Before JWST, there was Hubble: with the good coverage of its Wide Field Camera 3 (WFC3) in the near infrared (0.8–1.7 µm), it was the reference telescope for the characterization of exoplanetary atmospheres. JWST has a wider wavelength range and higher precision than Hubble, but how much of that effectively translates into better results, and which is the main improving factor? Chloe Fisher, Jake Taylor and colleagues perform a systematic comparison to answer this question.

The authors analyse spectra of the reference hot Jupiter WASP-39 b obtained by both WFC3 and JWST/NIRISS in its single object slitless mode (0.6–2.8 µm), using the same state-of-the-art Bayesian model. They look in particular at the water bands, the only ones detectable in the WFC3 range. NIRISS does indeed do a better job at constraining both the water detection confidence (21.3σ versus 9.0σ) and its abundance, which is substantially overestimated by WFC3. In addition, NIRISS can tentatively detect other species (CO₂, K) and favours the non-isothermal + non-grey cloud model of the atmospheric structure, whereas WFC3 is not able to distinguish between different atmospheric profiles. The authors show that the dominant factor in this improvement is the extended NIRISS wavelength range, which provides more reliable continuum information.

在 JWST 之前，有哈勃望远镜：凭借其宽视场相机 3 （WFC3） 在近红外 （0.8-1.7 μm） 的良好覆盖范围，它是表征系外行星大气的参考望远镜。JWST 比哈勃望远镜具有更宽的波长范围和更高的精度，但其中有多少能有效地转化为更好的结果，哪个是主要的改进因素？Chloe Fisher、Jake Taylor 及其同事进行了系统比较来回答这个问题。

作者使用相同的最先进的贝叶斯模型分析了 WFC3 和 JWST/NIRISS 在其单目标无狭缝模式 （0.6–2.8 μm） 下获得的参考热木星 WASP-39 b 的光谱。他们特别关注水带，这是 WFC3 范围内唯一可以检测到的水带。NIRISS 在限制水检测置信度（21.3σ 对 9.0σ）及其丰度方面确实做得更好，WFC3 大大高估了这一水平。此外，NIRISS 可以初步检测其他物质 （CO₂， K） 并倾向于大气结构的非等温 + 非灰色云模型，而 WFC3 无法区分不同的大气剖面。作者表明，这种改进的主导因素是扩展的 NIRISS 波长范围，它提供了更可靠的连续谱信息。