Corot 7b成為迄今為止人類發現的首顆太陽系外岩石行星

據英國《每日郵報》9月16日報道，天文學家宣布發現首顆太陽系以外的岩石行星——Corot7b，這為人類尋找宇宙生命燃起了新的希望。該行星在體積上與地球相當，但白天的溫度高達1500攝氏度，這意味著不大可能有生物在那里繁衍。然而，這項發現卻提示人們，銀河中或許存在像地球一樣宜居的行星。

　　據介紹，Corot7b在麒麟座星群中圍繞一顆500光年以外的恆星做公轉運動，它上面到處是赤熱的熔岩和洶涌的海洋。盡管科學家已經在太陽系以外發現了330多顆行星，其中包括12顆固態行星，但Corot7b卻是迄今為止體積最小、也是第一顆被證實的岩石行星。

　　“我們等待這一天已經很久了，”任教于美國麻省理工學院的天文學家西格爾說。“它（指Corot7b）是太陽系以外的第一個岩石世界，並為我們今後的研究提供了一個新的方向。我們為此感到非常非常激動。”

　　科學界認為，生命的開始需要一個固體的表面作為支撐。這顆新行星距離它的恆星較近，只有150萬英里（約合240萬公里）；而地球與太陽之間的距離則有9300萬英里（約合1.49億公里）之遙。研究人員表示，它的質量是地球的5倍左右。

　　“這正是科學研究中最令人興奮和驚異的時刻，”從事該研究的小組組長、瑞士天文學家奎洛茲說。絕大多數太陽系以外的行星都是氣態的，就像我們熟悉的木星和土星那樣。像地球這樣的固態行星大約只有12個。然而這一次，科學家首次精確計算出了上述新行星的密度，從而證明它是一顆地地道道的岩石行星。據悉，研究人員借助一個高精度視向速度行星掃描儀完成了這個工程，歷時70個小時。

　　“可以說我們就住在一塊岩石上，”研究小組成員之一、德國天文學家哈茨爾斯說，“它（指Corot7b）是我們目前發現的最類似地球的行星，只不過它距離自己公轉的恆星太近了。”據介紹，這顆今年年初被發現的 “希望之星”只需20個小時就能完成一次公轉，而即便是太陽系里距離太陽最近的水星也要花上88天才能繞日走完一圈。“它的溫度太高了，因此我們叫它"熔岩行星"，”哈茨爾斯表示。

　　由於Corot7b距離地球很遙遠，人們無法用普通的天文望遠鏡觀測到它。因此，研究人員只能測量它的恆星在其引力作用下出現的“搖擺程度”，從而計算出Corot7b本身的質量、體積和密度。


An international team (A) of astronomers, among which astronomers from Paris Observatory, have measured the mass and density of the smallest exoplanet. The longest set of HARPS measurements ever made has firmly established the nature of the smallest and fastest-orbiting exoplanet ever found, CoRoT-7b, revealing its mass as five times that of Earth's. Combined with CoRoT-7b's known radius, which is less than twice that of our terrestrial home, this tells us that the exoplanet's density is quite similar to the Earth's, suggesting a solid, rocky world. The extensive data set also reveals the presence of another so-called super-Earth in this foreign solar system.

"This is science at its thrilling and amazing best," says Didier Queloz, leader of the team who made the observations. "We did everything we could to learn what the object discovered by the CoRoT satellite looks like and we found a unique system."
In February 2009, a small exoplanet was discovered around a rather unremarkable star named TYC 4799-1733-1 by the French CNES CoRoT satellite. The star, known since as CoRoT-7, is located towards the constellation Monoceros (the Unicorn) at a distance of about 500 light-years. Slightly smaller and cooler than our Sun, CoRoT-7 is thought to be younger as well, with an age about 1.5 billion years.
Every 20.4 hours, the planet eclipses a small fraction of the light of the star for a little over one hour by one part in 3,000 [1]. This planet, designated CoRoT-7b, is only 2.5 million kilometres away from its host star, or 23 times closer than Mercury is to the Sun. It has a radius of less than twice that of Earth.

The initial set of measurements, however, could not provide the mass of the exoplanet. Such a result requires extremely precise measurements of the velocity of the star, which is pulled a tiny amount by the gravitational tug of the orbiting exoplanet. The problem with CoRoT-7b is that these tiny signals are jumbled by stellar activity in the form of 'starspots' (just like sunspots on our Sun), which are cooler regions on the surface of the star. Therefore, the main signal is linked to the rotation of the star, with makes one complete revolution in about 23 days.

To get an answer, astronomers had to call upon the best exoplanet-hunting device in the world, the High Accuracy Radial velocity Planet Searcher (HARPS) spectrograph attached to the ESO 3.6-metre telescope at the La Silla Observatory in Chile.
"Even though HARPS is certainly unbeaten when it comes to detecting small exoplanets, the measurements of CoRoT-7b proved to be so demanding that we had to gather no less than 70 hours of observations on the star," says co-author François Bouchy.
HARPS delivered, allowing the astronomers to tease out the 20.4-hour signal in the data. This figure led them to infer that CoRoT-7b has a mass of about five Earth masses, placing it in rare company as one of the lightest exoplanets yet found.

"Thanks to the fact that it is transiting it is the smallest mass ever precisely measured for an exoplanet [2]," says team member Claire Moutou. "Moreover, as we have both the radius and the mass, we can determine the density and get a better idea of the internal structure of this planet."
With a mass much closer to that of Earth than for example ice giant Neptune's 17 Earth masses, CoRoT-7b belongs to the exoplanetary category of 'super-Earths'. About a dozen of these bodies have been detected, though in the case of CoRoT-7b, this is the first time that the density has been measured for such a small exoplanet. The calculated density is close to Earth's, suggesting that the planet.s composition is similarly rocky.
"CoRoT-7b resulted in a 'tour de force' of astronomical measurements. The superb light curves of the space telescope CoRoT gave us the best radius measurement, and HARPS the best mass measurement for an exoplanet. Both were needed to discover a rocky planet with the same density as the earth," says co-author Artie Hatzes.

CoRoT-7b earns another distinction as the closest known exoplanet to its host star, which also makes it the fastest - it orbits its star at a speed of 216 kilometres per second- .In fact, CoRoT-7b is so close that the place may well look like Dante's Inferno. With a probable temperature well above 1000 degrees, the planet may have lava or boiling oceans on its surface. Such a 'sauna planet' is definitively not a place for life to develop, says Queloz.



As a further testament to HARPS. sublime precision, the astronomers found from their dataset that CoRoT-7 hosts another exoplanet slightly further away than CoRoT-7b. Designated CoRoT-7c, it circles its host star in 3 days and 17 hours and has a mass about eight times that of Earth, so it too is classified as a super-Earth. Unlike CoRoT-7b, this sister world does not pass in front of its star as seen from Earth, so astronomers cannot measure its radius and thus its density.
Given these findings, CoRoT-7 stands as the first star known to have a planetary system made of two short period super-Earths with one that transits its host. "These are very exciting times," concludes Queloz. "We may be very close to finding a sister planet to Earth, especially when new instruments, in particular the ESPRESSO spectrograph planned for ESO's Very Large Telescope, will be online."