The interaction between a magma ocean and a primordial atmosphere is increasingly recognized as a key process in shaping planetary envelope compositions. This coupling should strongly influence gas accretion, yet its role during the disk-embedded stage remains poorly constrained. We develop a time-dependent model that couples solid accretion, nebular-gas accretion, and water enrichment and partitioning through magma-atmosphere interactions, along with post-disk thermal evolution and escape. We find that, for super-Earth-mass planets, water production is generally limited by the magma oxygen budget and typically ceases before disk dispersal. Subsequent nebular-gas accretion dilutes the envelope toward hydrogen-dominated compositions, largely independent of the initial magma redox state. This establishes an upper bound on the envelope water fraction -- the oxygen exhaustion limit -- primarily set by the reactive-oxygen inventory and the planet mass. After disk dispersal, degassing increases the water fraction only in Earth-mass planets undergoing strong escape, while super-Earths exhibit little change because surface pressures are hardly affected by escape. Magma-atmosphere coupling alone therefore cannot maintain water-rich envelopes in sub-Neptunes and produces a strong mass-composition relation imposed by the oxygen exhaustion limit. Highly enriched sub-Neptunes would therefore imply additional mechanisms such as late volatile delivery or post-disk giant impacts. The relation between planetary radius and envelope composition offers a means to infer magma properties, providing a pathway to connect present-day observables with early formation histories.
Thermochemistry and vertical mixing in the tropospheres of Uranus and Neptune: How convection inhibition can affect the derivation of deep oxygen abundances
Escape of Water- and Metal-enriched Atmospheres from compact Hot mini-Neptunes with CHAIN
University of Bern（伯尔尼大学）；Space Research Institute, Austrian Academy of Sciences（奥地利科学院空间研究所）；European Space Agency (ESA), European Space Research and Technology Centre (ESTEC)（欧洲空间局（ESA）欧洲空间研究与技术中心（ESTEC））；Department of Astronomy & Astrophysics, University of Chicago（芝加哥大学天文与天体物理系）
Contrasting C/O ratios in Uranus and Neptune from disequilibrium chemistry: A clue to distinct evolutionary pathways?
Aix-Marseille Université（艾克斯-马赛大学）；Southwest Research Institute（西南研究院）；Laboratoire d’Astrophysique de Bordeaux, Univ. Bordeaux（波尔多大学天体物理实验室）；Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université Paris Cité（巴黎天文台、巴黎文理研究大学、法国国家科学研究中心、索邦大学、巴黎西岱大学）；University of Science and Technology of China（中国科学技术大学）；Dublin Institute for Advanced Studies（都柏林高等研究所）；Space Science Institute（空间科学研究所）
JWST NIRSpec G395H Transmission Spectra of Two Similar Sub-Neptunes in the TOI-125 System
Johns Hopkins University（约翰斯·霍普金斯大学）；Space Telescope Science Institute（太空望远镜科学研究所）；University of Oxford（牛津大学）；Cavendish Laboratory, JJ Thomson Avenue, Cambridge CB3 0HE, UK（剑桥大学卡文迪许实验室）；University of California San Diego（加利福尼亚大学圣地亚哥分校）
Lithium Enrichment in a Subgiant Star with a Brown Dwarf Companion: A Planetary Engulfment Candidate
University of Michigan（密歇根大学）；University of Wisconsin–Madison（威斯康星大学麦迪逊分校）；Massachusetts Institute of Technology（麻省理工学院）；University of California, Santa Cruz（加州大学圣克鲁兹分校）；Texas Advanced Computing Center, University of Texas, Austin（德克萨斯大学奥斯汀分校高级计算中心）；University of Wisconsin-Whitewater（威斯康星大学惠特沃分校）
