| An exoplanet, or extrasolar planet, is a planet outside of our solar system that usually orbits another star in our galaxy. 一颗系外行星是一颗围绕着非太阳的恒星公转的恒星，坐落于太阳系之外。  系外行星大概长这样。。。Youtube视频介绍详见： https://www.youtube.com/embed/0ZOhJe_7GrE?flag=1&enablejsapi=1&html5=1&origin=https://exoplanets.nasa.gov Most of the exoplanets discovered so far are in a relatively small region of our galaxy, the Milky Way. ("Small" meaning within thousands of light-years of our solar system; one light-year equals 5.88 trillion miles, or 9.46 trillion kilometers.) That is as far as current telescopes have been able to probe. We know from NASA’s Kepler Space Telescope that there are more planets than stars in the galaxy. 迄今为止发现的大多数系外行星都位于银河系中一个相对较小的区域（“小”是指距离太阳系数千光年以内的地方；一光年等于5.88万亿英里，或9.46万亿公里。）这是目前望远镜所能探测到的距离。通过开普勒太空望远镜观测到的数据可以得出，银河系中的行星数量大于恒星数量。  Proxima Centauri b的模拟成像图。欲获得3D模型，请前往： https://exoplanets.nasa.gov/resources/2211/proxima-b-3d-model/ Although exoplanets are far – even the closest known exoplanet to Earth, Proxima Centauri b, is still about 4 light-years away – scientists have discovered creative ways to spot these seemingly tiny objects. 尽管系外行星距离地球很远——即使是已知的离地球最近的系外行Proxima Centauri b也距离我们大约 4 光年——科学家们已经找到了发现这些宇宙中的沧海一粟的创造性方法。 How do we find exoplanets? There are five methods scientists commonly use to discover exoplanets. 我们如何去发现系外行星呢？科学家们一共有五种方法来探测系外行星的存在。  01 Radial Velocty径向速度法 Orbiting planets cause stars to wobble in space, changing the color of light astronomers see when observing a star. Stars are affected by the gravitational tug of their orbiting planets and, when observed through a telescope, this affects the star's light spectrum. If the star moves in the direction of the observer it will appear to be shifted toward blue. If it is moving away from the observer, it will shift toward the red. Observing this is known as the radial velocity method. 绕行的行星会导致恒星在太空中微小的晃动，从而改变天文学家在观察恒星时看到的光的颜色。恒星受到其行星的引力效应的影响，所以通过望远镜观察时，恒星的光谱发生微小的变化。如果恒星向观察者的方向移动，可见光中的谱线将向蓝色移动。如果它远离观察者，就将向红色移动。观察这一点被称为径向速度法。  可视化gif：恒星运动时会压缩其发出光的波长，导致频率变化，产生红移或者蓝移。 02 Transit穿越探测法 When a planet passes directly between an observer and the star it orbits, it blocks some of that starlight. For a brief period of time, that star’s light actually gets dimmer. It's a tiny change, but it's enough to clue astronomers in to the presence of an exoplanet around a distant star. This is known as the transit method. 当一颗行星直接从地球和其围绕的恒星之间经过时，它会挡住一些恒星的光线。在短时间内，观察者看到的那颗恒星的亮度变暗了。这是一个微小的变化，但足以让天文学家了解一颗遥远恒星周围存在系外行星。这被称为穿越探测法。  可视化gif：行星穿过恒星与地球的视线时会将恒星部分发出的光挡住，使得观测的亮度有规律的减少。 03 Direct Imaging直接成像法 Exoplanets are far away, and they are millions of times dimmer than the stars they orbit. So, unsurprisingly, taking pictures of them the same way you'd take pictures of, say Jupiter or Venus, is exceedingly hard. 系外行星距离我们很远，它们比它们环绕的恒星暗数百万倍。因此，毫不奇怪的是，以与拍摄木星或金星相同的方式拍摄它们是非常困难的。  可视化gif：当恒星前的行星直接经过恒星与地球的视线时拍照，会产生恒星表面上的小黑点。 The major problem astronomers face in trying to directly image exoplanets is that the stars they orbit are millions of times brighter than their planets. Any light reflected off of the planet or heat radiation from the planet itself is drowned out by the massive amounts of radiation coming from its host star. It's like trying to find a flea in a lightbulb, or a firefly flitting around a spotlight. However, there are two main methods astronomers use to block the light of a star. 天文学家用望远镜直接对系外行星进行成像时面临的主要问题是，它们环绕的恒星比它们的行星亮数百万倍。任何从行星反射的光或来自行星本身的热辐射都会被来自其主星的大量辐射所掩盖。这就像试图在发光的灯泡表面寻找跳蚤，或者在聚光灯下寻找萤火虫。然而，天文学家发明了两种方法来解决这个问题。  日冕仪实例，差不多长这样。 One, called coronography, uses a device inside a telescope to block light from a star before it reaches the telescope's detector. Coronagraphs are built as internal add-ons to telescopes, and are now being used to directly image exoplanets from ground-based observatories. 一种叫做日冕仪，它使用望远镜内部的一种装置，在恒星发出的光到达望远镜的光敏传感器之前将其挡住。日冕仪是望远镜的内部构建，现在是地面天文台直接对系外行星进行成像的核心部件。  可视化gif：恒星光帆展开之后挡住恒星的光芒，使得望远镜可以调整适当光圈对行星进行拍照。 Another method is to use a 'starshade', a device that's positioned to block light from a star before it even enters a telescope. For a space-based telescope looking for exoplanets, a starshade would be a separate spacecraft, designed to position itself at just the right distance and angle to block starlight from the star astronomers were observing. 另一种方法是使用恒星光罩，这是一种在恒星进入望远镜之前就可以阻挡光线的装置。对于寻找系外行星的太空望远镜来说，星罩将是一个单独的航天器，旨在将自身定位在恰到好处的距离和角度，以阻挡后面望远镜正在观察的恒星发出的星光。 04 Gravitational Microlensing微引力透镜 Among his many insights, Albert Einstein rethought the concept of gravity, defining it less as a mysterious attraction between objects and more as a geometric property of spacetime. 在他的众多理论思考中，阿尔伯特爱因斯坦重新思考了引力的意义，并非将其定义为物体之间的神秘吸引力，而是定义为时空的几何特性。  可视化gif：在广义相对论中，行星附近会产生时空的弯曲，使得恒星发出的光线汇聚，从而使得光线突然变亮。 In other words, big objects warp the fabric of space. This effect causes light to distort and change direction when affected by the gravity of a massive object, like a star or a planet. This change of direction can cause some pretty interesting things to happen. Sometimes, gravity can bend and focus light like a lens in a magnifying glass or pair of glasses. 具体来讲，大质量物体扭曲了时空结构。当受到大质量物体（如恒星或行星）的重力影响时，这种效应会导致光线扭曲并改变其传播方向。这种方向的改变会导致一些非常有趣的事情发生。有时，引力场可以像放大镜或眼镜中的透镜一样弯曲和聚焦光线。  可视化gif：天文学家真正观测到的被引力透镜的星体是这样的。成像星体、透镜星体、地球三者恰好成一条直线时地球上的观察者会观测到一个亮环，是因为围绕透镜星体一周的全部光纤都汇聚在一起，被称为爱因斯坦环。 Gravitational microlensing happens when a star or planet's gravity focuses the light of another, more distant star, in a way that makes it temporarily seem brighter. 当一颗恒星或行星的引力场使得另一颗更远的恒星的光线发生汇聚时，就会发生微引力透镜效应，使遥远星体暂时看起来更亮。 05 Astrometry天体测量法 Doppler shifts aren't the only way astronomers can find stars that are wobbling due to the gravity of their planets. The wobble can also be visible as changes in the star's apparent position in the sky. In other words, scientists can actually detect the star's position wiggling around in space. 多普勒频移并不是天文学家发现因行星引力而摇摆不定的恒星的唯一方式。随着恒星在天空中位置的明显变化，较大幅度的晃动也可以直接被观测到。或者说，天文学家直接探测到了一颗恒星在其邻域空间上发生了小幅度位置的变化。 Astrometry, as this method is called, is still amazingly hard to do. Stars wobble such a minute distance that it's very difficult to accurately detect the wobble from planets, especially small ones the size of Earth. 天体测量非常难直接观测到，因为行星的质量相比于恒星十分微小，产生的恒星摆动的距离很短，以至于很难准确地检测行星的轨道和质量，尤其是像地球大小的小体积行星。  可视化gif：受到周围行星扰动，恒星的相对位置会发生轻微的变化。 In order to track the movement of these stars, scientists take a series of images of a star and some of the other stars that are near it in the sky. In each picture, they compare the distances between these reference stars and the star they're checking for exoplanets. 为了追踪这些恒星的运动，科学家们拍摄了一系列恒星和天空中靠近它的其他一些恒星的图像。在每张图片中，他通过比较一段时间内目标恒星与周边参考恒星的视角距离来检验该目标恒星是否产生了微小晃动。 If the target star has moved in relation to the other stars, astronomers can analyze that movement for signs of exoplanets. 如果目标恒星相对于周围恒星产生了移动，天文学家就可以根据移动的距离、周期推算出系外行星的质量与轨道半径。  以上五种方法就是科学家们测量一个系外行星的方法，不知道是不是要助力开启人类星际穿越时代的文明了呢？ |
