发现三个行星在可居住区的系统的意义

欧洲南天观察台（ the European Southern Observatory【ESO】）利用他们在智利的直径3.6米的HARPS（ the High Accuracy Radial velocity Planet Searcher）望远镜，发现了一个至少有六个行星系统，其中三个行星在可居住区内。

这个系统位于天蝎座内（constellation of Scorpius ）。其宿主恒星是Gliese 667。

该恒星离太阳系大约是22光年。

这个恒星其实是一个三星系统。Gliese 667A、 Gliese 667B和Gliese 667C。而Gliese 667A 和Gliese 667B的质量分别占太阳质量的73% 和69%。它们二者互相围绕旋转。Gliese 667C的质量仅占太阳质量的31%。它围绕A和B星旋转。这是轨道稳定的三星系统的标准形式。

这是艺术家的想像图。远处的双星系统是Gliese 667A和Gliese 667B。

三者都是主序星。Gliese 667C仅发射太阳光度的1.4%。它的表面温度仅有3700K。所以，它属于红矮星。现在发现的至少有六个行星都是属于Gliese 667C。行星的编号是其宿主恒星的名字后面加一小写的字母。Gliese 667Cb、Gliese 667Cc、Gliese 667Cd、Gliese 667Ce、Gliese 667Cf、Gliese 667Cg。所以说至少六个行星，因为Gliese 667Ch 还没有被确认。这台望远镜发现的所有行星的方法都是对观测到的数据作小心翼翼地精细处理，最后才得到了结论。如果处理数据方法有微小的偏差，就可能把子虚乌有的误差信号当作是行星。

为什么目前Gliese 667A和Gliese 667B 没有发现行星？这两个恒星的轨道平面和地球与它们之间的视线成128度的夹角。这样的两个恒星若有行星，它们的轨道平面应当和两颗恒星的轨道平面的夹角很小。当初如果它们的行星的轨道和恒星的轨道夹角太大的话，在轨道的调整期内，那些行星不是被拉得改变了轨道平面，就是被靠近的恒星吃掉了，或者被踢出了该恒星系统。调整后稳定的轨道大致是：行星的轨道平面和恒星的轨道平面的夹角不会太大。这样来推测，那两颗恒星若有行星，行星的轨道平面与地球视线夹角在128度左右。而ESO的这台望远镜探测行星的方法对于行星的轨道平面与地球视线间的夹角为90度时，完全无效——至少，理论上是这样的。其二，这两颗恒星的质量比Gliese 667C大许多。而该望远镜探测行星的方法对于行星和恒星的质量比大的，容易探测到，而这个比率越小，探测越困难。这应当是Gliese 667A和B目前未探测到行星的一种解释。

目前被确认的Gliese 667C的六颗行星，其中的三颗大致在可居住区内。下面是它们之间的大小对比图。注意：其中的大小是按比例画的；而它们之间的位置距离不是按比例画的。ESO拥有此图的credit。

这是各个行星间相对大小和排列。

按照各个行星离恒星的距离，由近及远的顺序是b、c、f、e、d和g，它们相应的质量是地球质量的倍数：5.94-12；3.86-7.8；1.94-4；2.68-5.4；5.21-10.1；4.41-8.8。它们的Semi-major axis分别是，单位是AU（天文单位，是地球与太阳间的距离：1.5亿公里）：0.050432；0.12507；0.15575；0.21257；0.2758；0.5389。它们的倾角都大于30度。

其中，特别是Gliese 667Cc，它接收了太阳光能是地球接收的90%。推测它上面有液体水的可能性是很大的。也就是有生命的可能性是很大的。

一个恒星周围若形成行星，它们之间的距离和相对质量的大小是具有某种规律的。这个系统的发现，说明了我们太阳系的行星系统是一个“标准”的例子。银河系中，质量比太阳小的恒星的数量是很庞大的，它们远多于比太阳质量大的恒星数量。一个很简单的推理是：我们这样的系统在银河系中和整个宇宙中，应当是很普遍的现象。那种认为地球在行星系统的范围内来说，在银河系甚至在宇宙中都是“罕见”的观点，该寿终正寝啦！

另外，科学家们研究太阳系的行星系统的诞生和演化，不再是仅有一个孤立的例子啦，至少有一个参照对比的例子。这具有目前无法估计的伟大意义。

主镜的直径达39米的European Extremely Large Telescope (E-ELT)正在建设中。在未来十余年时间内，当它投入使用后，完全有能力分辨这些行星上是否有大气层，并能分辨大气层的成分。那时，弄清楚这些行星上是否有生命，应当不是太难的事。

我们期待未来有更惊人的发现。

下面是ESO网站上关于此次发现的报道全文。

Three Planets in Habitable Zone of Nearby Star

team of astronomers has combined new observations of Gliese 667C with existing data from HARPS at ESO’s 3.6-metre telescope in Chile, to reveal a system with at least six planets. A record-breaking three of these planets are super-Earths lying in the zone around the star where liquid water could exist, making them possible candidates for the presence of life. This is the first system found with a fully packed habitable zone.

Gliese 667C is a very well-studied star. Just over one third of the mass of the Sun, it is part of a triple star system known as Gliese 667 (also referred to as GJ 667), 22 light-years away in the constellation of Scorpius (The Scorpion). This is quite close to us — within the Sun’s neighbourhood — and much closer than the star systems investigated using telescopes such as the planet-hunting Kepler space telescope.

Previous studies of Gliese 667C had found that the star hosts three planets (eso0939, eso1214) with one of them in the habitable zone. Now, a team of astronomers led by Guillem Anglada-Escudé of the University of Göttingen, Germany and Mikko Tuomi of the University of Hertfordshire, UK, has reexamined the system. They have added new HARPS observations, along with data from ESO's Very Large Telescope, the W.M. Keck Observatory and the Magellan Telescopes, to the already existing picture [1]. The team has found evidence for up to seven planets around the star [2]. 

These planets orbit the third fainter star of a triple star system. Viewed from one of these newly found planets the two other suns would look like a pair of very bright stars visible in the daytime and at night they would provide as much illumination as the full Moon. The new planets completely fill up the habitable zone of Gliese 667C, as there are no more stable orbits in which a planet could exist at the right distance to it. 

“We knew that the star had three planets from previous studies, so we wanted to see whether there were any more,” says Tuomi. “By adding some new observations and revisiting existing data we were able to confirm these three and confidently reveal several more. Finding three low-mass planets in the star’s habitable zone is very exciting!”

Three of these planets are confirmed to be super-Earths — planets more massive than Earth, but less massive than planets like Uranus or Neptune — that are within their star’s habitable zone, a thin shell around a star in which water may be present in liquid form if conditions are right. This is the first time that three such planets have been spotted orbiting in this zone in the same system [3].

“The number of potentially habitable planets in our galaxy is much greater if we can expect to find several of them around each low-mass star — instead of looking at ten stars to look for a single potentially habitable planet, we now know we can look at just one star and find several of them,” adds co-author Rory Barnes (University of Washington, USA).

Compact systems around Sun-like stars have been found to be abundant in the Milky Way. Around such stars, planets orbiting close to the parent star are very hot and are unlikely to be habitable. But this is not true for cooler and dimmer stars such as Gliese 667C. In this case the habitable zone lies entirely within an orbit the size of Mercury's, much closer in than for our Sun. The Gliese 667C system is the first example of a system where such a low-mass star is seen to host several potentially rocky planets in the habitable zone.

The ESO scientist responsible for HARPS, Gaspare Lo Curto, remarks: “This exciting result was largely made possible by the power of HARPS and its associated software and it also underlines the value of the ESO archive. It is very good to also see several independent research groups exploiting this unique instrument and achieving the ultimate precision.”

And Anglada-Escudé concludes: “These new results highlight how valuable it can be to re-analyse data in this way and combine results from different teams on different telescopes.”

Notes

[1] The team used data from the UVES spectrograph on ESO’s Very Large Telescope in Chile (to determine the properties of the star accurately), the Carnegie Planet Finder Spectrograph (PFS) at the 6.5-metre Magellan II Telescope at the Las Campanas Observatory in Chile, the HIRES spectrograph mounted on the Keck 10-metre telescope on Mauna Kea, Hawaii as well as extensive previous data from HARPS (the High Accuracy Radial velocity Planet Searcher) at ESO’s 3.6-metre telescope in Chile (gathered through the M dwarf programme led by X. Bonfils and M. Mayor 2003–2010 described here).

[2] The team looked at radial velocity data of Gliese 667C, a method often used to hunt for exoplanets. They performed a robust Bayesian statistical analysis to spot the signals of the planets. The first five signals are very confident, while the sixth is tentative, and seventh more tentative still. This system consists of three habitable-zone super-Earths, two hot planets further in, and two cooler planets further out. The planets in the habitable zone and those closer to the star are expected to always have the same side facing the star, so that their day and year will be the same lengths, with one side in perpetual sunshine and the other always night.

[3] In the Solar System Venus orbits close to the inner edge of the habitable zone and Mars close to the outer edge. The precise extent of the habitable zone depends on many factors.

More information

This research was presented in a paper entitled “A dynamically-packed planetary system around GJ 667C with three super-Earths in its habitable zone”, to appear in the journal Astronomy & Astrophysics.

The team is composed of G. Anglada-Escudé (University of Göttingen, Germany), M. Tuomi (University of Hertfordshire, UK), E. Gerlach (Technical University of Dresden, Germany), R. Barnes (University of Washington, USA), R. Heller (Leibniz Institute for Astrophysics, Potsdam, Germany), J. S. Jenkins (Universidad de Chile, Chile), S. Wende (University of Göttingen, Germany), S. S. Vogt (University of California, Santa Cruz, USA), R. P. Butler (Carnegie Institution of Washington, USA), A. Reiners (University of Göttingen, Germany), and H. R. A. Jones (University of Hertfordshire, UK).

ESO is the foremost intergovernmental astronomy organisation in Europe and the world’s most productive ground-based astronomical observatory by far. It is supported by 15 countries: Austria, Belgium, Brazil, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Portugal, Spain, Sweden, Switzerland and the United Kingdom. ESO carries out an ambitious programme focused on the design, construction and operation of powerful ground-based observing facilities enabling astronomers to make important scientific discoveries. ESO also plays a leading role in promoting and organising cooperation in astronomical research. ESO operates three unique world-class observing sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope, the world’s most advanced visible-light astronomical observatory and two survey telescopes. VISTA works in the infrared and is the world’s largest survey telescope and the VLT Survey Telescope is the largest telescope designed to exclusively survey the skies in visible light. ESO is the European partner of a revolutionary astronomical telescope ALMA, the largest astronomical project in existence. ESO is currently planning the 39-metre European Extremely Large optical/near-infrared Telescope, the E-ELT, which will become “the world’s biggest eye on the sky”.