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Exoplanets 太阳系外行星

Small, furry creatures from Alpha Centauri?  阿尔发人马座来的矮小的绒毛般的生物?

Sadly, no. But one of Earth’s nearest stellar neighbours has an Earth-sized planet going round it, and a more distant planet has four suns 最好别!要不然惨了,可最邻近地球的星球群中一颗行星居然有着地球尺寸大小围着阿尔发人马座转,但相对还是更加遥远的一颗行星却居然有4个太阳。

Oct 20th 2012 | from the print edition





EXOPLANETS—those that orbit stars other than the sun—are one of the hottest topics in astronomy. The first was found in 1992, going round a neutron star (a strange beast made of matter as dense as that in an atomic nucleus) 1,500 light-years from Earth. Planets orbiting more conventional stars turned up three years later, and since then the initial trickle of discoveries has become a flood. The present tally is 843 confirmed and another couple of thousand probables.



So it takes a lot, these days, for news of a freshly discovered planet to raise eyebrows. But it still happens from time to time. October 16th was one of those times. On that day Nature published a paper in which a team from Geneva University claimed to have found an Earth-sized planet orbiting a star called Alpha Centauri B.



If the name sounds familiar, that is because, at just over four light-years away, the Alpha Centauri system is the sun’s nearest stellar neighbour. The planet, therefore, is not only the closest to Earth ever discovered, it is just about as close to Earth as any exoplanet can be. Only if it were orbiting Proxima Centauri, the third member of the system after Alpha Centauri A and B, could it be closer. It is small, too, with a minimum estimated mass just 1.13 times that of Earth. And the star it circles is much like the sun: 93% as massive, and with a surface temperature just a few hundred degrees cooler.

假如名称听起来熟悉是因为阿尔法星系是离太阳系最近的星系群,只不过4光年。这颗行星不仅是发现的离地球最近的,也是能作为太阳系外行星离地球最近的。只要是在比邻星轨道运行的,经过阿尔法星座AB星系中有1/3成员都会离得更近。这颗行星也很小,估价其密度只有地球密度的1.13倍。它围绕运转的行星就像太阳:密度93% 且表面温度是零下数百度,更加冰冷。



There, though, the similarities end. The new planet is extremely close to its parent star, completing an orbit every 3.2 Earth days. (By way of comparison Mercury, the scorched innermost planet in the Solar System, has a year that is 88 days long.) It is also almost certainly tidally locked to its parent, meaning that one side is constantly baked and blasted by stellar radiation, while the other faces the freezing blackness of space. Life on such a body is, presumably, out of the question.




That Xavier Dumusque and his colleagues were able to find the thing at all is, however, newsworthy in itself. They ran their quarry to ground using what is known as the radial-velocity method, which examines stars for tiny wobbles caused by the gravity of planets that circle them. This method is well established. But the smaller a planet is, the smaller the wobble it induces, which makes looking for Earth-sized tiddlers particularly difficult. To be exact, the team managed to discover a wobble of 0.5 metres a second in a star that is roughly 41,300,000,000,000km away.



Indeed, the tininess of the measurement is cause for caution. In the past, several apparent planets have evaporated on closer inspection. In this case, a great deal of statistical gymnastics was necessary to extract the planet-induced wobble from bigger ones caused by the star’s internal churnings, noise in the instruments, the motion of Earth through space, and contamination of the signal with light from Alpha Centauri A, the biggest member of the Alpha Centauri system.




Mr Dumusque and his colleagues reckon, however, that the probability of a false alarm is less than one in 500. Their observations were painstaking, and spread over 3½ years. One condition of their paper’s publication by Nature was that their data be made immediately available to other researchers, who will be able to double-check the team’s sums.




Assuming the planet is real, other astronomers will be keen to study it further. As the paper points out, enough exoplanets have now been found for statistical patterns to emerge. One such is that if a star has one smallish planet it is quite likely to have several more. So confirmation of the team’s discovery would boost the odds that more worlds are circling Alpha Centauri B, perhaps even in the so-called habitable zone where the temperature would allow liquid water to exist on the surface.



The team’s technique is sensitive enough to detect a planet about four times the mass of Earth within the habitable zone, although it would require a decade of observations to confirm such a discovery. Better telescopes than exist now might then study the atmospheres of such planets, looking for evidence about their surface chemistry and (inevitably) for signs of life. Most intriguingly, Alpha Centauri’s proximity means that a sufficiently committed and luxuriantly financed group of scientists could send a probe there to take a closer look. Such a trip would take decades—possibly more than a century—but there are several designs for craft that could do it using existing technology.




Exciting as Mr Dumusque’s news was, though, not all eyes have been turned to Alpha Centauri. On October 15th, at a planetary-science conference in Reno, Nevada, hosted by the American Astronomical Society, another unusual planet was announced. This object, named PH1, is notable both for the fact that it boasts four suns and for the method by which it was found. As is described in a paper to be published in the Astrophysical Journal by Meg Schwamb, an astronomer at Yale, and her team, it was discovered by two members of Planet Hunters (whence its name), a web-based citizen-science project in which ordinary people are given access to data from NASA’s planet-hunting Kepler space telescope and encouraged to look for things that the computer programs which crunch Kepler’s data might have missed.

尽管不是所有人的视线都会转向阿尔法星座,但令Dumusque先生兴奋的消息是由美国天文协会主持在1015日召开的星系科学年会上又有一颗不同寻常的行星被发现。这颗行星被命名为 PH1,就是举世瞩目的那个宣称有4个太阳的行星,正是采用了这个团队的方法寻找到的。正如发表在天文学杂志上由耶鲁大学的Meg Schwamb女士和她的团队撰写的论文中描述那样,这颗行星是由两个行星搜寻者发现的,这个项目是基于网络平台所举办的民众科学项目,允许普通人接触从美国国家宇航局猎星探测宇宙望远镜 Kepler数据并鼓励人们去寻找计算机程序漏掉的那些Kepler望远镜或许已经丢失的数据。



The planet hunters in question are Kian Jek, a web developer, and Robert Gagliano, a doctor. Their discovery was a textbook example of the usefulness of having people check the data alongside computers, says Chris Lintott, an astronomer at Oxford who helps to manage the Planet Hunters project. Kepler detects planets by noting the drop in light caused when one of them passes in front of its parent star. That works well for single-star systems, and even for binaries (two stars revolving around a common centre of gravity). In PH1’s case, however, the variations generated by a planet’s interactions with four separate stars baffled the computers—but not the eagle-eyed humans.

这些受质疑的行星猎寻者们是Kian Jek,一位网络开发人员和Robert Gagliano,一位医生。牛津大学的一位帮助管理猎星项目的天文学家Chris Lintott讲他们的发现会成为有效利用人去检查计算机数据的教科书式的典范。Kelper 望远镜通过记录这些行星中的一颗在其母星系前滑落的光来探寻这些行星的。对于单一星系会有效,甚至对双星(两颗星围绕共同的引力中心运行)。然而针对PH1的情况来看,由4颗独立的星球同一个行星相互之间作用所产生的不同变化会使计算机迷惑,而对有鹰一般锐眼的人类来说应该不是问题。


Besides being fascinating in its own right, such an exotic find is a test of astronomers’ theories about how planets form. PH1’s four parents are actually a pair of binaries: each binary revolves around a common centre, like an ordinary binary, and the two binary systems then revolve around each other. Planetary-formation theory holds that worlds condense out of a disc of dust early in a star’s life. But in this case, according to Dr Lintott, computer models suggest that the stars’ gravitational dance would have disrupted the disc and stopped the formation of planets. Reality thus disagrees with the models—and that is how science advances.

除了陶醉在其自身外,如此奇妙的发现也证实了天文学家们有关行星形成的理论。PH14颗母星实际上是两对双星系就像两个普通的双星那样围绕一个共同的中心运行,这两个双星系彼此之间相互围绕运行。 行星形成理论认为这些星系浓缩了早期从一颗星的生命体中洒发出来的一缕尘埃而形成的。 可针对这一说法据Lintott博士的看法是计算机模型表明了星系群的引力跳动会干扰尘埃从而也就阻断了行星的形成。而现实中很多人不同意这些模,为此人们关注的是科学如何向前发展的问题。