Atmospheric characterization of HIP 67522 b with VLT/CRIRES+. VLT/CRIRES+ suggests a heavier planet and hints at deuterium fractionation

TL;DR

This study uses VLT/CRIRES+ high-resolution transmission spectroscopy to characterize the atmosphere of HIP 67522 b, detecting H2O at $20\sigma$ and CO at $5\sigma$ and inferring a day-to-night wind of $-2.9 \pm 0.2$ km s$^{-1}$ with an isothermal atmosphere. Bayesian retrievals yield a planet mass of $29.8 \pm 2.9$ $M_\oplus$ and a C/O ratio of $0.83 \pm 0.09$, though JWST-based mass estimates imply a discrepancy that remains unresolved without joint analyses; a subsolar $[\mathrm{C+O/H}] = -0.8 \pm 0.4$ is also inferred with cloud degeneracies discussed. A tentative HDO detection at 2σ suggests an extreme D/H enrichment (~1000) and potential strong atmospheric escape, requiring confirmation and further observations. Overall, the work demonstrates the power of ground-based high-resolution spectroscopy to probe extended, young exoplanet atmospheres and highlights tensions with space-based results that motivate integrated, multi-instrument analyses.

Abstract

Young transiting exoplanets provide unique opportunities to probe planetary atmospheres during the critical early phases of evolution. HIP~67522~b, a 17~Myr old hot Jupiter with an extraordinarily low bulk density, represents an ideal target for high-resolution transmission spectroscopy. We aim to characterize the atmospheric composition, thermal structure, and dynamics of HIP~67522~b using ground-based high-resolution near-infrared spectroscopy. We obtained high-resolution spectra with VLT/CRIRES+ in the K2166 band during a transit on 30 January 2025. We applied cross-correlation techniques and Bayesian nested sampling retrievals to constrain molecular abundances, temperature structure, and atmospheric dynamics. We detect H$_2$O at 20$σ$ and CO at 5$σ$, confirming the extremely extended atmosphere of this low-mass giant. A velocity offset of $-2.9 \pm 0.2$~km~s$^{-1}$ indicates day-to-night winds. The rotation velocity is constrained to $<1.8$~km~s$^{-1}$ at 3$σ$, consistent with tidal locking. Retrieval analysis suggests a planetary mass of 29.8 $\pm$ 3 Earth masses and a vertically isothermal atmosphere. This mass is two times larger than the mass estimated from JWST atmospheric observations and inconsistent at 3$σ$ hence leaving a doubt on the actual planetary density of the planet. Using the mass derived derived from the CRIRES+ data, we derive a C/O ratio of $0.83 \pm 0.09$, about 1.5 times solar, and a subsolar metallicity [C+O/H]~$= -0.8 \pm 0.4$ which can be increased if the atmosphere is cloudy, a degeneracy our data alone cannot resolve. We report a tentative 2$σ$ detection of HDO with an extreme enrichment factor of $\sim$1000 relative to the protosolar D/H ratio. If confirmed, this would be the first detection of deuterium in an exoplanet atmosphere and would require intense escape rate to be explained.

Figures (7)

• Figure 1: Evolution of the transit window, the median SNR per exposure, seeing, and airmass as a function of time from the first exposure. The median SNR and median seeing for the full time-series are indicated in red on the second and third panel respectively. The UT time at the start of the first exposure $t_0$ is indicated on the fourth panel.
• Figure 2: Model isothermal spectra for each of the chemical species we investigated in the atmosphere of HIP 67522 b. The gray shaded areas represent the wavelength coverage of CRIRES+ with six spectral orders each spread over three detectors.
• Figure 3: Top panel: Phase-$v_\textrm{sys}$ maps for the detected species H$_2$O (left) and CO (right) in the stellar reference frame. The red dashed line indicate the predicted velocity trail of the planet. The colorbar are expressed in standard deviations away from both planet and tellurics signals (excluding the square in the K$_p$-V$_\mathrm{sys}$ map defined by K$_p \in [0,300]$ and V$_\mathrm{sys} \in [-10,30]$). Bottom panel: K$_p$-$v_{\textrm{sys}}$ map for H$_2$O (left) and CO (right), the vertical dashed line indicate stellar reference frame, the horizontal dashed line indicated K$_p = 120$ km s$^{-1}$. The maximum value of the map is indicated by the + symbol in black with the associated value, the minimum is indicated by the blue cross.
• Figure 4: Corner plot of the posterior distribution of our isothermal models using a fixed mass of 29.8 Earth masses.
• Figure 5: Posterior distribution for the 4 point temperature-pressure (TP) profile. The solid vertical line indicates the median temperature at a given pressure, the grey shaded lines define the 1$\sigma$, 2$\sigma$, and 3$\sigma$ confidence interval (from dark grey to light gray).