科學家發現迄今最像地球的太陽系外行星
歐洲天文學家宣布，在距離地球 20.5 光年（194 兆公里）的太空，發現了一個「超級地球」，不但氣候和地球類似，而且有水和重力，這是人類在太陽系以外發現的第一個類似地球的行星，科學家已把這裡視為尋找外星生物的重要目標。
圖1. Artist's impression of the system of three planets surrounding the red dwarf Gliese 581. One of them is the first rocky planet lying in the habitable zone to have been discovered. (Credit: ESO)
天文學家尚未直接觀察到圍繞紅矮星 Gliese 581 公轉的這顆行星。但研究人員在寫給《天文和天體物理學》（Astronomy and Astrophysics）編輯的信中表示，
對 Gliese 581 進行的測量工作顯示，存在一顆體積比地球稍大一些的行星。
「這是迄今找到的第一顆（行星）可能同時存在岩地、水，以及與恆星間的距離適宜液態水的存在，」領導該項研究的日內瓦天文臺尤迪（Stephane Udry）表示。
「我們估計這種條件意味著，這顆類似地球行星的平均氣溫約在 0-40 攝氏度之間，因此水可能是以液態形式存在。」也就是說，雖然和紅矮星的距離比地球和太陽的距離近14倍，但因紅矮星溫度較低（比太陽低50倍），所以並沒有被輻射烤焦，而繞行一圈只要13天。其表面重力為地球兩倍，足以留住水份。
迄今發現的約 200 顆太陽系之外的行星，大多是類似木星的氣態巨大行星。但這一行星體積相對較小。
它的質量似乎約為地球的五倍。對該行星的研究團隊中，包括有在 1995 年發現被普遍視為首次找到太陽系外行星的研究人員。
Gliese 581 是距離地球最近的 100 顆恆星之一，位於距地球約 20.5 光年的天秤座。
新聞連結：
1. http://tw.news.yahoo.com/article/url/d/a/070425/16/deq0.html
2. http://tw.news.yahoo.com/article/url/d/a/070425/2/den9.html
紅矮星旁發現海王星大小的系外行星
法國和瑞士天文學家，利用歐南天文台（ESO）位在智利 La Silla 的 3.6 米望遠鏡加上 HARPS 儀器，發現一顆到目前為止最輕的系外行星。更讓這些天文學家驚訝的，是這顆系外行星的母星居然是一顆質量很小的紅矮星（red dwarfs）。由於紅矮星在宇宙中相當普遍，這項發現代表著宇宙中可能到處充斥著行星，而且目前已有探測這類系外行星的科技能力，未來探測系外行星的重點可能終將如SETI 科學家所言--是 M 型星的天下。
圖2. 智利安地斯山拉席拉的天文望遠鏡
這顆紅矮星名為 Gl 581（Gliese 581 的縮寫），位在天秤座中，離地球約 20.5 光年，質量僅為我們太陽的 1/3 而已。這種紅矮星所發出的總亮度，比我們的太陽還暗 50 倍以上，且是銀河系中數量最多的星種：舉例來說，在太陽附近 100 顆恆星中，就有至少 80 顆是紅矮星。Gliese 星表是列出太陽附近 25 秒差距（parsecs）、相當於 81.5 光年範圍內的恆星，Gliese 於 1969 年首度出版，後來 Gliese 和 Jahreiss 於 1991 年出版修正版。Gl 581 就是 Gliese 星表中第 581 顆星。
既然這類恆星如此普遍，天文學家便想知道這類恆星究竟會不會擁有行星。先前的巡天研究卻只在 200 顆紅矮星中，找到其中 2 顆擁有行星。不過，日內瓦天文台（Geneva Observatory）Stéphane Udry 表示：先前的研究不代表全部，因為當時的觀測精度可能不足以找到這些行星，所以他們後來才會發展超級精密的第二代儀器--HARPS 光譜儀（High Accuracy Radial Velocity for Planetary Searches，高精度徑向速度行星搜尋光譜儀），這台光譜儀的精密度最高可測到每秒 1 公尺的徑向速度變化。現在事實證明，這個決定相當正確。
圖3. Gliese 581 的徑向速度曲線
上圖是紅矮星 Gl 581 的徑向速度變化曲線圖。從圖中可看到，Gl 581 沿著我們視線方向的速度分量，有一個 5.366 天的週期變化，變化幅度約為每秒 13.2 公尺左右，應該是受到行星的重力牽曳的影響所致。且因變化曲線相當對稱，因此行星的軌道應該相當接近圓形。經計算之後，這顆行星的質量約為地球的 17 倍，相當於海王星的質量，因此是到目前為止，所有發現的 170 顆系外行星中質量最小的。它相當靠近母星，僅 600 萬公里遠，公轉一圈只有 5.4 天；太陽系中最靠近太陽的水星，距離也至少 5600 萬公里，公轉一圈約 88 天。由於它如此靠近母星，天文學家估計這顆系外行星的表面溫度可能高達攝氏 150 度以上。
相關論文發表在天文與天文物理期刊快訊（Astronomy and Astrophysics Letter）中。 ("The HARPS search for southern extra-solar planets. VI. A Neptune-mass planet around the nearby M dwarf Gl 518", by X. Bonfils et al., A&A 443, L15, 2005).
這些天文學家目前還在努力搜尋其他紅矮星周圍的系外行星，其中有5顆系外行星候選者的質量可能比Gl 581 的行星還小，這 5 顆分別是 Gliese 876d（7.3倍地球質量）、HD 160691 d（14 倍地球質量）、 55 Cnc e （14.4 倍地球質量）、HD 212301 b（14.4 倍地球質量）、HD 4308 b（15 倍地球質量）。不過這些都僅是估計的質量下限，有可能會因行星軌道面傾角的不同而變大。
資料來源：
1. http://www.tam.gov.tw/news/2005/200512/05120101.htm
2. http://www.eso.org/outreach/press-rel/pr-2005/pr-30-05.html, 2005.11.30
Astronomers Find First Habitable Earth-like Planet
Science Daily — Astronomers have discovered the most Earth-like planet outside our Solar System to date, an exoplanet with a radius only 50% larger than the Earth and capable of having liquid water. Using the ESO 3.6-m telescope, a team of Swiss, French and Portuguese scientists discovered a super-Earth about 5 times the mass of the Earth that orbits a red dwarf, already known to harbour a Neptune-mass planet. The astronomers have also strong evidence for the presence of a third planet with a mass about 8 Earth masses.
圖1. Artist's impression of the system of three planets surrounding the red dwarf Gliese 581. One of them is the first rocky planet lying in the habitable zone to have been discovered. (Credit: ESO)
This exoplanet - as astronomers call planets around a star other than the Sun -- is the smallest ever found up to now [1] and it completes a full orbit in 13 days. It is 14 times closer to its star than the Earth is from the Sun. However, given that its host star, the red dwarf Gliese 581 [2], is smaller and colder than the Sun -- and thus less luminous -- the planet nevertheless lies in the habitable zone, the region around a star where water could be liquid!
圖2. 紅矮星 Gliese 581
"We have estimated that the mean temperature of this super-Earth lies between 0 and 40 degrees Celsius, and water would thus be liquid," explains Stéphane Udry, from the Geneva Observatory (Switzerland) and lead-author of the paper reporting the result. "Moreover, its radius should be only 1.5 times the Earth's radius, and models predict that the planet should be either rocky -- like our Earth -- or covered with oceans," he adds.
"Liquid water is critical to life as we know it," avows Xavier Delfosse, a member of the team from Grenoble University (France). "Because of its temperature and relative proximity, this planet will most probably be a very important target of the future space missions dedicated to the search for extra-terrestrial life. On the treasure map of the Universe, one would be tempted to mark this planet with an X."
The host star, Gliese 581, is among the 100 closest stars to us, located only 20.5 light-years away in the constellation Libra ("the Scales"). It has a mass of only one third the mass of the Sun. Such red dwarfs are intrinsically at least 50 times fainter than the Sun and are the most common stars in our Galaxy: among the 100 closest stars to the Sun, 80 belong to this class.
"Red dwarfs are ideal targets for the search for low-mass planets where water could be liquid. Because such dwarfs emit less light, the habitable zone is much closer to them than it is around the Sun," emphasizes Xavier Bonfils, a co-worker from Lisbon University. Planets lying in this zone are then more easily detected with the radial-velocity method [3], the most successful in detecting exoplanets.
Two years ago, the same team of astronomers already found a planet around Gliese 581 (see ESO 30/05). With a mass of 15 Earth-masses, i.e. similar to that of Neptune, it orbits its host star in 5.4 days. At the time, the astronomers had already seen hints of another planet. They therefore obtained a new set of measurements and found the new super-Earth, but also clear indications for another one, an 8 Earth-mass planet completing an orbit in 84 days. The planetary system surrounding Gliese 581 contains thus no fewer than 3 planets of 15 Earth masses or less, and as such is a quite remarkable system.
圖3. Gliese 581 的徑向速度曲線
The discovery was made thanks to HARPS (High Accuracy Radial Velocity for Planetary Searcher), perhaps the most precise spectrograph in the world. Located on the ESO 3.6-m telescope at La Silla, Chile, HARPS is able to measure velocities with a precision better than one metre per second (or 3.6 km/h)! HARPS is one of the most successful instruments for detecting exoplanets and holds already several recent records, including the discovery of another 'Trio of Neptunes' (ESO 18/06, see also ESO 22/04).
The detected velocity variations are between 2 and 3 metres per second, corresponding to about 9 km/h! That's the speed of a person walking briskly. Such tiny signals could not have been distinguished from 'simple noise' by most of today's available spectrographs.
"HARPS is a unique planet hunting machine," says Michel Mayor, from Geneva Observatory, and HARPS Principal Investigator. "Given the incredible precision of HARPS, we have focused our effort on low-mass planets. And we can say without doubt that HARPS has been very successful: out of the 13 known planets with a mass below 20 Earth masses, 11 were discovered with HARPS!"
HARPS is also very efficient in finding planetary systems, where tiny signals have to be uncovered. The two systems known to have three low mass planets -- HD 69830 and Gl 581 -- were discovered by HARPS.
"And we are confident that, given the results obtained so far, finding a planet with the mass of the Earth around a red dwarf is within reach," affirms Mayor.
This research is reported in a paper submitted as a Letter to the Editor of Astronomy and Astrophysics ("The HARPS search for southern extra-solar planets : XI. An habitable super-Earth (5 MEarth) in a 3-planet system", by S. Udry et al.)
The team is composed of Stéphane Udry, Michel Mayor, Christophe Lovis, Francesco Pepe, and Didier Queloz (Geneva Observatory, Switzerland), Xavier Bonfils (Lisbonne Observatory, Portugal), Xavier Delfosse, Thierry Forveille, and C.Perrier (LAOG, Grenoble, France), François Bouchy (Institut d'Astrophysique de Paris, France), and Jean-Luc Bertaux (Service d'Aéronomie du CNRS, France)
Notes
[1] Using the radial velocity method, astronomers can only obtain a minimum mass (as it is multiplied by the sine of the inclination of the orbital plane to the line of sight, which is unknown). From a statistical point of view, this is however often close to the real mass of the system. Two other systems have a mass close to this. The icy planet around OGLE-05-390L, discovered by microlensing with a network of telescopes including one at La Silla (ESO 03/06), has a (real) mass of 5.7 Earth masses. It, however, orbits much farther from its small host star than the present one and is hence much colder. The other is one of the planets surrounding the star Gliese 876. It has a minimum mass of 5.89 Earth masses (and a probable real mass of 7.53 Earth masses) and completes an orbit in less than 2 days, making it too hot for liquid water to be present.
[2] Gl 581, or Gliese 581, is the 581th entry in the Gliese Catalogue, which lists all known stars within 25 parsecs (81.5 light years) of the Sun. It was originally compiled by Gliese and published in 1969, and later updated by Gliese and Jahreiss in 1991.
[3] This fundamental observational method is based on the detection of variations in the velocity of the central star, due to the changing direction of the gravitational pull from an (unseen) exoplanet as it orbits the star. The evaluation of the measured velocity variations allows deducing the planet's orbit, in particular the period and the distance from the star, as well as a minimum mass.
Note: This story has been adapted from a news release issued by European Southern Observatory.
原始報導：
ScienceDaily：Astronomers Find First Habitable Earth-like Planet
相關連結：
1. Wiki 百科：Gliese 581