下个地球在哪里？ Where is the Next Earth?
当你仰望夜空时，你有没有想过宇宙中还有像我们一样的生物吗?
甚至另外一个像地球一样的地方? 我想过。
When looking up the night sky, do you ever wonder if there are others like us in the universe?
or even places like the Earth somewhere out there?
I certainly do.
然而，远在我们之前，古希腊古典时期的哲学家们就已经有了两种说法: 亚里士多德(公元前384-322)说“不能有多于一个的世界”，而伊壁鸠鲁(公元前460-370)则有相反的观点“有无限的世界，既像我们的这个世界，也不像我们的这个世界”。
However, long before us, philosophers in the classical period of ancient Greece had two theories: Aristotle (384-322 B.C.) said “There cannot be more worlds than one”, while Epicurus’s (~460-370 B.C.) had an opposite view of “There are infinite worlds both like and unlike this world of ours”.
图为夜晚的星空 The stary night sky
千年后的新发现
Discoveris After Millenniums
2000多年后的今天，感谢科学家们所努力的工作，我们现在知道伊壁鸠鲁的观点更接近真相。就在几个月前，来自美国宇航 (NASA)的Ames研究中心的Steve Bryson和他的团队在人类寻找“另一个地球”的路上又向前迈进了一步。
Today, over 2000 years after those predications, thanks to all the work done by scientists, we now know Epicurus was much closer to the truth. And just a few months ago, Steve Bryson and his team, from the NASA’s Ames Research Centre, has taken us one closer step to finding ‘another Earth’.
利用开普勒太空望远镜和盖亚任务提供的数据，发现在我们的银河系中，可能在大约一半的类太阳恒星的宜居带(HZ)内拥有岩石状、可能宜居的行星。
Using the data form the Kepler Space Telescope and Gaia mission, it was found that around half of Sun-like stars in our Milky Way might host rocky, potentially habitable planets within their habitable zones (HZ).
宜居带是指距离主恒星的特定距离，在那里水会以液态水池的形式存在于行星的表面。
Habitable zone is the range of distances from the host star, where liquid water would exist as pools on the surface of the orbiting planet.
如果行星离恒星太近，水就会变热便蒸发，以气体的形式存在。另一方面，如果离恒星太远，水就会因为低温而结冰。因此，水的状态是确定系外行星是否适合居住的关键因素。
If the planet is too close to the star, the water would be to hot and evaporate, hence only existing as gas. On the other hand, if its too far away from the star, the water would freeze due to low temperature. Therefore, the state of water is a key factor in identifying if the exoplanet is habitable.
图为一颗接近地球大小的系外行星 - Kepler-452b, 他是第一颗被发现的在宜居带的系外行星 An illustration of Kepler-452b, an exoplanet near the size of Earth, it was the first exoplanet to be discovered in the habitable zone
工作历程
The Work Done
NASA的开普勒太空望远镜于2009年首次发射，旨在探索系外行星（太阳系之外的行星）的一部分。其中它的主要目标是找到在或靠近主恒星的宜居带的岩石行星，并使用数据，分析存在这种行星的存在频率。
NASA (The National Aeronautics and Space Administration)’s Kepler Space Telescope was first launched in 2009, set to explore a portion of the Milk Way for exoplanets, which are planets out of our solar system. One if it’s primary objective to find rocky planets in or near habitable zone of their host star and using the data to find the existence frequency of possible stars hosting such planets.
在探测期间，它从观测到的530,506颗恒星中发现了2393颗已确认的系外行星。
During its time, it detected 2393 confirmed exoplanets from the 530,506 stars it observed.
开普勒的探索方法是在太阳轨道上用高灵敏度的光度计，连续观测15万颗类似太阳的主序矮星(与太阳年龄和大小相似的恒星)。然后通过凌日法来识别行星。
Kepler’s method was to continuously observe >150,000 solar-like main sequency dwarf stars (stars similar to the age and size of the Sun) with a highly sensitive photometer in the Solar orbit. Planets would then be identified through the transit method.
凌日现象的探测主要监测当一颗系外行星从地球和主星之间经过时恒星亮度的下降程度。我们可以想象，由于检测的恒星都非常遥远，亮度变化会很小，所以对检测仪的要求也会相当高。
Detection of transits is all about monitoring the dips of brightness of the stars when an exoplanet travel passes between the Earth and that host Star. As we can imagen, since the stars are all so far away, the changes are pretty small.
对于像木星这样的气态巨行星，亮度会下降约1%，而对于像地球这样的小型岩石行星，亮度变化就更小了，只会下降0.1%。
For gas giants like Jupiter, brightness decreases by ~1% and for smaller rocky ones like the Earth, it only a 0.1% change.
开普勒望远镜经过9年半的工作，终于在燃料耗尽后于2018年退役，但在工作期间，当时一半以上的已知恒星都是它探测到的，所以退役后科学家们依然从它的数据中不断做出新发现。
After nine and half year of work, Kepler finally retired in 2018 after running out of fuel, but with its detection more than half of the planets known at the time, its retirement does not stop scientist keeping making new discoveries from its data.
当然，虽然开普勒退役了，盖亚任务仍在欧洲航天局(ESA)的管理下进行着。与开普勒类似，它的任务也是观察银河系，但任务更为雄心勃勃。它的目标是绘制我们星系的3D地图，还准备推算出银河系的演化、组成和形成过程。
The Gaia mission, however, is still on going and managed by ESA (European Space Agency). Similar to Kepler, its mission is also having a look at the Milky Way but more ambitiously. It plans to chart a 3D map of our galaxy. It is also going to reveal the evolution, composition and formation of our galaxy.
图为EAS 发布的盖亚的第一张星空图 Gaia's first sky map from EAS
对类地行星的探测的重要性不止于发现一个未来合适移居的地方；同时，这也是在银河系中发现文明的一条重要线索。在德雷克方程中，每颗恒星有多少能够有生命的行星也是相当重要的。德雷克方程是一个用来估算银河系内的有多少能与人类交流的文明的数量的一个公式。虽然但是其中没有一项变量是准确知晓的，大多数都是基于观测，但这次NASA的发现至少能让这个等式离成功更进一步。
The detection of Earth-like planets is not only important for us to see where a suitable place might be to move to one day; but also, an indication for possibility of detecting civilization in the Milky Way. So, the number of planets that could support life per star with planets plays an important role in the Drake equation. It is a function to estimate the number of civilizations human could communicate to within the Milky Way. However, none of its terms are known certainty yet, mostly of them are based on observations and this recent discovery by NASA could potentially improve the equation.
图为德雷克方程中的变量 Various terms in the Drake equation
新算法的新成果
New Results Done the New Ways
在比较了开普勒和盖亚的数据后，Steve Bryson的团队进行了详细的分析，为目前探测到的系外行星和恒星绘制了一幅更完整的蓝图。以前，在估计类地球的行星的出现率时，只考虑它与主恒星的距离。
After comparing the data from both Kepler and Gaia, the team of Steve Bryson performed a detailed analysis, painting a better picture of current detection of exoplanets and stars. Previously, when estimating Earth-like planets’ occurrence rate, only its distance from the host star was considered.
但是他们的新分析中，“恒星日晒通量”，指即从其宿主恒星到行星上的能量总量，也首次被考虑在内。来自盖亚的恒星光度和有效温度数据在他们的分析中也有着重要影响。
However, with their new analysis, ‘instellation flux’ was also considered for the first time, which is the amount of energy falling on the planet from its hosting star. The luminosity and effective temperature data from Gaia were especially important here.
在一系列的模拟和算法中，Steve Bryson的团队寻找和分析的行星有两个标准：半径为地球0.5到1.5倍，以及行星的住宿恒星相似于太阳的年龄和温度(4500-6000摄氏度)。*团队还考虑了两种情况，使用了‘保守’和‘乐观’的内部和外部边界。
Using a range of models and methods, the analysis was limited to exoplanets being 0.5 to 1.5 Earth’s radius and having a host stars similar in age and temperature as the Sun (4500-6000 Celsius).* The team also considered two scenarios, using the conservative and optimistic inner and outer boundaries.
HZ的‘保守’边界是由‘失控温室’和‘最大温室’的限制定义的，而‘乐观’的边界是‘近期金星’和‘早期火星’限制。因此，理论上在‘保守’限制范围内的环境更接近于地球环境。
The conservative edges of the HZ are defined by the ‘runaway greenhouse’ and ‘maximum greenhouse’ limits, while the optimistic boundaries are the ‘recent Venus’ and ‘early Mars’ limits. So, in theory the conservative HZ would be more similar to the Earth’s environment.
根据他们的分析，以及上述标准，观察结果是：在‘保守’限制里，每颗恒星拥有的类地行星的平均数量为0.37 – 0.60，在‘乐观’限制里为0.58 – 0.88。*这意味着，即使用来更严格的‘保守’限制标准，银河系中也大概率有3亿甚至更多颗潜在地球。他们的结果还显示，其中很有可能至少有4颗‘地球’距离我们只有30光年。
From their analysis, using the criteria stated above, the average number of Earth-like planets per star is 0.37-0.60 for conservative HZ and 0.58-0.88 for the optimistic HZ.*the This means, even using the stricter conservative limits, there could be 300million or even more potential Earth out there in the Milky Way. Their results also showed that there is a high probability of having at least 4 of them 30 light-years away from us.
图表为以‘乐观’限制和‘保守’限制为标准的行星发现概率* The frequency of discovering planets within the conservative and optimistic boundaries*
但是，正如我们看到结果那样，尽管该团队对开普勒和盖亚的数据进行了非常仔细的分析，不确定性仍然相对较大。这是因为开普勒实际探测到的岩石类系外行星数量很少，尽管宇宙中有无数的系外行星，岩石类系外行星只占据了一小部分。
Although the team carried out very careful analyses of the data from Kepler and Gaia, as we can from those results, the uncertainties are still relatively large. That is due to the small number of rocky exoplanets actually detected by Kepler, even though there are countless exoplanets in the universe, rocky planets are only a small fraction.
当然盖亚于2013年才发射，并计划到2022年退役，所以还会有更多的数据来帮助科学家完善现有的模型和未来的星空蓝天。
However, with Gaia launched in 2013 and planned to not retire until 2022, there will much more data to help the scientists refine the estimates.
视频为盖亚在太空中旋转扫描天空的模拟形态 Animation of Gaia spinning in space scanning the sky
在了解了银河系中每种类型的系外行星的普遍情况后，科学家将更好地设计未来的太空任务，并寻找我们的下一个家园。
With the knowledge of the how common each type of exoplanets is in the Milky Way, scientist will better design future space missions and searching for our next home.
所以在我们做好搬家的准备之前，
我们要尽量地保护好地球，
给予他们更多时间进行研究！
So before we know to get there safely,
better give them more time by
protecting our Earth!
图表Breakthough Starshot项目的超高速光驱动纳米飞船模拟图。用来以最快速度到达数光年外的行星，探索未来家园 Illustration of the ultra-fast light-driven nanocraft by Breakthough Starshot, it will be used to travel to exoplanets light years away to discover our new home
图片和视频来自Independent, Gaia, ESA, NASA, Breakthrough Starshot官网*部分来
自NASA Ames Research Center的“The Occurrence of Rocky Habitable Zone Planets Around Solar-Like Stars from Kepler Data"论文
Pictures from official website of Independent, Gaia, ESA, NASA, Breakthrough Starshot.
*Parts are from “The Occurrence of Rocky Habitable Zone Planets Around Solar-Like Stars from Kepler Data" by NASA Ames Research Center
