The carbon/oxygen (C/O) ratio in exoplanetary atmospheres is often used as a diagnostic tool for the formation history of giant planets. Chih-Chun Hsu and colleagues measure the C/O ratio of the protoplanet PDS 70 b, which is still embedded in the cavity of its protoplanetary disc and thus allows us, together with the other protoplanet PDS 70 c, to test our theories of in situ planetary formation directly.

The authors used the newly upgraded Keck Planet Imager and Characterizer (KPIC) to extract spectra of PDS 70 b between 2.29 and 2.49 μm at high spectral resolution. The spectra are rather noisy, but the authors could detect combined CO+H₂O molecules at 4.2 σ and constrain the C/O ratio to\({\,0.28}_{-0.12}^{+0.20}\) (≲0.63 at 95% confidence level).

系外行星大气中的碳/氧 （C/O） 比值通常被用作巨行星形成历史的诊断工具。Chih-Chun Hsu 和同事测量了原行星 PDS 70 b 的 C/O 比，它仍然嵌入其原行星盘的空腔中，因此使我们能够与另一颗原行星 PDS 70 c 一起直接测试我们的原位行星形成理论。

作者使用新升级的 Keck 行星成像仪和表征仪 （KPIC） 以高光谱分辨率提取 2.29 至 2.49 μm 之间的 PDS 70 b 光谱。光谱相当嘈杂，但作者可以在 4.2 σ处检测到 CO+H₂O 分子的组合，并将 C/O 比限制为\（{\，0.28}_{-0.12}^{+0.20}\）（在 95% 置信度下为 ≲0.63）。