發現三個行星在可居住區的系統的意義

歐洲南天觀察台（ the European Southern Observatory【ESO】）利用他們在智利的直徑3.6米的HARPS（ the High Accuracy Radial velocity Planet Searcher）望遠鏡，發現了一個至少有六個行星系統，其中三個行星在可居住區內。

這個系統位於天蠍座內（constellation of Scorpius ）。其宿主恆星是Gliese 667。

該恆星離太陽系大約是22光年。

這個恆星其實是一個三星系統。Gliese 667A、 Gliese 667B和Gliese 667C。而Gliese 667A 和Gliese 667B的質量分別占太陽質量的73% 和69%。它們二者互相圍繞旋轉。Gliese 667C的質量僅占太陽質量的31%。它圍繞A和B星旋轉。這是軌道穩定的三星系統的標準形式。

這是藝術家的想像圖。遠處的雙星系統是Gliese 667A和Gliese 667B。

三者都是主序星。Gliese 667C僅發射太陽光度的1.4%。它的表面溫度僅有3700K。所以，它屬於紅矮星。現在發現的至少有六個行星都是屬於Gliese 667C。行星的編號是其宿主恆星的名字後面加一小寫的字母。Gliese 667Cb、Gliese 667Cc、Gliese 667Cd、Gliese 667Ce、Gliese 667Cf、Gliese 667Cg。所以說至少六個行星，因為Gliese 667Ch 還沒有被確認。這台望遠鏡發現的所有行星的方法都是對觀測到的數據作小心翼翼地精細處理，最後才得到了結論。如果處理數據方法有微小的偏差，就可能把子虛烏有的誤差信號當作是行星。

為什麼目前Gliese 667A和Gliese 667B 沒有發現行星？這兩個恆星的軌道平面和地球與它們之間的視線成128度的夾角。這樣的兩個恆星若有行星，它們的軌道平面應當和兩顆恆星的軌道平面的夾角很小。當初如果它們的行星的軌道和恆星的軌道夾角太大的話，在軌道的調整期內，那些行星不是被拉得改變了軌道平面，就是被靠近的恆星吃掉了，或者被踢出了該恆星系統。調整後穩定的軌道大致是：行星的軌道平面和恆星的軌道平面的夾角不會太大。這樣來推測，那兩顆恆星若有行星，行星的軌道平面與地球視線夾角在128度左右。而ESO的這台望遠鏡探測行星的方法對於行星的軌道平面與地球視線間的夾角為90度時，完全無效——至少，理論上是這樣的。其二，這兩顆恆星的質量比Gliese 667C大許多。而該望遠鏡探測行星的方法對於行星和恆星的質量比大的，容易探測到，而這個比率越小，探測越困難。這應當是Gliese 667A和B目前未探測到行星的一種解釋。

目前被確認的Gliese 667C的六顆行星，其中的三顆大致在可居住區內。下面是它們之間的大小對比圖。注意：其中的大小是按比例畫的；而它們之間的位置距離不是按比例畫的。ESO擁有此圖的credit。

這是各個行星間相對大小和排列。

按照各個行星離恆星的距離，由近及遠的順序是b、c、f、e、d和g，它們相應的質量是地球質量的倍數：5.94-12；3.86-7.8；1.94-4；2.68-5.4；5.21-10.1；4.41-8.8。它們的Semi-major axis分別是，單位是AU（天文單位，是地球與太陽間的距離：1.5億公里）：0.050432；0.12507；0.15575；0.21257；0.2758；0.5389。它們的傾角都大於30度。

其中，特別是Gliese 667Cc，它接收了太陽光能是地球接收的90%。推測它上面有液體水的可能性是很大的。也就是有生命的可能性是很大的。

一個恆星周圍若形成行星，它們之間的距離和相對質量的大小是具有某種規律的。這個系統的發現，說明了我們太陽系的行星系統是一個“標準”的例子。銀河系中，質量比太陽小的恆星的數量是很龐大的，它們遠多於比太陽質量大的恆星數量。一個很簡單的推理是：我們這樣的系統在銀河系中和整個宇宙中，應當是很普遍的現象。那種認為地球在行星系統的範圍內來說，在銀河系甚至在宇宙中都是“罕見”的觀點，該壽終正寢啦！

另外，科學家們研究太陽系的行星系統的誕生和演化，不再是僅有一個孤立的例子啦，至少有一個參照對比的例子。這具有目前無法估計的偉大意義。

主鏡的直徑達39米的European Extremely Large Telescope (E-ELT)正在建設中。在未來十餘年時間內，當它投入使用後，完全有能力分辨這些行星上是否有大氣層，並能分辨大氣層的成分。那時，弄清楚這些行星上是否有生命，應當不是太難的事。

我們期待未來有更驚人的發現。

下面是ESO網站上關於此次發現的報道全文。

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”.