The Dense Volatile Atmosphere and Deep Mantle of HD 21749 b
Orbiting an orange K-type dwarf star approximately 53 light-years away in the southern constellation Reticulum , the exoplanet HD 21749 b occupies a critical structural niche in contemporary planetary science. First identified in photometric data collected by NASA’s Transiting Exoplanet Survey Satellite (TESS) and subsequently confirmed through precision radial velocity measurements from the High Accuracy Radial velocity Planet Searcher (HARPS) and the Planet Finder Spectrograph (PFS), this body serves as a foundational benchmark for the sub-Neptune regime . Unlike many previously characterized sub-Neptunes that exhibit inflated, puffy outer envelopes due to fierce stellar irradiation, HD 21749 b maintains a remarkably compact and dense architecture that challenges early models of intermediate-mass planetary structure. With a physical radius measured at roughly 2.61 times that of Earth and an astonishing mass of approximately 22.7 Earth masses, HD 21749 b displays a bulk density nea...