## 寻找下一个地球

2015.08.01 by kingcos
Preface · 序

Distant Planets 遥远的行星 July 23, 2015 2015 年 7 月 23 日

# 寻找下一个地球

A newly discovered exoplanet, Kepler-452b, comes the closest of any found so far to matching our Earth-sun system. This artist’s conception of a planetary lineup shows habitable-zone planets with similarities to Earth: from left, Kepler-22b, Kepler-69c, the just announced Kepler-452b, Kepler-62f and Kepler-186f. Last in line is Earth itself. Credits: NASA/Ames/JPL-Caltech

Of the 1,030 confirmed planets from Kepler, a dozen are less than twice the size of Earth and reside in the habitable zone of their host stars. In this diagram, the sizes of the exoplanets are represented by the size of each sphere. These are arranged by size from left to right, and by the type of star they orbit, from the M stars that are significantly cooler and smaller than the sun, to the K stars that are somewhat cooler and smaller than the sun, to the G stars that include the sun. The sizes of the planets are enlarged by 25 times compared to the stars. The Earth is shown for reference. Credits: NASA/Ames/JPL-Caltech

The discovery of a super-Earth-sized planet orbiting a sun-like star brings us closer than ever to finding a twin of our own watery world. But NASA’s Kepler space telescope has captured evidence of other potentially habitable planets amid the sea of stars in the Milky Way galaxy. 运行轨道围绕类日（sun-like恒星的超级地球（super-Earth-sized行星的发现使我们更加易于寻找类似我们自己赖以生存的淡水世界。NASA 的开普勒太空望远镜（Kepler space telescope已经在星罗云布的银河系中（Milky Way galaxy找到了其它潜在的宜居行星（存在）的证据。

To take a brief tour of the more prominent contenders, it helps to zero in on the “habitable zone” around their stars. This is the band of congenial temperatures for planetary orbits – not too close and not too far. Too close and the planet is fried (we’re looking at you, Venus). Too far and it’s in deep freeze. But settle comfortably into the habitable zone, and your planet could have liquid water on its surface – just right. Goldilocks has never been more relevant. Scientists have, in fact, taken to calling this water-friendly region the “Goldilocks zone.” 为了简单叙述更加突出的（地球）竞争者，（让我们）把聚焦于它们恒星的“宜居带”（habitable zone）。这是具有适合人类生存所需温度的行星公转轨道——不太近也不太远（公转轨道半径恰好适中）。太近（公转轨道半径太小）行星将被炙烤（我们说的就是金星（Venus）。太远（公转轨道半径太大）却又会是一个冰冻星球（译者注：这里的冰，并不单一指 H2O 的固态形式，例如：甲烷等一些气体在温度低于熔点也可凝结成冰，地球上也天然存在这样的冰例如可燃冰（天然气水合物））。但恰好位于宜居带（habitable zone）的行星表面将会有液态水——恰如其分。Goldilocks（译者注：Goldilocks 原意：金发姑娘）与此并没有关系。但实际上，科学家已开始将水以液态存在的（空间）区域称为古迪洛克区（Goldilocks zone译者注：也称“适居带”）。

The zone can be a wide band or a narrow one, and nearer the star or farther, depending on the star’s size and energy output. For small, red-dwarf stars, habitable zone planets might gather close, like marshmallow-roasting campers around the fire. For gigantic, hot stars, the band must retreat to a safer distance. 这个区域可能很宽也可能很拥挤（译者注：这是指此类似环形区域的内外圆半径之差），紧邻恒星又或远离恒星（译者注：这是指此类似环形区域的内圆半径长度），这些是由恒星的体积大小和能量输出来决定的。对于体积小的红矮星（red-dwarf stars来说，其处在宜居带（habitable zone）的行星彼此更加靠近，就像围在焰火旁烤棉花糖（译者注：marshmallow-roasting 扩展：棉花糖－烧烤，这可能是美国人在野外烧烤的一道佳肴）吃的野营者。对于巨大，炽热的恒星，它的这个区域必须后撤到一个更加安全的距离。

About a dozen habitable zone planets in the Earth-size ballpark have been discovered so far – that is, 10 to 15 planets between one-half and twice the diameter of Earth, depending on how the habitable zone is defined and allowing for uncertainties about some of the planetary sizes. 迄今为止有大约 12 个类地（Earth-size大小且位于宜居带（habitable zone）的行星被发现——即有 10 到 15 个行星直径为地球的 1.5 到 2 倍之间，这取决于宜居带（habitable zone）是如何定义并宽容一些不规则大小的行星。

The new discovery, Kepler-452b, fires the planet hunter’s imagination because it is the most similar to the Earth-sun system found yet: a planet at the right temperature within the habitable zone, and only about one-and-a-half times the diameter of Earth, circling a star very much like our own sun. The planet also has a good chance of being rocky, like Earth, its discoverers say. Kepler-452b 的最新发现，激发了行星探索者们的想象，因为这是迄今为止发现的最类似于我们的日地系统（Earth-sun system）的星球：一个位于宜居带（habitable zone）、温度适宜、且直径仅为地球的 1.5 倍的星球，还围绕着一颗极为相似于太阳的恒星。同样，它有很大可能是岩质行星（rocky ／terrestrial planet），像地球一样，它的发现者说。

Kepler-452b is more similar to Earth than any system previously discovered. And the timing is especially fitting: 2015 marks the 20th anniversary of the first exoplanet confirmed to be in orbit around a typical star. Kepler-452b 比以往发现的任何星体系统都更类似于地球。而且（发现的）时机尤其适合，2015 年是（人类）发现首颗围绕一个典型恒星运转的系（译者注：太阳系）外行星（exoplanet被确认的 20 周年纪念日。

But several other exoplanet discoveries came nearly as close in their similarity to Earth. 但是（也有）许多其它系（译者注：太阳系）外行星（exoplanet在个别方面类似于地球。

Before this, the planet Kepler-186f held the “most similar” distinction (they get the common moniker, “Kepler,” because they were discovered with the Kepler space telescope). About 500 light-years from Earth, Kepler-186f is no more than 10 percent larger than Earth, and sails through its star’s habitable zone, making its surface potentially watery. 在此之前，行星 Kepler-186f 是“最为相似（于地球）”的星球（它们的名字拥有相同的前缀，“Kepler”，因为它们均为开普勒太空望远镜（Kepler space telescope所发现的）。Kepler-186f 距离地球 500 光年（译者注：light-years 扩展：光年意思基本可以理解为：光传播一年的距离，即光速乘以相应单位年，这个名词是表示距离。在宇宙中（依据大爆炸假说），空间在起点急剧膨胀，在膨胀了约百亿多年后，形成了今天这样的宇宙，天体之间距离很远，便采取光传播一年的距离作为衡量天体距离的单位。，（体积）不超过地球的 1.1 倍，处在其恒星的宜居带（habitable zone）使其表面可能有水。

But its 130-day orbit carries it around a red-dwarf star that is much cooler than our sun and only half its size. Thus, the planet is really more like an “Earth cousin,” says Thomas Barclay of the Bay Area Environmental Research Institute at NASA’s Ames Research Center, Moffett Field, California, a co-author of the paper announcing the discovery in April 2014. 但它的公转周期为 130 天，其所围绕的红矮星（red-dwarf stars比太阳的温度要低得多，并且大小只有太阳的一半。因此，这个行星确实十分像“地球的姊妹”，工作在位于墨菲特联邦机场（Moffett Field／Moffett Federal AirfieldNASA 阿姆斯研究中心（NASA’s Ames Research Center中的海湾地区环境研究所（Bay Area Environmental Research Institute宣布此发现的 2014 年 4 月论文的共同作者之一的 Thomas Barclay 说到。

Kepler-186f gets about one-third the energy from its star that Earth gets from our sun. And that puts it just at the outside edge of the habitable zone. Scientists say that if you were standing on the planet at noon, the light would look about as bright as it does on Earth an hour before sunset. Kepler-186f 获得其恒星的能量约为地球获得太阳能量的 1/3。并且，它位于宜居带（habitable zone）的外缘。科学家称，如果中午时刻站在那个星球上，看到其恒星的亮度将会和地球上日落前一个小时的太阳亮度相当。

That doesn’t mean the planet is bereft of life, although it doesn’t mean life exists there, either. 这并不意味着这颗星球完全不能够居住，同样也不意味着那里没有生命。

Before Kepler-186f, Kepler-62f was the exoplanet known to be most similar to Earth. Like the new discovery, Kepler-62f is a “super Earth,” about 40 percent larger than our home planet. But, like Kepler-186f, its 267-day orbit also carries it around a star that is cooler and smaller than the sun, some 1,200 light-years away in the constellation Lyra. Still, Kepler-62f does reside in the habitable zone. 在 Kepler-186f 之前，Kepler-62f 是当时已知最类似地球的行星。就像新的发现，Kepler-62f 是一个“超级地球”，体积大约是我们地球妈妈的 1.4 倍。但是，如 Kepler-186f 一样，Kepler-62f 公转周期为 267 天，其所围绕的恒星温度同样要比太阳低得多，体积也比太阳小得多，在距离我们约 1200 光年的天琴座（constellation Lyra中。尽管如此，Kepler-62f 确实仍位于宜居带（habitable zone）。

Kepler-62f’s discovery was announced in April 2013, about the same time as Kepler-69c, another super Earth – though one that is 70 percent larger than our home planet. That’s the bad news; astronomers are uncertain about the planet’s composition, or just when a “super Earth” becomes so large that it diminishes the chance of finding life on its surface. That also moves it farther than its competitors from the realm of a potential Earth twin. The good news is that Kepler-69c lies in its sun’s habitable zone, with a 242-day orbit reminiscent of our charbroiled sister planet, Venus. Its star is also similar to ours in size with about 80 percent of the sun’s luminosity. Its planetary system is about 2,700 light-years away in the constellation Cygnus. Kepler-62f 在 2013 年 4 月被宣布发现，大约在同一时间，Kepler-69c——另一个超级地球被发现——虽然其大小约为地球的 1.7 倍。这不是个好消息；天文学家不确定这个星球的组成，而且当这个“超级地球”体积如此巨大，其表面生命存在可能性便大大降低。因此将它排在潜在地球孪生姐妹的名单中更加靠后的位置。好消息是，Kepler-69c 位于其恒星的宜居带（habitable zone），242 天的公转周期让我们联想起地球的姐妹星，金星（Venus。它的恒星大小与太阳相似，亮度约为太阳的 80%。它的行星系统在距离我们约 2700 光年的天鹅座（constellation Cygnus

Kepler-22b also was hailed in its day as the most like Earth. It was the first of the Kepler planets to be found within the habitable zone, and it orbits a star much like our sun. But Kepler-22b is a sumo wrestler among super Earths, about 2.4 times Earth’s size. And no one knows if it is rocky, gaseous or liquid. The planet was detected almost immediately after Kepler began making observations in 2009, and was confirmed in 2011. This planet, which could have a cloudy atmosphere, is 600 light-years away, with a 290-day orbit not unlike Earth’s. Kepler-22b 同样在当时被认为最像地球。它是被找到的第一个位于宜居带（habitable zone）的开普勒星球，并且其公转所围绕的恒星十分类似于我们的太阳。但是 Kepler-22b超级地球中身材类似于相扑选手，约为地球体积的 2.4 倍。并且没有人知道它是岩质，气态还是液态的星球。这颗行星在 2009 年开普勒太空望远镜（译者注：简称：Kepler开始进行观测时就几乎被立刻发现，并在 2011 年确认存在。这个星球，拥有多云的大气层，距离我们 600 光年，并且公转周期 290 天不同于地球。

Not all the planets jostling to be most like Earth were discovered using Kepler. A super Earth known as Gliese 667Cc also came to light in 2011, discovered by astronomers combing through data from the European Southern Observatory’s 3.6-meter telescope in Chile. The planet, only 22 light-years away, has a mass at least 4.5 times that of Earth. It orbits a red dwarf in the habitable zone, though closely enough – with a mere 28-day orbit – to make the planet subject to intense flares that could erupt periodically from the star’s surface. Still, its sun is smaller and cooler than ours, and Gliese 667Cc’s orbital distance means it probably receives around 90 percent of the energy we get from the sun. That’s a point in favor of life, if the planet’s atmosphere is something like ours. The planet’s true size and density remain unknown, however, which means it could still turn out to be a gas planet, hostile to life as we know it. And powerful magnetic fluxes also could mean periodic drop-offs in the amount of energy reaching the planet, by as much as 40 percent. These drop-offs could last for months, according to scientists at the University of Oslo’s Institute of Theoretical Astrophysics in Norway. 当然，并不只是由开普勒太空望远镜（译者注：简称：Kepler所观测到的所有行星都争抢着最类似于地球。在 2011 年，一个命名为 Gliese 667Cc超级地球同样被曝光，这是一位天文学家凭借坐落于智利的欧洲南方天文台（European Southern Observatory口径 3.6 米的望远镜观察后并梳理数据得来的。这颗行星，距离我们只有 22 光年，质量约为地球的 4.5 倍。它位于宜居带（habitable zone）并围绕着一颗红矮星（red dwarf公转，而且足够紧邻——公转周期 28 天，使得这颗星球遭受来自其恒星表面周期性的强烈耀斑（flare。尽管如此，其恒星比我们的太阳体积更小，温度更低，并且 Gliese 667Cc的公转轨道距离意味着它将可能接收到其恒星约 90% 的能量。这颗行星的大气层是否类似于地球将是生命是否存在的关键。这颗行星真实的大小和密度仍然未知，这意味着它仍有可能是一个气态行星（gas planet／giant，如我们所知一样不适宜生命存在。并且强大的磁场（magnetic fluxes可能导致到达此行星的能量将周期性衰减 40%。这些衰减将会持续数月，据位于挪威的奥斯陆大学（University of Oslo理论物理研究所（Institute of Theoretical Astrophysics所说。

Deduct two points. 推论两点。

Too big, too uncertain, or circling the wrong kind of star: Shuffle through the catalog of habitable zone planets, and the closest we can come to Earth – at least so far – appears to be the new kid on the interstellar block, Kepler-452b. （体积）太大，（性质）太不确定，或者围绕着一颗错误类型的恒星：遍历处在宜居带（habitable zone）的所有行星，并且要最接近地球——至少目前为止——一个新的行星出现在星际区域，即 Kepler-452b。

NASA’s Ames Research Center in Moffett Field, California, manages the Kepler and K2 missions for NASA’s Science Mission Directorate. NASA’s Jet Propulsion Laboratory in Pasadena, California, managed Kepler mission development. Ball Aerospace & Technologies Corp. operates the flight system with support from the Laboratory for Atmospheric and Space Physics at the University of Colorado in Boulder. 位于加利福尼亚州（California墨菲特联邦机场（Moffett Field／Moffett Federal AirfieldNASA 阿姆斯研究中心（NASA’s Ames Research Center，为 NASA 科学任务部门（NASA's Science Mission Directorate担任开普勒（计划）和 K2 任务（译者注：详见参考资料 3）。位于加利福尼亚州（California）帕萨迪纳市（Pasadena）的 NASA 喷气推进实验室（译者注：Jet Propulsion Laboratory 简称：JPL管理开普勒计划的发展。鲍尔航天科技公司（Ball Aerospace & Technologies Corp.在位于科罗拉多大学波德分校（University of Colorado (in) Boulder大气和空间物理实验室（Laboratory for Atmospheric and Space Physics提供的支持下管理飞行系统。

JPL is managed by The California Institute of Technology for NASA. 喷气推进实验室（JPL／Jet Propulsion LaboratoryThe California Institute of Technology（译者注：加州理工大学 为 NASA 管理。

A related news release about Kepler’s latest planetary find is online at: 在线获取近期相关 Kepler 所发现的最新行星的新闻请访问： http://www.nasa.gov/press-release/nasa-kepler-mission-discovers-bigger-older-cousin-to-earth

Written by Pat Brennan, PlanetQuest 作者：Pat Brennan, PlanetQuest Whitney Clavin Jet Propulsion Laboratory, Pasadena, Calif. 818-354-4673 whitney.clavin@jpl.nasa.gov

2015-244

Last Updated: Aug. 1, 2015 网页最后更新于：2015-08-01 （译者注：此非原文的最后更新时间，而是本人翻译时打开此网页的最后时间。） Editor: Tony Greicius 编辑：Tony Greicius

1. NASA 官网