One Star Over, a Planet That Might Be Another Earth
Another Earth could be circling the star right next door to us.
Astronomers announced on Wednesday that they had detected a planet orbiting Proxima Centauri, the closest neighbor to our solar system. Intriguingly, the planet is in the star’s “Goldilocks zone,” where it may not be too hot nor too cold. That means liquid water could exist at the surface, raising the possibility for life.
Although observations in recent years, particularly by NASA’s Kepler planet-finding mission, have uncovered a bounty of Earth-size worlds throughout the galaxy, this one holds particular promise because it might someday, decades from now, be possible to reach. It’s 4.2 light-years, or 25 trillion miles, away from Earth, which is extremely close in cosmic terms.
One astronomer likened it to a flashing neon sign. “I’m the nearest star, and I have a potentially habitable planet!” said R. Paul Butler, an astronomer at the Carnegie Institution for Science and a member of the team that made the discovery.
一位天文学家将它比作正在闪烁的霓虹招牌。“我是最近的恒星，我有一颗可能宜居的行星！”卡内基科学研究所(Carnegie Institution for Science)的天文学家R·保罗·巴特勒(R. Paul Butler)说。正是他所在的团队做出了这一发现。
Guillem Anglada-Escudé, an astronomer at Queen Mary University of London and the leader of the team that made the discovery reported in the journal Nature, said, “We know there are terrestrial planets around many stars, and we kind of expected the nearby stars would contain terrestrial planets. This is not exciting because of this. The excitement is because it is the nearest one.”
伦敦大学玛丽王后学院(Queen Mary University of London)的天文学家吉列姆·安格拉达-埃斯屈德(Guillem Anglada-Escudé)，是在《自然》(Nature)杂志上发表这一发现的研究团队的带头人。他表示，“我们知道，在很多恒星周围有类地行星，我们也差不多预计到那些临近的恒星周围会有类地行星。但令人振奋的并不是这个。让人激动的是这是最近的那颗。”
Beyond the planet’s size and distance from its parent star, much about it is still mysterious. Scientists are working off computer models that offer mere hints of what’s possible: Conditions could be Earthlike, but they could also be hellish like Venus, or cold and dry like Mars.
There is no picture of the planet, which has been designated Proxima b. Instead, Dr. Anglada-Escudé and his colleagues detected it indirectly, studying via telescope the light of the parent star. They zeroed in on clocklike wobbles in the starlight, as the colors shifted slightly to the reddish end of the spectrum, then slightly bluish. The oscillations, caused by the bobbing back-and-forth motion of the star as it is pulled around by the gravity of the planet, are similar to how the pitch of a police siren rises or falls depending on whether the patrol car is traveling toward or away from the listener.
From the size of the wobbles, the astronomers determined that Proxima b is at least 1.3 times the mass of Earth, although it could be several times larger. A year on Proxima b — the time to complete one orbit around the star — lasts just 11.2 days.
通过这种移动的大小，天文学家们确定Proxima b的质量至少是地球的1.3倍，尽管它也可能要大上好几倍。Proxima b上的一年——在环绕比邻星的轨道上运行一周的时间——仅为11.2个地球日。
Although the planet, lost in the glare of the star, cannot be viewed by current telescopes, astronomers hope to see it when the next generation is built a decade from now.
And the planet’s proximity to Earth gives hope that robotic probes could someday be zooming past the planet for a close-up look. A privately funded team of scientists and technology titans, led by the Russian entrepreneur Yuri Milner and the theoretical physicist Stephen Hawking, have announced Breakthrough Starshot Initiative, a project to develop and launch a fleet of iPhone-size spacecraft within two to three decades. Their proposed destination is the Alpha Centauri star system, which includes two larger, sunlike stars in addition to Proxima Centauri.
这颗行星与地球的相似性增加了一种希望：未来某一天，机器探测器可以掠过或近距离地观察它。在俄罗斯企业家尤里·米尔纳(Yuri Milner)和理论物理学家斯蒂芬·霍金(Stephen Hawking)牵头之下，私人资助的一群科学家与技术巨擘宣布启动突破·探星计划(Breakthrough Starshot Initiative)。这一项目的目标是在未来二三十年里开发并发射一系列iPhone大小的太空飞船。他们提出的目的地是半人马座阿尔法星系（Alpha Centauri，又称南门二），除了比邻星，其中还有两颗更大的类日恒星。
“We will definitely aim at Proxima,” said Avi Loeb, a Harvard astronomer who is the chairman of an advisory committee for Breakthrough Starshot. “This is like finding prime real estate in our neighborhood.”
This newly discovered planet is much closer to its parent star, about five million miles apart, than Earth is to the sun, 93 million miles. Even Mercury, the innermost planet of our solar system, is 36 million miles from the sun.
While Proxima b might be similar to Earth, its parent star, Proxima Centauri, is very different from the sun. It is tiny, belonging to a class of stars known as red dwarfs, with only about 12 percent of the mass of the sun and about 1/600th the luminosity — so dim that it cannot be seen from Earth with the naked eye.
Thus Proxima b, despite its closeness to the star, receives less warmth than Earth, but enough that water could flow on the surface. Whether the planet has liquid water or an atmosphere is “pure speculation at this point,” Dr. Anglada-Escudé said in a news conference.
If the planet formed close to the star, it could be dry and airless, but it might also have formed farther out and migrated inward to its current orbit. It is also possible that the planet formed dry and was later bombarded by comets or ice-rich asteroids.
“There are viable models and stories that lead to a viable Earthlike planet today,” Dr. Anglada-Escudé said.
The discovery could provide impetus for planet-finding telescopes. Ruslan Belikov of the NASA Ames Research Center in Mountain View, Calif., has proposed a small space telescope costing less than $175 million dedicated to the search for planets in Alpha Centauri. While it would not be powerful enough to spot Proxima b, its existence would give more confidence that terrestrial planets also orbit the two sunlike stars there.
这一发现也许会给搜寻行星的望远镜提供推动力。在加州山景城的美国航空航天局艾姆斯研究中心(Ames Research Center)供职的鲁斯兰·别利科夫(Ruslan Belikov)提出，建设一座小型太空望远镜，成本不到1.75亿美元，专门用来搜寻半人马座阿尔法星系的行星。虽然它并不会强大到足以看见Proxima b，但它的存在也许能提供更多信心：在该星系的两颗类日恒星周围，也存在类地行星。
“It just raises the public awareness there’s a new world just next door,” Dr. Belikov said. “It’s a paradigm shift in people’s minds.”