Astronomers Have Detected an Exomoon for the First Time
Astronomers Have Detected an Exomoon for the First Time
天文学家首次探测到系外卫星
An international research team has discovered evidence of a moonlike body. It marks the first time such an object has been detected outside our solar system—though just how close it comes to being a moon is a question researchers are still grappling with. 一个国际研究团队发现了一个类似卫星的天体证据。这是人类首次在太阳系外探测到此类天体，尽管它在多大程度上能被定义为“卫星”，仍是研究人员正在探讨的问题。
Although more than 6,000 exoplanets have been discovered to date, there’s been a lack of definitive evidence of moons in other planetary systems. The new findings challenge that by focusing on an object orbiting the brown dwarf CD-35 2722 B in a system 73 light-years away. 尽管迄今为止已经发现了超过 6,000 颗系外行星，但一直缺乏其他行星系统中存在卫星的确凿证据。这项新发现通过聚焦于一颗距离地球 73 光年、围绕褐矮星 CD-35 2722 B 运行的天体，挑战了这一现状。
A brown dwarf is a celestial body that lies between a planet and a star that lacks the mass required to become a star. CD-35 2722 B has a mass about 37 times that of Jupiter and orbits the star CD-35 2722, which has about 40 percent of the sun’s mass; they are separated by an apparent distance of about 2.8 arc seconds. 褐矮星是一种介于行星和恒星之间的天体，其质量不足以演化成恒星。CD-35 2722 B 的质量约为木星的 37 倍，围绕着质量约为太阳 40% 的恒星 CD-35 2722 运行；两者之间的视距离约为 2.8 角秒。
The research team observed CD-35 2722 B over a total of 23 nights between October 2023 and February 2026 using the CRIRES+ infrared spectrograph mounted on the European Southern Observatory’s Very Large Telescope. Using a technique known as the Doppler method, the researchers captured how CD-35 2722 B wobbles as it is slightly pulled by the gravity of another celestial body orbiting nearby. 研究团队在 2023 年 10 月至 2026 年 2 月期间，利用安装在欧洲南方天文台甚大望远镜（VLT）上的 CRIRES+ 红外光谱仪，对 CD-35 2722 B 进行了总计 23 个夜晚的观测。研究人员利用“多普勒方法”捕捉到了 CD-35 2722 B 的晃动，这种晃动是由附近另一颗天体的引力牵引所致。
The technique uses the Doppler effect observed in a celestial body’s spectrum to record changes in radial velocity—essentially looking for those—to estimate an exoplanet’s mass, orbital period, and orbital eccentricity. This method was also used in 1995 when the first exoplanet orbiting a sunlike star was discovered. 该技术利用在天体光谱中观测到的多普勒效应来记录径向速度的变化，从而估算系外行星的质量、轨道周期和轨道离心率。1995 年发现首颗围绕类太阳恒星运行的系外行星时，也使用了这种方法。
Since CD-35 2722 B is sufficiently distant from its host star, it was possible to obtain its spectrum directly with virtually no interference from the host star’s light. Similar attempts have been made with other brown dwarfs and planets. In this study, the combination of an instrument with higher wavelength resolution and minimal spectral contamination enabled measurements that were up to 100 times more precise than previous ones. 由于 CD-35 2722 B 与其宿主恒星距离足够远，研究人员得以直接获取其光谱，且几乎不受宿主恒星光线的干扰。此前针对其他褐矮星和行星也进行过类似尝试。在本研究中，高波长分辨率仪器与极低光谱污染的结合，使得测量精度比以往提高了 100 倍。
The analysis, published in Nature last month, revealed a strong signal indicating the presence of a large moon orbiting over a period of approximately 170 days and having a mass of at least 0.9 times that of Jupiter. In terms of mass ratio, this body accounts for about 2.5 percent of the mass of the brown dwarf it orbits, which is a significantly higher value compared with moons in our solar system. The system with the highest mass ratio among the planets and their moons in the solar system is Earth and the moon (approximately 1.2 percent); the mass ratios of the moons orbiting Jupiter and Saturn are much smaller. 上个月发表在《自然》杂志上的分析显示，一个强信号表明存在一颗大型卫星，其公转周期约为 170 天，质量至少为木星的 0.9 倍。从质量比来看，该天体占其所绕褐矮星质量的约 2.5%，这一比例远高于太阳系内的卫星。太阳系中行星与卫星质量比最高的是地球和月球（约 1.2%）；而围绕木星和土星运行的卫星质量比则要小得多。
To confirm that the observed signal was indeed caused by a moon, the research team also examined other possible factors. These included apparent periods resulting from errors in corrections for Earth’s orbital motion, seasonal variations in atmospheric conditions, and the effects of the brown dwarf’s own rotation. 为了确认观测到的信号确实是由卫星引起的，研究团队还排除了其他可能的因素，包括地球轨道运动校正误差导致的视周期、大气条件的季节性变化，以及褐矮星自身旋转的影响。
An artist’s concept of a moonlike object (center) orbiting the brown dwarf CD-35 2722 B (right). Researchers refer to this object as an “exosatellite” and describe it as a massive gas giant with a mass at least equal to that of Jupiter. 艺术家笔下的系外卫星概念图（中），它正围绕褐矮星 CD-35 2722 B（右）运行。研究人员将其称为“系外卫星”（exosatellite），并将其描述为一颗质量至少相当于木星的巨大气态巨行星。
Furthermore, calculations of the Roche limit—the boundary beyond which a satellite would be torn apart by the brown dwarf’s tidal forces—and the Hill radius—the radius of the brown dwarf’s gravitational influence—confirmed that the satellite’s orbit falls within a range where it can exist in a physically stable manner. 此外，通过计算洛希极限（超过该边界，卫星会被褐矮星的潮汐力撕碎）和希尔半径（褐矮星引力影响的范围），研究人员确认该卫星的轨道处于物理上稳定的范围内。
According to the researchers, this is the first evidence of a satellite orbiting a brown dwarf companion obtained using this method. A few months earlier, another team had gathered clues suggesting the presence of a satellite during observations of the HD 206893 star system using the VLT Interferometer but had not yet achieved a definitive detection. 据研究人员称，这是首次利用该方法获得围绕褐矮星伴星运行的卫星证据。几个月前，另一个团队在使用甚大望远镜干涉仪（VLTI）观测 HD 206893 星系时，也曾收集到暗示卫星存在的线索，但尚未实现确切的探测。
The object discovered in this study occupies a unique position that cannot be fully understood using the conventional framework for moons in our solar system. “We have a clear delineation between the planets and the Sun in the Solar System, so defining things like moons is simple,” says Alice Zurlo, an astrophysicist at Diego Portales University, in a news release. “In the CD-35 2722 system, where we are blurring the lines between stars, planets, and moons, the whole thing becomes more complicated to describe.” 本研究中发现的天体处于一个独特的位置，无法用太阳系卫星的传统框架完全理解。迭戈波塔利斯大学的天体物理学家爱丽丝·祖洛（Alice Zurlo）在新闻发布会上表示：“在太阳系中，行星和太阳之间的界限非常清晰，因此定义卫星等事物很简单。但在 CD-35 2722 系统中，我们模糊了恒星、行星和卫星之间的界限，整个描述过程变得更加复杂。”
It remains unclear whether this object should be called a moon. ESO also notes that there is no officially recognized definition for exomoons and the researchers use the term “exosatellite.” The paper itself acknowledges that it is uncertain whether this object will meet future criteria to be considered a moon, but that the discovery is a step toward creating a definitive detection. 目前尚不清楚该天体是否应被称为“卫星”。欧洲南方天文台也指出，目前尚无官方认可的“系外卫星”定义，因此研究人员使用了“系外卫星”（exosatellite）这一术语。论文本身也承认，该天体未来是否符合卫星的定义尚不确定，但这一发现是迈向确切探测的重要一步。
Researchers believe this discovery will serve as a catalyst for identifying new directions in future theories of planet formation and celestial mechanics. Furthermore, if there are smaller, rocky moons like the one in the new paper, they could be subjected to tidal heating from brown dwarfs, potentially creating environments suitable for life even at greater distances from their stars—a development that might also have implications for the search for extraterrestrial life. 研究人员认为，这一发现将成为探索行星形成理论和天体力学新方向的催化剂。此外，如果存在像文中提到的那种较小的岩石卫星，它们可能会受到褐矮星的潮汐加热，从而在远离恒星的地方创造出适合生命生存的环境——这一进展也可能对寻找地外生命产生影响。
It’s also possible that once the next-generation Extremely Large Telescope equipped with a 39-meter primary mirror is completed, it will be possible to detect even smaller exomoons. 此外，一旦配备 39 米主镜的下一代极大望远镜（ELT）建成，人类或许将能够探测到更小的系外卫星。