Probing Superhabitable Worlds: Modeling Exoplanetary Atmospheres for simulated JWST Observations

In our search for life in the Universe, there may be planetary bodies that are more conducive to life than Earth. Even Earth's habitability has varied enormously throughout the eons. We call worlds that are more habitable than Earth today ‘superhabitable’. In the pursuit of superhabitable worlds, K dwarf stars emerge as promising candidates due to their stable luminosity evolution, offering prolonged stability within the habitable zone conducive to life's emergence and sustainability (Arney 2019; Heller and Armstrong 2014; Schulze-Makuch, Heller, and Guinan 2020). A planet with up to 2 times Earth’s mass and around 25% larger than Earth orbiting such a K dwarf star could qualify as superhabitable. Such dimensions would result in surface pressures of roughly 1.2 bar, exceeding Earth's current pressure by 20%. This configuration would offer expanded living space and enable the retention of a denser atmosphere, providing the necessary mass and energy to support a more extensive biosphere. Moreover, the denser atmosphere would enhance detectability through remote observations. A superhabitable planet would also likely exhibit a slightly warmer climate than present-day Earth, with temperatures elevated by approximately 5 degrees Celsius. This modest increase aligns with historical trends, as biodiversity flourished during warmer epochs, with tropical zones hosting the majority of Earth's current biodiversity (Vilović, Schulze-Makuch, and Heller 2023). In terms of atmospheric compositions, such planets would exhibit heightened oxygen concentrations which contribute to expanded metabolic networks and support larger body sizes among organisms. 

In our most recent study, we tested the effects of simulated K-dwarf radiation on the phototrophic organisms garden cress and cyanobacteria using an LED stellar simulator. We found that both organisms are capable of growing under this modified radiation environment, with cyanobacteria exhibiting significantly better responses to K dwarf compared to solar radiation (Vilović et al. 2024). Expanding upon these laboratory results, we now turn to theoretical models to assess the detectability of superhabitability with the James Webb Space Telescope (JWST). We combine the results of the 1D coupled climate-photochemistry model Atmos for modeling superhabitable atmospheres (Kopparapu et al. 2013) as well as the POSEIDON forward modeling code to calculate synthetic planetary spectra (MacDonald and Madhusudhan 2017; MacDonald 2023), with the PandExo tool for simulating observations of transiting exoplanets with the JWST (Batalha et al. 2017). Preliminary results indicate that superhabitable conditions positively impact the observability of key spectral features, including the oxygen features at 0.69, 0.77 and 1.24 micrometers, as well as the carbon dioxide feature at 4.3 micrometers and the ozone feature at 9.6 micrometers. Furthermore, these spectral features may require fewer transits for detection with the JWST compared to a modern Earth counterpart. This underscores the importance of prioritizing exoplanets orbiting K dwarf stars within the center of their habitable zones in our search for life outside of the Solar System using state of the art instrumentation. 

 

References

Arney, Giada N. 2019. “The K Dwarf Advantage for Biosignatures on Directly Imaged Exoplanets.” The Astrophysical Journal Letters 873 (1): L7.

Batalha, Natasha E., Avi Mandell, Klaus Pontoppidan, Kevin B. Stevenson, Nikole K. Lewis, Jason Kalirai, Thomas Greene, Loïc Albert, Louise D. Nielsen, and Nick Earl. 2017. “PandExo: A Community Tool for Transiting Exoplanet Science with JWST & HST.” arXiv [astro-ph.IM]. arXiv. http://arxiv.org/abs/1702.01820.

Heller, René, and John Armstrong. 2014. “Superhabitable Worlds.” Astrobiology. https://doi.org/10.1089/ast.2013.1088.

Kopparapu, Ravi Kumar, Ramses Ramirez, James F. Kasting, Vincent Eymet, Tyler D. Robinson, Suvrath Mahadevan, Ryan C. Terrien, Shawn Domagal-Goldman, Victoria Meadows, and Rohit Deshpande. 2013. “Habitable Zones around Main-Sequence Stars: New Estimates.” The Astrophysical Journal 765 (2): 131.

MacDonald, Ryan J. 2023. “POSEIDON: A Multidimensional Atmospheric Retrieval Code for Exoplanet Spectra.” Journal of Open Source Software 8 (81): 4873.

MacDonald, Ryan J., and Nikku Madhusudhan. 2017. “HD 209458b in New Light: Evidence of Nitrogen Chemistry, Patchy Clouds and Sub-Solar Water.” Monthly Notices of the Royal Astronomical Society 469 (August): 1979–96.

Schulze-Makuch, Dirk, René Heller, and Edward Guinan. 2020. “In Search for a Planet Better than Earth: Top Contenders for a Superhabitable World.” Astrobiology 20 (12): 1394–1404.

Vilović, Iva, Dirk Schulze-Makuch, and René Heller. 2023. “Variations in Climate Habitability Parameters and Their Effect on Earth’s Biosphere during the Phanerozoic Eon.” Scientific Reports 13 (1): 12663.

Vilović, I., Schulze-Makuch, D. & Heller, R. (2024). Observation of Significant Photosynthesis in Garden Cress and Cyanobacteria under Simulated Illumination from a K Dwarf Star. International Journal of Astrobiology. (In Review)