Intermediate-mass planets, from super-Earth to Neptune-sized bodies, are the most common types of planet in the Galaxy¹. The prevailing theory of planet formation—core accretion²—predicts the existence of substantially fewer intermediate-mass giant planets than have been observed^3,4. The competing mechanism for planet formation—disk instability—can produce massive gas giant planets on wide orbits, such as HR 8799⁵, by direct fragmentation of the protoplanetary disk⁶. Previously, fragmentation in magnetized protoplanetary disks has been considered only when the magneto-rotational instability is the driving mechanism for magnetic field growth⁷. However, this instability is naturally superseded by the spiral-driven dynamo when more realistic, non-ideal magneto-hydrodynamic conditions are considered^8,9. Here, we report on magneto-hydrodynamic simulations of disk fragmentation in the presence of a spiral-driven dynamo. Fragmentation leads to the formation of long-lived bound protoplanets with masses that are at least one order of magnitude smaller than in conventional disk instability models^10,11. These light clumps survive shear and do not grow further owing to the shielding effect of the magnetic field, whereby magnetic pressure stifles the local inflow of matter. The outcome is a population of gaseous-rich planets with intermediate masses, while gas giants are found to be rarer, in qualitative agreement with the observed mass distribution of exoplanets.

中等质量的行星，从超级地球到海王星大小的天体，是银河¹中最常见的行星类型。流行的行星形成理论——核心吸积^2——预测存在的中等质量巨行星比观测到的要少得多^3,4。^{行星形成的竞争机制——盘的不稳定性——可以通过原行星盘6}的直接破碎，在宽轨道上产生巨大的气态巨行星，例如 HR 8799 ⁵。以前，只有当磁旋转不稳定性是磁场增长的驱动机制时，才考虑磁化原行星盘中的碎片⁷. 然而，当考虑更现实的、非理想的磁流体动力学条件时，这种不稳定性自然会被螺旋驱动的发电机取代^8,9。在这里，我们报告了在存在螺旋驱动发电机的情况下磁盘碎片的磁流体动力学模拟。碎片化导致形成寿命长、质量至少比传统盘不稳定性模型小一个数量级的原行星^10,11. 由于磁场的屏蔽作用，这些轻团块经受住剪切并且不会进一步生长，从而磁压力抑制了物质的局部流入。结果是一群具有中等质量的富含气体的行星，而气体巨星被发现更稀有，与观察到的系外行星质量分布在质量上一致。