生命”的家园泰坦也可能有一个地下海洋。如果我们太阳系中的其他行星和卫星存在生命形式，这一点并不明显——但NASA和其他航天机构正在积极调查这种可能性。
“我们对自己的宇宙后院研究得越多，发现的惊喜就越多，”南加州NASA喷气推进实验室天体生物学和海洋世界小组的研究员摩根·凯布尔说。“我很兴奋。当我们继续将我们的感官延伸到外太阳系和更远的地方时，我们会越来越惊讶。”
到目前为止，天空中到处都是发现的系外行星，它们与我们自己的世界相比如何
科学家已经确认银河系中有4000多颗系外行星，其中许多可能是地球大小范围内的岩石行星。预计未来几年还会有数千人得到确认。但有一种非常特殊的行星仍在我们的视线之外：一个与地球大小相当的世界，围绕着一颗类似太阳的恒星运行，距离相当于我们的一年。
虽然这类行星可能会变得非常罕见，但另一个因素可以更好地解释这个谜团。目前用于搜寻系外行星的技术，主要是太空望远镜及其携带的仪器，很难解决此类系统。
Future, more powerful space telescopes could help bridge this gap, or peer into the atmospheres of Earth-sized worlds that have been found in abundance – those in close orbits around red-dwarf stars – to look for signs of habitability.
“The day we detect life on an exoplanet will be nothing short of a Copernican revolution,” Hudgins said. “It will changes the way human beings view our place in the universe forever.”
The chances of finding life elsewhere
We could say that the chances of finding life somewhere else in the galaxy are improving. While scientists have confirmed thousands of exoplanets so far, the Milky Way likely holds trillions. A good percentage of these exoplanets are in Earth’s size range, and believed to be of similar composition.
Yet the cosmos is stubbornly silent on the question. Exoplanet-hunting technology, though developing rapidly, probably is not yet sufficient to detect signs of possible life in exoplanet atmospheres.
We’ve seen or heard no credible indications of a technological species among the sea of stars; a question more than half a century old, “Where is everybody?” still has no answer.
Concepts like the “habitable zone” – the orbital distance from a star allowing a planet, with a suitable atmosphere, to retain liquid water on its surface – are helping astronomers sort through the many possibilities to find likelier candidates for life-bearing worlds. Still, planets with life could be far outside this zone if there were, for example, an ice-covered planet with a deep ocean supporting aquatic organisms.
未来，更强大的太空望远镜可能有助于弥合这一差距，或者观察大量发现的地球大小的星球的大气层——那些在红矮星附近轨道上的星球——以寻找宜居的迹象。
哈金斯说：“我们在系外行星上发现生命的那一天，简直就是哥白尼革命。”。“它将永远改变人类看待我们在宇宙中的地位的方式。”
在别处找到生命的机会
我们可以说，在银河系其他地方发现生命的机会正在增加。虽然科学家迄今已确认了数千颗系外行星，但银河系可能拥有数万亿颗系外行星。这些系外行星中有很大一部分在地球的大小范围内，并且被认为具有相似的组成。
然而，宇宙在这个问题上却固执地保持沉默。外行星搜寻技术虽然发展迅速，但可能还不足以探测外行星大气中可能存在生命的迹象。
我们在星海中没有看到或听到任何可靠的迹象表明有一种技术物种；一个半个多世纪前的问题，“每个人都在哪里？”仍然没有答案。
像“可居住区”这样的概念——即与恒星的轨道距离，允许一个具有合适大气层的行星在其表面保留液态水——正在帮助天文学家梳理各种可能性，以找到更可能的生命世界候选。尽管如此，如果有一颗被冰覆盖的行星，有着支持水生生物的深海，那么有生命的行星可能会远离这个区域。
“To search for life anywhere, we have this ‘follow the water’ approach,” said Shawn Domagal-Goldman, a research astronomer at Goddard. “Anywhere you find water on Earth, you find life. Whether it’s life on Mars, ocean worlds, or exoplanets, water is the first signpost we’re looking for.”
Technology now under development that will allow us to peer into exoplanet atmospheres to look for signs of life
We’re on the brink of a new era in exoplanet science: sifting through the atmospheres of distant worlds to look for combinations of gases that could reveal a living planet.
First in line is the James Webb Space Telescope, targeted for launch in October 2021. The Webb telescope will be a cosmic multi-tasker, looking deep into the universe – and deep into its past – to discover clues to its origin and early formation.
The Webb telescope also will capture starlight shining through the atmospheres of exoplanets, which provides a kind of profile of the gases present. That will pave the way for future, more powerful space telescopes to look in on small, rocky planets perhaps resembling our own.
戈达德的研究天文学家肖恩·多玛加尔·戈德曼说：“为了在任何地方寻找生命，我们都有这种‘跟着水走’的方法。”。“地球上任何地方只要有水，就有生命。无论是火星上的生命、海洋世界还是系外行星，水都是我们寻找的第一个路标。”
目前正在开发的技术将允许我们观察系外行星的大气层，寻找生命迹象
我们正处在系外行星科学的一个新时代的边缘：在遥远世界的大气层中进行筛选，寻找可能揭示一个活行星的气体组合。
排在第一位的是詹姆斯·韦伯太空望远镜，计划于2021发射。韦伯望远镜将是一个宇宙多任务望远镜，深入研究宇宙——以及它的过去——以发现其起源和早期形成的线索。
韦伯望远镜还将捕捉穿过系外行星大气层的星光，从而提供现有气体的一种轮廓。这将为未来更强大的太空望远镜观测可能与我们类似的岩石小行星铺平道路。
目前正在开发的技术可以揭示系外行星大气中可能存在生命的迹象——生物信号。
With more advanced technology, astronomers could detect atmospheric chemicals that are considered “biosignatures,” potentially indicating the influence of lifeforms.
This new era of characterizing exoplanets will continue with the launch of the Nancy Grace Roman telescope in the mid-2020s. An intricate instrument onboard called a coronagraph will help blot out the glare of parent stars to reveal orbiting planets.
That will mean direct images of large, gaseous planets. Those targets are unlikely to be habitable, but demonstrating this technology will open the door to future such instruments with greater resolving power. A future telescope might even find a small, rocky world with an atmosphere of oxygen, methane, and carbon dioxide – in other words, an atmosphere that reminds us of home.
“We could actually find out: Is Earth common or rare? Is life common or rare?” said Aki Roberge, an astronomer at Goddard. “We honestly have zero clue what habitable or inhabited planets look like in general. We really need to just take a look.”
借助更先进的技术，天文学家可以探测到被认为是“生物信号”的大气化学物质，这可能表明生命形式的影响。
随着南希·格雷斯·罗马望远镜在20世纪20年代中期的发射，这一表征系外行星的新时代将继续下去。船上一个叫做日冕仪的复杂仪器将有助于掩盖母星的眩光，从而揭示绕轨道运行的行星。
这将意味着直接拍摄大型气态行星的图像。这些目标不太可能适合居住，但展示这项技术将为未来更高分辨率的此类仪器打开大门。未来的望远镜甚至可能发现一个由氧气、甲烷和二氧化碳组成的岩石小世界——换句话说，一个让我们想起家的大气层。
“我们实际上可以发现：地球是普通的还是稀有的？生命是普通的还是稀有的？”戈达德的天文学家阿基·罗伯奇说。“老实说，我们不知道可居住或有人居住的行星总体上是什么样子。我们真的需要看看。”
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