Hydrocarbon Hazes on the Sub-Neptune K2-18 b?
Super-Earths and sub-Neptunes (planets with radii between 1 and 1.8 Earth radii, and between 1.8 and 3.5 Earth radii, respectively) rank among the most common exoplanets out of the more than 6400 planets discovered to date. Nonetheless, their internal composition remains largely elusive, and various models have been proposed so far: rocky cores enveloped by extended primordial atmospheres dominated by molecular hydrogen (H2), planets with a Neptune-like bulk composition, or volatile-rich “water worlds”.
Any exoplanet whose atmospheric composition and physical structure can be characterized becomes critical to understanding the true nature of this class of planets. This can be achieved through the transmission spectroscopy technique, which compares spectroscopic observations of the host star taken at two distinct epochs: during the planet’s transit across our line of sight and out of transit. During transit, a fraction of the starlight filters through the planetary atmosphere and is absorbed at specific wavelengths by the molecules populating it.
A research team led by astrophysicist Rouhan Liu from University College London analyzed transmission spectroscopy observations of the temperate sub-Neptune K2-18 b acquired with the James Webb Space Telescope (JWST).
Using advanced atmospheric models to interpret the spectra gathered by the MIRI, NIRISS, and NIRSpec instruments, the authors identified robust evidence supporting a molecular-hydrogen-rich atmosphere characterized by photochemical hydrocarbon hazes, alongside significant abundances of methane and carbon dioxide—reminiscent of the hazes found in Titan’s atmosphere.
The study is detailed in the paper “Hydrocarbon Hazes on Temperate Sub-Neptune K2-18b Supported by Data from the James Webb Space Telescope“ published in The Astronomical Journal. Among the co-authors is astrophysicist Jesús Maldonado of the INAF – Palermo Astronomical Observatory.
The cover image (click here to view it in full) shows an artist’s impression of the planet K2-18 b in the upper panel; the lower panel displays the spectrum obtained with NIRISS, highlighting the molecules and atoms responsible for the observed signal.
