中国科技网7月28日报道（张微 编译）美国宇航局最近发现的12颗系外行星，包括最像地球的那个地球2.0，这使得已发现的系外行星总数接近2000颗。现在我们认为，几乎所有的恒星都有一个行星系统，地球只是我们星系中数10亿颗行星中的一个。

我们已经发现的许多系外行星与太阳系中的行星是完全不同的：“热木星”是轨道非常接近其恒星的巨行星，而“超级地球”是一个岩石行星，其质量是地球的10倍。最新发现的Kepler-452b是首个与地球大小非常相似的系外行星，而且位于宜居带内，围绕恒星运转，温度适宜，可能存在支撑生命的条件。

除了它们的质量、密度和它们与各自恒星的距离，我们对这些系外行星了解甚少。它们是由什么组成的?它们是如何形成的?那里的天气如何?我们的小型、快速跟踪观测卫星将致力于研究系外行星，“闪烁”将努力回答这些问题。

因为系外行星距离我们如此遥远，所以这个任务是一个巨大的挑战。大多数已经发现的系外行星都是间接发现的，当系外行星在它们的恒星面前经过时，恒星的亮度发生变化，或通过寻找由于轨道恒星的重力拖曳导致的恒星的摆动。这些系外行星很少被直接拍摄到，由于它们与地球的距离太过遥远，从地球上看它们不过是宇宙中的微光。

然而，即使是这点点星光，也可以揭示大量的信息。近年来，我们的创新技术也能够提取信息，从系外行星经过它们的恒星时透过大气层的星光中发现蛛丝马迹。

光谱分析

光谱能够让我们将光(在这个意义上，是指的整个电磁频谱，不只是人类肉眼的可见光)分解成不同颜色的组成部分，我们就可以详细地研究它。周期表中的元素分子吸收特定波长的电磁频谱，会留下一个独特的线型模式，有点像条形码。通过检测和分离这些条形码，我们就可以识别出元素出现的印迹，由此分析出系外行星大气层包含的气体成分。

一颗系外行星的大气成分可以揭示出一个行星是否在其目前运行的轨道形成的，还是从行星系统的不同地方迁移过来的。进化、物理和化学过程对系外行星大气层的驱动，强烈影响其与母恒星的距离。较轻分子的损失，与其它天体如彗星或小行星的碰撞，火山活动，甚至生命都显著改变原始大气层的组成。因此一个行星大气层的组成可以追溯它的历史，并给出一个暗示，这颗星球是否宜居甚至存在生命。

太空中的眼镜

除了哈勃和斯皮策太空望远镜，这两个即将退役的望远镜之外，目前没有合适的设备来研究，更不用说发现系外行星。太空任务如ARIEL，欧洲的一个备选任务，将于2026年发射，在未来的十年里也不可能加以利用。即将升空的大天文台，杰姆斯•韦伯太空望远镜或E-ELT可能有一些我们所需的能力，但它们研究系外行星的时间是有限的。

这就是我们要开发“闪烁”的目的：一个小型的，相对低成本的致力于研究系外行星的商业任务。“闪烁”卫星将在英国建造，使用萨里卫星技术有限公司设计的平台和伦敦大学学院设计的仪表。

用一个低成本的方法研究太空科学

从距离地球上空轨道700公里的制高点，“闪烁”将观察100多个围绕遥远恒星运行的行星，它的仪器分析可见光和近红外波段光(从0.5到5微米)，它能够监测一系列分子包括水蒸气，二氧化碳和外来金属化合物，以及甲烷、乙炔和乙烷等有机分子。而且，它也对前体氨基酸——构成生命的基石——如氨和氰化氢也非常敏感。

通过测量这些系外行星反射的可见光和它们发射的红外线热，“闪烁”能够计算出行星的能量平衡，它的温度以及大气层中是否存在云。对于围绕明亮恒星运转的体积大的行星，“闪烁”甚至能够获取温度和云层的二维地图。通过5年寿命期内的反复观测，“闪烁”能够告诉我们那些行星上的气候和天气情况。

作为一个私人和公共混合资金资助的独立任务，“闪烁”为天文任务开创了一个全新的模式。该航天器的结构是一个高分辨率的地球成像平台，它的设备将利用有现货供应的器件实现，并重复利用现有的软件来降低成本，提高可靠性。随着研究工作的进行，仪器设备将在年底完成，发射时间定在2019年。

资助研究的机构包括15家英国研究机构和公司，资助单位的数量还在增加，希望开启一个系外行星科学研究的新时代，同时也证明小型、灵活和低成本科学项目研究的可行性。

英文原文：

UK satellite Twinkle will boost search for Earth-like exoplanets

NASA's recent discovery of 12 more exoplanets, including the most Earth-like yet, brings the number of exoplanets – those outside our solar system – discovered to nearly 2,000. It's now thought that almost every star has a planetary system, with Earth just one of several billion planets in our galaxy alone.

Many of the exoplanets we've found are quite different to those in oursolar system: "hot-Jupiters" are giant planets orbiting very close to their star, while "super-Earths" are rocky planets up to ten times the mass of Earth. The newly discovered Kepler-452b is the first exoplanet that is relatively similar to Earth in size and within the habitable zone around its star – not too hot and not too cold – that might be able to support life.

But really we know very little about these alien worlds beyond their mass, density and distance from their star. What are they made of? How did they form? What's the weather like there? Our small and fast-track satellite observatory dedicated to studying exoplanets, Twinkle, aims to answer these questions.

It's a huge challenge, since exoplanets are so far away. Most have been detected only indirectly – by a star's dip in brightness as a planet passes in front of it, or by looking for a wobble in a star's position caused by an orbiting planet's gravitational tug. A very few have been imaged directly but, due to their enormous distance from Earth, they are no more than pinpricks of light.

However, even a tiny amount of light can reveal a huge amount of information. In recent years, we have pioneered techniques to extract information about exoplanets from starlight filtered through their atmospheres as they pass in front of their star.

It's all in the waves

Spectroscopy allows us to split light – in this sense, the entire electromagnetic spectrum, not just that visible to the human eye – into a "rainbow" of its constituent parts so it can be examined in detail. Molecules formed from the periodic table's elements absorb specific wavelengths from the electromagnetic spectrum, leaving a unique pattern of lines, a bit like a barcode. By detecting and separating out these barcodes we can identify the tell-tale footprints of the elements present, and therefore which gases the exoplanets' atmospheres contain.

The composition of an exoplanet's atmosphere can reveal whether a planet formed in its current orbit, or whether it migrated from a different part of its planetary system. The evolution, chemistry and physical processes driving an exoplanet's atmosphere are strongly affected by the distance from its parent star. The loss of lighter molecules, impacts with other bodies such as comets or asteroids, volcanic activity, or even life can significantly alter the composition of primordial atmospheres. So a planet's atmospheric composition traces its history, and gives an indication as to whether it might be habitable – or even host life.

Eyes in the sky

However, aside from the Hubble and Spitzer space telescopes, both nearing the end of their lives, there is currently a gap in facilities suitable for studying, rather than finding, exoplanets. Space missions such as ARIEL, a European candidate mission competing for launch in 2026, won't be available for a decade or more. Upcoming general observatories, like the James Webb space telescope or E-ELT may have some of the capabilities needed, but time available on these for exoplanet research will be limited.

This is what led us to develop Twinkle: a small, relatively low-cost at £50m commercial mission dedicated to studying exoplanets. The Twinkle satellite will be built in the UK using a platform designed by Surrey Satellite Technology Ltd and instrumentation led by UCL.

Putting a low cost approach to work for science in space

From a vantage point in orbit 700km above the Earth, Twinkle will observe more than 100 planets orbiting distant stars, its instruments analysing light in the visible and near-infrared wavelengths (from 0.5 to 5 micrometers). It will be able to detect a range of molecules including water vapour, carbon dioxide and exotic metallic compounds, and organic molecules such as methane, acetylene and ethane. It will also be sensitive to precursors to amino acids – the building blocks of life – such as ammonia and hydrogen cyanide.

By measuring the visible light reflected by an exoplanet and the infrared heat that it emits, Twinkle will work out the planet's energy balance, its temperature and whether clouds are present or absent in the atmosphere. For very large planets orbiting very bright stars, Twinkle will even be able to obtain 2-D maps of temperature and clouds. With repeated observations over the five-year lifetime of the mission, this will tell us about climate and weather on those planets.

As an independent endeavour funded through a mixture of private and public sources, Twinkle is pioneering a new model for astronomy missions. The spacecraft's structure will be a platform developed for high-resolution Earth imaging, while the instruments will use off-the-shelf components and reuse existing software to bring down costs and increase reliability. With studies already underway, the instruments should be completed by the end of this year, the aim being to launch in 2019.

The consortium includes more than 15 UK research institutions and companies so far and continues to grow – hopefully kickstarting a new era of exoplanet science, but also demonstrating the feasibility of small, nimble and cost-effective science projects.

来源：中国科技网