Kepler-51d’s Low Density Atmosphere Explained by Hazes or Rings.

Observations of Kepler-51d, a low-density super-puff exoplanet, reveal a low-metallicity atmosphere containing high-altitude hazes of submicron particles, or potentially a tilted ring system with a short lifespan. The host star exhibits hotter and more extensive starspots than our Sun.

The exceptionally low density of exoplanet Kepler-51d has challenged conventional models of planetary formation and atmospheric composition. This gas giant, orbiting a Sun-like star 500 million years old, possesses a density so slight it has prompted investigation into explanations ranging from extensive hydrogen-helium envelopes to the presence of substantial ring systems. New observations utilising the James Webb Space Telescope’s Near-Infrared Spectrograph (NIRSpec) have provided crucial data to constrain these possibilities. The research, detailed in a paper titled ‘The James Webb Space Telescope NIRSpec-PRISM Transmission Spectrum of the Super-Puff, Kepler-51d’, is the work of Jessica E. Libby-Roberts, Caleb I. Cañas, Aaron Bello-Arufe, Zachory K. Berta-Thompson, Yayaati Chachan, Renyu Hu, Yui Kawashima, Catriona Murray, Kazumasa Ohno, Armen Tokadjian, Suvrath Mahadevan, Kento Masuda, Leslie Hebb, Caroline Morley, Guangwei Fu, Peter Gao, and Kevin B. Stevenson, representing institutions including The Pennsylvania State University, the Jet Propulsion Laboratory, and the California Institute of Technology.

Observations from the James Webb Space Telescope’s Near-Infrared Spectrograph (NIRSpec) instrument are providing new insights into the exceptionally low-density exoplanet Kepler-51d, challenging conventional planet formation models. Researchers analysed the planet’s transmission spectrum – the alteration of starlight as it passes through the planet’s atmosphere – to determine atmospheric composition and structure, seeking to understand the origins of this unusual world. This investigation reveals a spectrum best characterised by a linear slope, prompting detailed analysis of potential atmospheric constituents and configurations.

Kepler-51d orbits a young G-type star and presents a significant challenge to current planetary formation theories due to its unusually large radius of 9.32 Earth radii and low mass of 5.6 Earth masses, resulting in a remarkably low density of just 0.038 g/cm³. Researchers meticulously analysed the transmission spectrum, spanning wavelengths from 0.6 to 5.3 microns, employing both forward modelling and atmospheric retrievals to interpret the observed data. The analysis reveals a spectrum best characterised by a linear slope, indicating a relatively featureless absorption profile and suggesting the presence of either a hazy atmosphere or a tilted ring system.

Forward modelling and atmospheric retrievals indicate the presence of a low-metallicity atmosphere, coupled with high-altitude hazes composed of submicron-sized particles, effectively scattering light and contributing to the observed spectral slope. These hazes extend across a pressure range of 1-100 microbars, creating a diffuse scattering layer that dominates the transmission spectrum.

However, the observed spectrum also admits an alternative explanation: a tilted ring system surrounding Kepler-51d. Modelling demonstrates that a ring system can also reproduce the observed spectral slope, although such a configuration is predicted to have a relatively short lifespan – approximately 0.1 million years, raising questions about its long-term stability. Researchers explored a range of ring parameters, including inclination, particle size, and optical depth, to determine the best fit to the observed data, carefully considering the physical plausibility of each configuration.

Investigation of the host star, Kepler-51, indicates significant stellar activity, influencing the observed planetary signal and necessitating careful consideration during data analysis. Researchers determined that Kepler-51 exhibits starspots considerably hotter than those observed on our Sun, with covering fractions ranging from 1 to 10 per cent, dependent on assumed parameters. Careful modelling of stellar activity is crucial for accurately interpreting the planetary transmission spectrum, as stellar features can mimic or mask atmospheric signals, and researchers employed sophisticated techniques to remove the effects of stellar activity from the observed data.

Future observations with higher spectral resolution and sensitivity will be crucial for definitively determining the nature of Kepler-51d’s atmosphere, seeking to identify specific spectral features that could differentiate between a hazy atmosphere and a tilted ring system. Researchers plan to utilise the James Webb Space Telescope to obtain additional observations of Kepler-51d, focusing on searching for absorption features associated with specific atmospheric constituents. Additionally, long-term monitoring of Kepler-51d will be necessary to assess the stability of any potential ring system.

The combination of high-resolution spectroscopy and long-term monitoring will provide a comprehensive understanding of Kepler-51d’s atmosphere, revealing the secrets of this unusual exoplanet and shedding light on the diversity of planetary systems beyond our own.