Interstellar interlopers are bodies formed outside of the Solar System but observed passing through it. The first two identified interlopers, 1I/‘Oumuamua and 2I/Borisov, exhibited unexpectedly different physical properties. 1I/‘Oumuamua appeared unresolved and asteroid-like, whereas 2I/Borisov was a more comet-like source of both gas and dust. Both objects moved under the action of nongravitational acceleration. These interlopers and their divergent properties provide our only window so far onto an enormous and previously unknown galactic population. The number density of such objects is ∼0.1 AU−3 which, if uniform across the galactic disk, would imply 1025 to 1026 similar objects in the Milky Way. The interlopers likely formed in, and were ejected from, the protoplanetary disks of young stars. However, we currently possess too little data to firmly reject other explanations.▪1I/‘Oumuamua and 2I/Borisov are both gravitationally unbound, subkilometer bodies showing nongravitational acceleration.▪The acceleration of 1I/‘Oumuamua in the absence of measurable mass loss requires either a strained explanation in terms of recoil from sublimating supervolatiles or the action of radiation pressure on a nucleus with an ultralow mass column density, ∼1 kg m−2.▪2I/Borisov is a strong source of CO and H2O, which together account for its activity and nongravitational acceleration.▪The interlopers are most likely planetesimals from the protoplanetary disks of other stars, ejected by gravitational scattering from planets. 1I/‘Oumuamua and 2I/Borisov have dynamical ages ∼108 and ∼109 years, respectively.▪Forthcoming observatories should detect interstellar interlopers every year, which will provide a rapid boost to our knowledge of the population.

中文翻译：

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星际闯入者


星际闯入者是在太阳系之外形成但观察到穿过太阳系的天体。前两个确定的闯入者 1I/'Oumuamua 和 2I/Borisov 表现出意想不到的不同物理特性。1I/'Oumuamua 似乎未解析并且像小行星，而 2I/Borisov 则更像彗星的气体和尘埃来源。两个物体都在非引力加速度的作用下移动。这些闯入者及其不同的特性为我们提供了迄今为止了解巨大且以前未知的银河系人口的唯一窗口。这些天体的数量密度为 ∼0.1 AU−3，如果在整个银河系盘中均匀，则意味着银河系中有 1025 到 1026 个类似的天体。闯入者可能在年轻恒星的原行星盘中形成，并从中被抛出。然而，我们目前拥有的数据太少，无法坚决拒绝其他解释。▪1I/'Oumuamua 和 2I/Borisov 都是无重力的亚公里天体，显示出非重力加速度。▪在没有可测量的质量损失的情况下，1I/'Oumuamua 的加速度需要根据升华的超级挥发物的反冲或辐射压力对具有超低质量柱密度的原子核的作用进行紧张的解释。 ∼1 kg m−2.2I ▪/Borisov 是 CO 和 H2O 的强来源，它们共同解释了其活性和非引力加速度。 ▪1I/'Oumuamua 和 2I/Borisov 的动态年龄分别为 ∼108 岁和 ∼109 岁，▪即将到来的天文台应该每年都会探测到星际闯入者，这将迅速增强我们对星际闯入者的了解。