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.