A Baby Hot Jupiter: Atmosphere, Winds, and a Hint of Heavy Water express
Planet HIP 67522 b is quite young: it's only 17 million years old, compared to Earth's 4.5 billion. It's a hot Jupiter — a gas giant that orbits very close to its star, so its atmosphere is heated and puffed up. Astronomers pointed the Very Large Telescope (VLT) in Chile at it and, during a transit across the star's disk, captured light that passed through the planet's atmosphere.
In that light, like a rainbow, the 'fingerprints' of molecules remained. The scientists found water and carbon monoxide — the water signal is 20 times stronger than the noise, a very confident detection. By measuring the shift of spectral lines, they determined that the atmosphere is moving: winds blow from the day side to the night side at about 3 km/s. The planet itself rotates slowly, as if it always shows the same face to its star, like the Moon does to Earth.
Then came the surprises. According to this telescope's data, the planet's mass is about 30 Earth masses — twice as much as estimated from observations by the James Webb Space Telescope. If we trust the new number, the planet is not as 'fluffy' as previously thought. Also, the atmosphere may contain heavy water — a molecule where a heavier cousin of hydrogen, deuterium, replaces the usual hydrogen. It appears to be a thousand times more abundant than in the material that formed the Solar System. For now, this is just a hint, but if confirmed, it would be the first detection of deuterium in the atmosphere of an alien planet, and it would mean that HIP 67522 b's atmosphere is rapidly escaping into space, like steam from a boiling pot.
Original abstract
as written by the authors arXiv:2602.03498
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$$\sigma$$ and CO at 5$$\sigma$$, 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$$\sigma$$, 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$$\sigma$$ hence leaving a doubt on the actual planetary density of the planet. Using the mass 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$$\sigma$$ 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.
Heavy water, the kind where deuterium replaces ordinary hydrogen, is rare on Earth: in the oceans, only one molecule in several thousand is heavy water. On HIP 67522 b, it might be a thousand times more abundant than that.