High-resolution spectroscopy
High-resolution spectroscopy (HRS) changed exoplanet atmospheric work by resolving molecular bands into individual lines. Telluric and stellar contamination can be separated in velocity space, and the Doppler motion of the planet itself becomes part of the measurement. The landmark CRIRES detection of CO in HD 209458b (Snellen et al. 2010), dayside emission from non-transiting τ Boötis b (Brogi et al. 2012), and ground-based water in HD 189733b (Birkby et al. 2013) established the toolkit. For imaged planets, the same approach delivered the spin of β Pic b (Snellen et al. 2014) and, later, the first exoplanet isotope ratio (Zhang et al. 2021).
Snellen’s 2025 Annual Review frames where the field stands now: HRS remains uniquely competitive alongside JWST, retrievals of hot and super-Jupiters increasingly point toward solar-like chemistry (though samples are still incomplete), and minor isotopes of C and O are emerging as formation diagnostics. The review also stresses the pitfalls — correlated noise, overconfident posteriors, and model choices that can bias abundances. That caution is not abstract; it shapes how I design observations and analyses.
Instruments I use
VLT/CRIRES+ ($R\sim10^{5}$) is the natural successor to the CRIRES programmes above. The K-band CO overtone near $2.3\,\mu\mathrm{m}$ is where carbon isotopes become practical for cool, self-luminous atmospheres, which is why it anchors the ESO SupJup Survey and my β Pic b campaign.
JWST/NIRSpec ($R\sim2700$) does not resolve lines as finely, but it covers $1$–$5\,\mu\mathrm{m}$ continuously — including the CO fundamental near $4.6\,\mu\mathrm{m}$. That baseline is ideal for joint atmosphere and disc modelling, and for high-contrast companions where residual speckles warp the continuum.
SPIRou at CFHT provides archival K-band spectra of nearby M dwarfs at the S/N needed for rare isotopologues — the stellar counterpart to the planetary isotope programme (Cristofari et al. 2022 developed the SPIRou parameter framework that makes such samples usable).
What I add methodologically
I couple petitRADTRANS (Mollière et al. 2019) to nested sampling and apply it at native resolution, rather than stopping at cross-correlation detections. For close-in companions I weight spectral extraction by cross-correlation strength after modelling residual starlight. Multi-epoch CRIRES+ data are retrieved night by night so systematic scatter is not mistaken for precision. With JWST, I fit residual IFU speckles or disc continuum jointly with the atmosphere, following the spirit of the Annual Review’s warning against overconfident retrievals.
The goal is straightforward: keep the resolving power that made HRS powerful, while making isotope and abundance posteriors honest enough to compare across objects.