HIP 67522 b
One of the youngest transiting planets with measured spin-orbit angle.
HIP 67522 b is a hot Neptune or sub-Saturn exoplanet that orbits a G-type star about 415 light-years away in Centaurus. It was discovered by the Transiting Exoplanet Survey Satellite (TESS). This planet is among the youngest transiting exoplanets known, and it is one of only four planets under 100 million years old—alongside AU Mic b, V1298 Tau c, DS Tuc Ab, and TOI-942 b—for which the tilt between its orbital plane and its star’s rotation has been measured. Studying it could shed light on how similar hot Neptunes develop. There are also signs that another planet may exist in the same system.
Because it is so young, HIP 67522 b has not yet settled into its final size. As it cools through the Kelvin–Helmholtz mechanism, its internal pressure drops, causing the planet to shrink. Its eventual size will depend on what its core is made of. In 2024, it was found to be one of the least dense planets known, with a density below 0.10 g/cm³. It may have formed beyond the water-snowline, where both rocky and icy materials mix. By 2025, it was confirmed that the planet triggers increased flaring from its host star through star-planet magnetic interactions, and those flares may have eroded some of the planet’s atmosphere. In 2026, observations with the Very Large Telescope (VLT) detected water vapor (H₂O) and carbon monoxide (CO) in its atmosphere with high confidence, along with day-to-night winds. Its mass was measured at 29.8 ± 3 Earth masses, a value that disagrees with an earlier estimate from the James Webb Space Telescope by three standard deviations. A tentative detection of semiheavy water (HDO) was also reported, which would mark the first discovery of deuterium in an exoplanet’s atmosphere if confirmed.
Quick Facts
- Extrasolarplanet
- yes
- Apsis
- astron
- Discoverer
- THYME (Rizzuto et al.)
- Discovered
- 2020
- Discovery Method
- Primary Transit
- Eccentricity
- 0.059 · 0.193 · 0.046
- Period
- 6.959503 · 0.000016 d
- Arg Peri
- 343.0 · 92.0 · 140.0 °
- Star
- HIP 67522
- Density
- (uncertain)
- Single Temperature
- 1174 · 21 K
Facts from the source article.
Lore & Background
HIP 67522 b was discovered using the Transiting Exoplanet Survey Satellite (TESS). Due to its young age, the planet has not reached its final size; the Kelvin–Helmholtz mechanism, occurring as the planet cools, causes its internal pressure to drop, leading to shrinkage. Its final size will depend on the composition of its core. In 2024, it was shown to be one of the least dense known planets, with a density less than 0.10 g/cm³. It might have formed beyond the water-snowline, where contamination by both rocky and icy materials frequently takes place. In 2025, it was revealed with certainty that HIP 67522 b triggers an increase in flares from the host star via star-planet magnetic interactions, which might have caused shrinkage of the planet's atmosphere. In 2026, using the VLT, the presence of water vapour (H₂O) and carbon monoxide (CO) in the planet's atmosphere was revealed with high confidence, along with day-to-night winds. The mass was measured at 29.8 ± 3 Earth masses, which is 3σ inconsistent with the previous estimate from JWST. A tentative detection of semiheavy water (HDO) was also reported, which would be the first detection of deuterium in an exoplanet's atmosphere if confirmed. There is also evidence that another planet might be present in the system.
Reader's Guide
HIP 67522 b is significant as one of the youngest transiting exoplanets known, with an age under 100 million years, placing it among a small group of only four other planets of similar youth for which the spin-orbit angle has been measured. Its extreme low density, less than 0.10 g/cm³, makes it one of the least dense planets known, offering a window into planetary composition and formation processes early in a system's history. The confirmed star-planet magnetic interactions that trigger flares from the host star provide direct evidence of such coupling in a young system, and these flares may have driven atmospheric shrinkage. The detection of water vapour and carbon monoxide in its atmosphere, along with day-to-night winds, adds to the understanding of atmospheric dynamics on young, inflated planets. The tentative detection of semiheavy water (HDO) is particularly notable, as it would mark the first detection of deuterium in an exoplanet's atmosphere if confirmed. The mass measurement, which is 3σ inconsistent with an earlier JWST estimate, highlights ongoing refinement in characterizing young planets. The possibility of an additional planet in the system further enhances its value as a laboratory for studying planetary system evolution.
Did You Know?
- It is one of only four planets less than 100 million years old with a measured spin-orbit angle.
- In 2025, it was confirmed that the planet triggers increased flares from its host star via magnetic interactions.
More in Transiting Exoplanets, Part 4 1-24
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