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Frank Wilczek

弗兰克·维尔切克是麻省理工学院物理学教授、量子色动力学的奠基人之一。因发现了量子色动力学的渐近自由现象，他在2004年获得了诺贝尔物理学奖。

作者 | Frank Wilczek

翻译 | 胡风、梁丁当

配音 | Betsy Devine

感谢Frank夫人Betsy Devine女士为本专栏配音！

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中文版

遥远星球上大气与温度的异常变化或许是存在外星生命的第一条线索。

通过系外行星探索，人们迄今已在太阳系外发现了4000多颗行星。对天文学家来说，探寻系外行星的最大挑战在于他们要找寻的并不是天空中最大、最亮或者最不寻常的那些星体。由于行星具有相对较小的质量和能量，它们比其环绕的恒星暗淡许多，所以很难辨认。但是天文学家们已经战胜了这个挑战。

目前系外行星天文学发展了很多新技术。这些技术将大大增加可搜索到的系外行星数目，并为我们提供它们的详细信息：不仅仅是它们的质量和大小，还包括它们的温度及大气成分。大气对于外星生命的探寻尤其重要，因为它可能会受到生化过程的影响--正是通过这种过程，地球生物的光合作用产生了地球大气中的几乎所有氧气。在天体生物学领域--即研究地球外生命存在的学科，寻找生命的迹象是一个核心课题。

相比之下，人们较少关注另一个更令人瞩目的可能性：外星科技。未来的系外行星探索能否发现太阳系以外先进技术的证据？这些证据又会以怎样的形式呈现？

如果一个行星上诞生了一种技术先进的文明，他们或许会想要通过殖民附近的其他星球来达到生存环境的多元化，就如埃隆·马斯克（Elon Musk）和杰夫·贝佐斯（Jeff Bezos）等大力倡导我们地球人所做的那样。为了让这些殖民星球适宜于大规模居住，可能需要重新改造它们的大气构成，使其与原来的星球相似。这些克隆系外行星会具有无法用其他原因解释的相似的大气，这或许是外星科技存在的一个迹象。

还有一种可能性是，外星文明会有意地利用温室效应来升高行星的温度。水蒸气和二氧化碳的积累会产生温室效应。失控的温室效应导致金星变成了地狱星球，也造成了众所周知的地球气候变暖。而一个先进的外星文明能够以一种可控的方式利用温室效应来升高冰冷星球的温度，使其能够支持液态水，从而有利于生命的形成。又或者，它可以改造大气来产生冷却效应。系外行星探索可能会发现这两种“非自然”形成的大气。

如果一颗系外行星呈现出异常的高温，这或许表明它正在使用人造能源。这可能是以核裂变或核聚变为动力的制造业造成的，也可能是因为存在类似于戴森球的构造。戴森球是以物理学家弗里曼·戴森命名的一个假想结构：即在太阳的外围制造一个由太阳能电池板组成的巨型装置，来截获恒星发出的大部分能量并将它传给行星。

搜寻地外文明计划（SETI）所采用的传统策略是寻找外星先进文明发射的信号。 但这是一种低效的策略，因为几乎所有潜在的信号都可能被我们的检测器漏掉。而系外天文学的发展为我们提供了另一种思路。想与外界联系的外星人可能通过其太阳系的异常来吸引系外行星天文学家的目光，从而有效地利用其母星来吸引注意力。这或许才是最实用的在宇宙中进行沟通的方法。

英文版

Unusual changes in the atmosphere and temperature of distant planets might be our first clue tothe existence of life beyond Earth.

The search for planets outside our own solar system has unearthed more than 4,000 of them sofar. The challenge in finding planets is that astronomers aren't looking for the biggest, brightest or most unusual objects in the sky, as they usually do. As sources of mass and energy planets are relatively insignificant, and they are much dimmer than the stars they orbit, making them hard to discern. But astronomers have risen to the challenge.

New techniques in exoplanetary astronomy now in development will vastly increase the count and give us more detailed information about the objects we discover-not just their masses and sizes but their temperatures and the chemical composition of their atmospheres. Atmospheres are especially significant in the search for alien life because they might be affected by biological processes, the way that photosynthesis on Earth produces nearly all of our planet's atmospheric oxygen. The search for signs of life is a core subject in astrobiology, the study of life beyond Earth.

But less consideration has been given to a more spectacular possibility: astrotechnology. Could future exoplanetary exploration turn up evidence of advanced technology outside our solar system? What would such evidence look like?

A technologically advanced civilization born on one planet might want to diversify its portfolio by colonizing other planets nearby. (Elon Musk and Jeff Bezos, among others, have advocated that we Earthlings get to work on it.) To make such colonies habitable on a large scale, it would probably be necessary to re-engineer the new planet's atmosphere to mimic the old one. Such clone exoplanets, with an otherwise inexplicable resemblance between their atmospheres, could be one signature of astrotechnology.

Another possibility is that an alien civilization might deliberately trigger a greenhouse effect to raise a planet's temperature. A runaway greenhouse effect caused by an accumulation of water vapor and carbon dioxide turned Venus into a hellish wasteland; notoriously, an accumulation of carbon dioxide is currently warming Earth's climate. An advanced alien civilization could use this technique in a more controlled fashion to raise the temperature of a cold planet, making it more favorable for life by allowing it to support liquid water. Alternatively, it might be possible to use atmospheric engineering to produce cooling effects. Exoplanetary exploration could reveal “unnatural” atmospheres of either kind.

Anomalously high temperatures on an exoplanet could also be a sign that it is using artificial energy sources. Manufacturing powered by nuclear fission or fusion could look like that. Closely related to this concept is the Dyson sphere, a hypothetical structure named after the physicist Freeman Dyson. It involves deploying solar panels in space, on a colossal scale, to trap a significant fraction of a star's energy output and beam that energy to planetary manufacturing centers.

The classic strategy in SETI, the Search for Extraterrestrial Intelligence, is to look for signals that an advanced technology might broadcast. But this is an inefficient strategy, since almost all of any potential signal would miss our detectors. The pursuit of exoplanetary astronomy suggests a different approach. An alien species that wants to communicate could draw the gaze of exoplanetary astronomers to anomalies in its solar system, effectively using its parent star to focus attention. This might turn out to be the most practical way to open communications across the universe.

关于“墨子沙龙”

墨子沙龙是由中国科学技术大学上海研究院主办、上海市浦东新区科学技术协会及中国科大新创校友基金会协办的公益性大型科普论坛。沙龙的科普对象为对科学有浓厚兴趣、热爱科普的普通民众，力图打造具有中学生学力便可以了解当下全球最尖端科学资讯的科普讲坛。