High-resolution doppler spectroscopy provides a powerful means for chemical detections in exoplanetary atmospheres. This approach involves monitoring hundreds of molecular lines in the planetary spectrum doppler shifted by the orbital motion of the planet. The molecules are detected by cross-correlating the observed spectrum of the system with a model planetary spectrum. The method has led to molecular detections of H 2 O, CO, and TiO in hot Jupiters using large ground-based telescopes. Critical to this method, however, is the accurate removal of the stellar and telluric features from the observed spectrum, also known as detrending. Previous molecular detections have relied on specific choices of detrending methods and parameters. However, the robustness of molecular detections across the different choices has not been investigated in detail. We conduct a systematic investigation of the effect of detrending algorithms, parameters, and optimizations on chemical detections using high-resolution spectroscopy. As a case study, we consider the hot Jupiter HD 189733 b. Using multiple methods, we confirm high-significance detections of H 2 O (4.8σ) and CO (4.7σ). Additionally, we report evidence for HCN at high significance (5.0σ). On the other hand, our results highlight the need for improved metrics and extended observations for robust confirmations of such detections. In particular, we show that detection significances of 4σ can be obtained by optimizing detrending at incorrect locations in the planetary velocity space; such false positives can occur in nearly 30% of cases. We discuss approaches to help distinguish molecular detections from spurious noise.

Molecular species in planetary atmospheres provide key insights into their atmospheric processes and formation conditions. In recent years, high-resolution Doppler spectroscopy in the near-infrared has allowed detections of H 2 O and CO in the atmospheres of several hot Jupiters. This method involves monitoring the spectral lines of the planetary thermal emission Doppler-shifted due to the radial velocity of the planet over its orbit. However, aside from CO and H 2 O, which are the primary oxygen-and carbon-bearing species in hot H 2 -rich atmospheres, little else is known about molecular compositions of hot Jupiters. Several recent studies have suggested the importance and detectability of nitrogen-bearing species in such atmospheres. In this Letter, we confirm potential detections of CO and H 2 O in the hot Jupiter HD 209458b using high-resolution spectroscopy. We also report a cross-correlation peak with a signal-to-noise ratio of 4.7 from a search for HCN. The results are obtained using high-resolution phase-resolved spectroscopy with the Very Large telescope CRyogenic high-resolution InfraRed Echelle Spectrograph (VLT CRIRES) and standard analysis methods reported in the literature. A more robust treatment of telluric contamination and other residuals would improve confidence and enable unambiguous molecular detections. The presence of HCN could provide constraints on the C/O ratio of HD 209458b and its potential origins.

High-resolution spectroscopy (HRS) has been used to detect a number of species in the atmospheres of hot Jupiters. Key to such detections is accurately and precisely modelled spectra for cross-correlation against the R ≳ 20 000 observations. There is a need for the latest generation of opacities which form the basis for high signal-to-noise detections using such spectra. In this study we present and make publicly available cross-sections for six molecular species, H2O, CO, HCN, CH4, NH3, and CO2 using the latest line lists most suitable for low- and high-resolution spectroscopy. We focus on the infrared (0.95–5 μm) and between 500 and 1500 K where these species have strong spectral signatures. We generate these cross-sections on a grid of pressures and temperatures typical for the photospheres of super-Earth, warm Neptunes, and hot Jupiters using the latest H2 and He pressure broadening. We highlight the most prominent infrared spectral features by modelling three representative exoplanets, GJ 1214 b, GJ 3470 b, and HD 189733 b, which encompass a wide range in temperature, mass, and radii. In addition, we verify the line lists for H2O, CO, and HCN with previous high-resolution observations of hot Jupiters. However, we are unable to detect CH4 with our new cross-sections from HRS observations of HD 102195 b. These high-accuracy opacities are critical for atmospheric detections with HRS and will be continually updated as new data become available.

High-resolution Doppler spectroscopy has been used to detect several chemical species in exoplanetary atmospheres. Such detections have traditionally relied on cross correlation of observed spectra against spectral model templates, an approach that is successful for detecting chemical species but not optimised for constraining abundances. Recent work has explored ways to perform atmospheric retrievals on high-resolution spectra (HRS) and combine them with retrievals routinely performed for low-resolution spectra (LRS) by developing a mapping from the cross correlation function to a likelihood metric. We build upon previous studies and report HyDRA-H, a hybrid retrieval code for simultaneous analysis of low-and high-resolution thermal emission spectra of exoplanets in a fully Bayesian approach. We demonstrate HyDRA-H on the hot Jupiter HD 209458b as a case study. We validate our HRS retrieval capability by confirming previous results and report a simultaneous hybrid retrieval using both HRS and LRS data. The LRS data span the HST WFC3 (1.1-1.7 µm) and Spitzer photometry (IRAC 3.6-8µm) bands, while the HRS data were obtained with CRIRES on VLT at 2.3 µm. The constraints on the composition and temperature profiles for the hybrid retrieval are more stringent than retrievals with either LRS or HRS datasets individually. We retrieve abundances of log(H 2 O) = −4.11 +0.91 −0.30 and log(CO) = −2.16 +0.99 −0.47 , and C/O = 0.99 +0.01 −0.02 , consistent with previous works. We constrain the photospheric temperature to be 1498 +216 −57 K, consistent with the equilibrium temperature. Our results demonstrate the significant advantages of hybrid retrievals by combining strengths of both HRS and LRS observations which probe complementary aspects of exoplanetary atmospheres.

We research the requirements of High Contrast Imaging when combined with the cross correlation (CC) of high resolution spectra with known spectroscopic templates for detecting and characterising exoplanets in reflected light. We simulate applying the technique to a potentially habitable Proxima b-like planet and show that the O 2 A-band spectral feature could feasibly be detected on nearby rocky exoplanets using future instruments on the ELT . The technique is then more widely analysed showing that detections of planets and O 2 with signal to noise in the CC function ( SNR CC ) > 3 can be obtained when the signal to noise of the simulated planet spectrum ( SNR spec ) is from 0.25 to 1.2. We place constraints on the spectral resolution, instrument contrast, point spread function (PSF), exposure times and systematic error in stellar light subtraction for making such detections. We find that accurate stellar light subtraction (with 99.99% removal) and PSFs with high spatial resolutions are key to making detections. Lastly a further investigation suggests the ELT could potentially discover and characterise planets of all sizes around different spectral type stars, as well as detecting O 2 on Super-Earths with habitable zone orbits around nearby M stars.