2026年4月14日的 ESA 新闻稿指出，Webb 观测了 29 Cygni b，这是一个约为木星质量 15 倍（15 M_J）的天体，绕一颗附近恒星运行，平均距离约 24 亿公里（约 16 AU，与天王星轨道相近），正位于行星类形成与恒星类形成边界重叠的区域，研究团队认为其起源更像行星而非恒星。此结果重要，因为标准模型中微小岩冰颗粒逐步聚集为颗粒与原行星再吸积形成大行星；当质量过高时自下而上形成越难，而盘碎裂可在盘外缘形成更高质量天体，这解释了两种形成机制的过渡区。

观测计划（编号 6905）使用 Webb 的 NIRCam 于日冕阻挡模式直接成像 4 个年轻目标，每个目标质量介乎 1–15 倍木星质量，且轨道距离主星都在约 150 亿公里（1.5×10^10 km）内。目标温度为 530–1000°C（约 803–1273 K）。研究者透过 CO2 与 CO 吸收线滤镜测得金属元素丰度，发现 29 Cygni b 相对于其类太阳恒星明显富金属，等效重元素含量约为 150 个地球质量，这不易由直接引力塌缩解释，反而符合快速吸积模型。

另一项由 CHARA（高角解析光学阵列）进行的测量显示，29 Cygni b 的轨道平面与恒星自转轴方向一致，这是原行星盘形成天体所预期的几何关系。结合化学成分与轨道对齐证据，团队认为该天体是在原行星盘中透过核心吸积形成，而非自上而下碎裂。研究团队将把同样方法套用至另外 3 颗行星，检验高、低质量物体是否存在组成差异，并测试与形成机制的关联。

A 14 April 2026 ESA release reports that Webb targeted 29 Cygni b, a ~15-Jupiter-mass body orbiting a nearby star at ~2.4×10^9 km (~16 AU, close to Uranus’ orbit), the region where planet-like and star-like formation boundaries overlap, and the team concluded it is more planet-like in origin than star-like. This is significant because standard models explain planetary growth as solid grains coagulating into pebbles, then protoplanets, then gas-giant accretion, while very massive objects near the upper end become harder to build by this bottom-up route and can also be explained by disk fragmentation.

The observing programme (ID 6905) used Webb’s NIRCam in coronagraphic mode to directly image four young targets with masses between 1 and 15 Jupiter masses, each orbiting within ~1.5×10^10 km of host stars. Their temperatures were 530–1000°C (~803–1273 K). CO2 and CO absorption filters were used to derive metallicity, and 29 Cygni b was found metal-enriched relative to its Sun-like host, corresponding to roughly 150 Earth masses of heavy elements—an amount difficult to reconcile with direct collapse alone and more consistent with rapid accretion.

A companion CHARA interferometric study found 29 Cygni b’s orbital plane aligned with its star’s spin axis, matching expectations for disk-born planets. Combining chemistry and orbital alignment, the team argues that 29 Cygni b formed through core accretion in a protoplanetary disk rather than top-down fragmentation. The team plans to apply this approach to the other three targets to test whether high- and low-mass objects show compositional differences linked to different formation pathways.