Astronomers may have found the first exomoon

天文学家可能发现了首颗系外卫星

Observations made with the European Southern Observatory’s Very Large Telescope (ESO’s VLT) have revealed evidence for a moon-like object in the CD-35 2722 system. Unlike moons in our Solar System, the newly found object does not orbit a planet, raising questions about what to name it. Instead, it circles a brown dwarf, an object larger than a planet, that orbits the CD-35 2722 star. If confirmed, this could be the first ‘moon’ discovered outside our Solar System. 利用欧洲南方天文台（ESO）的甚大望远镜（VLT）进行的观测，揭示了 CD-35 2722 系统中存在一个类似卫星的天体的证据。与我们太阳系中的卫星不同，这个新发现的天体并不围绕行星运行，这引发了关于如何命名它的争议。相反，它围绕着一颗褐矮星运行，而这颗褐矮星本身又围绕着 CD-35 2722 恒星运行。如果得到证实，这可能是人类在太阳系外发现的第一颗“卫星”。

Kevin Hoy, an ESO student in Chile and lead author of the study published today in Nature, describes the system he spent months analysing as “super weird” compared to our own. The biggest and most massive object in this young system is the star CD-35 2722, which has about half the mass of the Sun. The star is being orbited by a brown dwarf, an object too massive to be a planet but too small to be a star. The newly discovered object orbits this brown dwarf. 目前在智利工作的 ESO 学生、今日发表于《自然》杂志的研究论文第一作者凯文·霍伊（Kevin Hoy）将他花费数月分析的这个系统描述为与我们太阳系相比“超级怪异”。在这个年轻系统中，最大、质量最大的天体是 CD-35 2722 恒星，其质量约为太阳的一半。这颗恒星由一颗褐矮星环绕，褐矮星的质量太大而无法成为行星，但又太小而无法成为恒星。新发现的天体正是围绕这颗褐矮星运行。

“This system is somewhat hard to define using Solar-System-based words like ‘planet’ and ‘moon’,” states Hoy, who is also affiliated with the Universidad Diego Portales and the Millennium Nucleus of Young Exoplanets and their Moons (YEMS) in Chile. The new object, which the team call an exosatellite, is at least as massive as Jupiter while the brown dwarf has more than 30 times the mass of Jupiter. “使用基于太阳系的词汇，如‘行星’和‘卫星’，很难定义这个系统，”霍伊表示。他同时隶属于智利的迭戈波塔莱斯大学（Universidad Diego Portales）和年轻系外行星及其卫星千年核（YEMS）。研究团队将这个新天体称为“系外卫星”（exosatellite），其质量至少与木星相当，而那颗褐矮星的质量则是木星的 30 多倍。

“The exosatellite is clearly massive enough to be a planet, but it does not orbit a star, though it orbits an object that orbits a star,” says Hoy. “Being the third wheel in this system makes us want to call it a moon, even if it is nothing like the small, rocky moons we have in our system.” “这颗系外卫星的质量显然大到足以成为一颗行星，但它并不直接围绕恒星运行，尽管它围绕着一个围绕恒星运行的天体，”霍伊说。“作为这个系统中的‘第三者’，我们倾向于称它为卫星，即使它与我们太阳系中那些小型岩石卫星完全不同。”

This exosatellite or ‘exomoon’, a natural satellite outside our Solar System [1], is difficult to label, given the differences in this system compared to our own. Alice Zurlo, YEMS Director and collaborator on the study explains: “The satellite we report is a giant gaseous body orbiting a highly massive companion, itself several times the mass of Jupiter.” 鉴于该系统与我们太阳系的差异，这颗系外卫星或“系外月球”（即太阳系外的天然卫星 [1]）很难被归类。YEMS 主任兼该研究合作者爱丽丝·祖洛（Alice Zurlo）解释道：“我们报告的这颗卫星是一个巨大的气态天体，它围绕着一个质量极大的伴星运行，而该伴星本身的质量就是木星的数倍。”

“We have a clear delineation between the planets and the Sun in the Solar System, so defining things like moons is simple. 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,” adds Zurlo, who is also an astrophysicist at Universidad Diego Portales. “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，”同时担任迭戈波塔莱斯大学天体物理学家的祖洛补充道。

Regardless of what to call this object, astronomers have been trying to detect satellites outside our Solar System for years, but none has yet been confidently detected. Therefore, despite the over 6000 exoplanets discovered to date, only a few exomoon candidates have been spotted and the evidence to support them is limited. 无论如何称呼这个天体，天文学家多年来一直试图探测太阳系外的卫星，但至今尚未有确凿的发现。因此，尽管迄今已发现了 6000 多颗系外行星，但被发现的系外卫星候选者寥寥无几，且支持它们的证据也非常有限。

For the CD-35 2722 observations, Hoy, Zurlo and their team used the CRIRES+ instrument on ESO’s VLT, employing the method that was used to find the first exoplanet around a Sun-like star. They applied this radial velocity method to detect small wobbles on the brown dwarf caused by the object orbiting it, finding what the team believe to be strong evidence for this ‘moon’. 在对 CD-35 2722 的观测中，霍伊、祖洛及其团队使用了 ESO 甚大望远镜上的 CRIRES+ 仪器，采用了发现首颗类太阳恒星系外行星时所用的方法。他们利用这种径向速度法，探测到了褐矮星因受该天体环绕而产生的微小摆动，并发现了团队认为支持这颗“卫星”存在的有力证据。

“As exotic as it is, this system is truly unique and represents a breakthrough: the first plausible detection of an exosatellite,” says Zurlo. Beyond the excitement of discovering new types of objects, detecting satellites in other planetary systems can help us understand how diverse their formation and evolution might be. “尽管它非常奇异，但这个系统确实独一无二，代表了一项突破：这是首次对系外卫星的合理探测，”祖洛说。除了发现新型天体带来的兴奋感之外，探测其他行星系统中的卫星还能帮助我们了解其形成和演化的多样性。

With its 39-metre mirror and advanced instrumentation, ESO’s upcoming Extremely Large Telescope (ELT) will allow astronomers to detect smaller exomoons. Discoveries with the ELT will make us further reconsider how we label planetary objects from systems different from our own. 凭借其 39 米的主镜和先进的仪器，ESO 即将建成的极大望远镜（ELT）将使天文学家能够探测到更小的系外卫星。利用 ELT 取得的发现将促使我们进一步反思，该如何定义那些来自与我们太阳系截然不同的系统的行星天体。