A recent study led by graduate student Pa Chia Thao has unveiled exciting new findings on a 17-million-year-old exoplanet, HIP 67522 b, using cutting-edge data from the James Webb Space Telescope (JWST). Originally classified as a “hot Jupiter” due to its size – about 10 times that of Earth – and its short orbital period of less than 7 days, the planet’s atmosphere tells a different story. Using transmission spectroscopy, a technique that analyzes how starlight filters through a planet’s atmosphere during transit, researchers can probe the atmosphere’s composition and structure by observing how transit depths vary across different wavelengths.
image credit: NASA/JPL-Caltech/R. Hurt (IPAC).
Signatures of water and carbon dioxide were detected, with absorption features 30-50% deeper than expected. By analyzing the strength of these spectral features and the planet’s atmospheric scale height, they determined that HIP 67522 b has a mass less than 15 times that of Earth – reclassifying this planet as a precursor to the more common sub-Neptune planets. Despite being about the size of Jupiter, the planet’s density is comparable to that of cotton balls. This revised mass also confirms HIP 67522 b as one of the least dense planets ever discovered. The planet is likely undergoing significant mass loss – between 0.01 and 0.03 Earth masses per million years –potentially leading to the destruction of its envelope within a billion years. These findings highlight the rapid and dramatic evolution taking place within the first 100 million years of the planet’s existence.
The attached plot shows the transmission spectrum of HIP 67522 b, observed with SOAR and JWST, compared to three atmospheric models: a clear model (purple), a photochemical model (pink), and a cloudy model (red). The strength of the molecular features in the spectrum constrains the planet’s mass to an upper limit of 15 Earth masses – significantly lower than expected for a planet of Jovian size (R = 10 Earth radii).
The study was published in Astrophysical Journal (https://iopscience.iop.org/article/10.3847/1538-3881/ad81d7).