The magnetosphere of an exoplanet has yet to be unambiguously detected. Investigations of star–planet interaction and neutral atomic hydrogen absorption during transit to detect magnetic fields in hot Jupiters have been inconclusive, and interpretations of the transit absorption non-unique. In contrast, ionized species escaping a magnetized exoplanet, particularly from the polar caps, should populate the magnetosphere, allowing detection of different regions from the plasmasphere to the extended magnetotail and characterization of the magnetic field producing them. Here we report ultraviolet observations of HAT-P-11 b, a low-mass (0.08 M_J) exoplanet showing strong, phase-extended transit absorption of neutral hydrogen (maximum and tail transit depths of 32 ± 4% and 27 ± 4%) and singly ionized carbon (15 ± 4% and 12.5 ± 4%). We show that the atmosphere should have less than six times the solar metallicity (at 200 bar), and the exoplanet must also have an extended magnetotail (1.8–3.1 au). The HAT-P-11 b equatorial magnetic field strength should be about 1–5 G. Our panchromatic approach using ionized species to simultaneously derive metallicity and magnetic field strength can now constrain interior and dynamo models of exoplanets, with implications for formation and evolution scenarios.

一颗系外行星的磁层还没有被明确地探测到。为探测热木星磁场而进行的恒星-行星相互作用和中性原子氢吸收的研究尚无定论，对过境吸收的解释也不是唯一的。相比之下，从磁化系外行星逃逸的电离物质，特别是从极冠中逃逸的物质，应该填充磁层，从而可以检测从等离子层到扩展磁尾的不同区域，并表征产生它们的磁场。在这里，我们报告了对 HAT-P-11 b 的紫外观测，这是一个低质量（0.08 M _J) 系外行星显示出对中性氢（最大和尾部过境深度分别为 32 ± 4% 和 27 ± 4%）和单电离碳（15 ± 4% 和 12.5 ± 4%）的强相延过境吸收。我们表明，大气层的金属丰度应该小于太阳金属丰度的六倍（在 200 bar 时），并且系外行星还必须有一个延长的磁尾（1.8-3.1  au）。HAT-P-11 b 赤道磁场强度应约为 1-5 G。我们使用电离物质同时推导金属丰度和磁场强度的全色方法现在可以约束系外行星的内部和发电机模型，对形成和演化情景产生影响.