Artist’s depiction of the vista from the oceans of a “Hycean” exoplanet. | Credit: Amanda Smith, Nikku Madhusudhan
Hycean Exoplanets: Promising Candidates for Alien Life Detection
Hycean worlds, a theorized class of exoplanet characterized by expansive oceans and a dense hydrogen-rich atmosphere, may present optimal opportunities for the James Webb Space Telescope (JWST) to identify biosignatures, according to recent research.
Methyl Halides: Potential Indicators of Extraterrestrial Life
These potential signs of life consist of methyl halides, a group of chemical compounds produced on Earth by certain bacteria and marine algae.
Eddie Schwieterman, an astrobiologist at the University of California, Riverside, stated, “Hycean planets offer a substantially clearer signal for biosignature detection, unlike Earth-like planets where atmospheric interference and telescope limitations complicate the process.”
The Concept of Hycean Planets
The existence of hycean planets is currently theoretical. The term “hycean,” a blend of “hydrogen” and “ocean,” was first introduced in 2021 by Nikku Madhusudhan, a planetary scientist at the University of Cambridge.
vCard.red is a free platform for creating a mobile-friendly digital business cards. You can easily create a vCard and generate a QR code for it, allowing others to scan and save your contact details instantly.
The platform allows you to display contact information, social media links, services, and products all in one shareable link. Optional features include appointment scheduling, WhatsApp-based storefronts, media galleries, and custom design options.
K2-18b: A Prime Hycean World Candidate
Hycean planets are predicted to orbit red dwarf stars. The exoplanet K2-18b is considered the foremost candidate. Classified as a “sub-Neptune” world, it resides within the habitable zone of a red dwarf star, approximately 124 light-years away in the Leo constellation.
In 2019, the Hubble Space Telescope detected water vapor in K2-18b’s atmosphere. Subsequently, JWST has identified carbon dioxide and methane, coupled with a scarcity of carbon monoxide and ammonia – findings consistent with the hycean planet hypothesis. Preliminary evidence suggests the presence of dimethyl sulfide, a compound solely produced by ocean plankton on Earth, although this remains debated.
Methyl Halides as Enhanced Biosignatures on Hycean Worlds
Researchers from the University of California, Riverside, and ETH Zurich have expanded on this research. They propose that methyl halides, generated by microbial ocean life on Earth, could serve as a more readily detectable biosignature in the atmospheres of hycean worlds compared to oxygen on Earth-like planets.
Michaela Leung, the lead author of the study from the University of California, Riverside, explained, “Oxygen detection on Earth-like planets is currently challenging. However, methyl halides on hycean worlds present a distinctive and technologically feasible detection prospect.”
Properties of Methyl Halides
Methyl halides are molecules containing carbon atoms and three hydrogen atoms bonded to a halogen atom (such as bromine, chlorine, or fluorine). Halogens comprise a group of reactive, non-metallic elements. On Earth, biological processes produce methyl halides, but their atmospheric concentration is low.
However, Leung’s team suggests that conditions on hycean worlds could facilitate substantial atmospheric accumulation of methyl halides. Furthermore, methyl halides exhibit strong infrared light absorption, within the observational capabilities of the JWST.
Leung noted, “A significant advantage of searching for methyl halides is the potential for rapid detection with James Webb, possibly within 13 hours. This is comparable to, or significantly less than, the telescope time needed for gases like oxygen or methane. Reduced observation time translates to lower costs.”
Challenges and Considerations for Hycean World Habitability
Leung’s team acknowledges two primary uncertainties. Firstly, the actual existence of hycean worlds remains unconfirmed. They were proposed as a potential explanation for certain characteristics of warm sub-Neptune planets with densities suggesting thick hydrogen atmospheres and deep liquid water oceans. Directly observing subsurface oceans remains technically challenging.
Secondly, the habitability of these oceans is uncertain. Hycean worlds are expected to be hot, and while the hydrogen envelope prevents ocean evaporation, temperatures might be prohibitive for life as we understand it. However, detecting methyl halides in a candidate hycean world’s atmosphere would strongly suggest the possibility of life in its depths.
Anaerobic Life on Hycean Worlds
Life on hycean worlds would likely be anaerobic, relying on hydrogen rather than oxygen.
Schwieterman explained, “Hypothetical microbes on these worlds would be anaerobic, adapted to vastly different environments, producing these gases as a plausible metabolic byproduct.”
Anaerobic life exists on Earth, suggesting this possibility isn’t entirely alien. While Earth-like planets around red dwarfs may be scarce due to stellar radiation, hycean worlds with protective hydrogen atmospheres might be more resilient.
Prevalence of Hycean Worlds in the Cosmos
Consequently, hycean worlds could be a dominant habitat for life in red dwarf systems. Given that red dwarfs constitute approximately three-quarters of the stars in the Milky Way galaxy, habitable hycean worlds might outnumber Earth-like worlds in the cosmos.
The findings from Leung’s team were published in the journal The Astrophysical Journal Letters on March 11.