Using a Range of Molecules to Detect Biosignatures in Exoplanet Atmospheres

Introduction

In the present era of exploring habitable exoplanets, advancements in space telescopes like the James Webb Space Telescope (JWST) and ground-based observatories offer new possibilities. A team of scientists from the University of Washington’s Department of Astronomy and Astrobiology program is engaged in research focused on the detection and characterization of terrestrial exoplanet atmospheres using ground-based, high-resolution spectroscopy. While an oxygenated atmosphere is a valuable indicator for life, it is crucial to investigate the detectability of a range of molecules to comprehensively assess the presence of life on distant worlds.

Next Generation Detectability

The progress made in space and ground-based observatories empowers scientists to study the atmospheres of terrestrial exoplanets and search for signs of life. The JWST, equipped with near-infrared (NIR) and mid-infrared range capabilities, provides insights into exoplanet atmospheres, such as the TRAPPIST-1 system. However, the JWST may have limitations in detecting certain molecules like oxygen and ozone, which are critical indicators of life. In contrast, the upcoming ground-based extremely large telescopes (ELTs) offer a complementary advantage by enabling the assessment of oxygen. In the next decade, it may be possible to investigate the oxygen content in Earth-like exoplanet atmospheres orbiting M-dwarf stars using next-generation, high-spectral-resolution ELTs.

Ground-based observations have advantages in resolving narrow features at shorter wavelengths, where oxygen absorption is stronger, and they encompass a broader range of M-dwarf targets. By combining observations from space-based telescopes like the JWST and ground-based telescopes, scientists can gain a more comprehensive understanding of terrestrial exoplanet atmospheres. The objective is to identify environments that may exhibit signs of planetary evolution influenced by abiotic or biological processes through analysis of the obtained spectra. However, it is important to note that while oxygen is a critical biomarker on Earth, its presence alone in an exoplanet atmosphere does not guarantee the presence of life.

Beyond Oxygen

To enhance the reliability of oxygen (O2) as a biosignature, it is essential to consider other molecules that provide contextual information about the planetary environment. Further exploration is needed to understand how ELTs can aid in understanding other aspects of the planet's environment, which could provide context for oxygen measurements. Expanding the detection of molecules beyond O2 is crucial to enhance the search for biosignatures and interpret their significance. Understanding the context behind these other molecules can help determine if a planet is habitable, ascertain whether O2 was produced abiotically, or reveal other biosignatures. For example, post-ocean-loss worlds can abiotically generate high levels of atmospheric O2, leading to false identification of biosignatures. Other abiotic sources of O2, such as CO2 photolysis, may be accompanied by the presence of specific molecules like carbon monoxide (CO) or the absence of water vapor.

Detecting CO2 can increase confidence in the planet's terrestrial nature and indicate a possible source for abiotic O2. Additional molecules like methane (CH4), ozone (O3), and water vapor (H2O) can help eliminate false positives for biological sources of O2 and improve the ability to interpret the potential presence of life. Exploring molecules in disequilibrium pairs, such as O2/CH4 or CH4/CO2, can provide further evidence of biological activity. For instance, searching for CO2 may also reveal biological levels of CH4 if present, forming a biosignature pair. The absence of CH4 may indicate an abiotic environment. Simultaneous non-detection of H2O or CH4 can also assist in distinguishing between abiotic and biogenic O2 scenarios. Furthermore, the detection of CO can aid in differentiating between abiotic O2 buildup from CO2 photolysis and O2 buildup from processes like ocean loss or outgassing.

Methods

The team of scientists conducted simulations using high-resolution spectroscopy to analyze the changes in light as it passes through the atmospheres of these planets during their transits. When studying M-dwarf planets, which are ideal for transmission spectroscopy, it is necessary to account for star-planet interactions and the effects of irradiance levels. These factors can influence atmospheric composition and affect the detectability of certain molecules. The simulations considered different atmospheric models, including those resembling early Earth with and without life, as well as planets without life but with naturally produced O2. The simulations were based on future telescopes such as the Giant Magellan Telescope, Thirty Meter Telescope, and European Extremely Large Telescope. The researchers employed a technique called cross-correlation to simulate spectra, taking into account various exoplanet environments and M-dwarf spectral types. The objective was to determine the number of transits required to detect major molecules such as O2, O3, CH4, CO2, CO, ethane (C2H6), and H2O using ELTs.

Adapted from Currie, M. H., Meadows, V. S., & Rasmussen, K. C. (2023). There’s more to life than O2: Simulating the detectability of a range of molecules for ground-based, high-resolution spectroscopy of transiting terrestrial exoplanets. The Planetary Science Journal, 4(5), 83. https://doi.org/10.3847/psj/accf86

Discussion

The results indicate that CO2 and CH4 are the easiest molecules to detect using ELTs, requiring only a few observed transits. One of the primary challenges is distinguishing exoplanet O2 absorption from Earth's O2 absorption. However, the simulations suggest that with well-constrained radial-velocity measurements from ELT spectrometers, this challenge can be overcome. Through cross-correlation analysis and simulations, the researchers determined that the number of transits needed to detect O2 for nearby targets may be less than 40. Detecting Earth-like levels of O2 in an atmosphere will be difficult but feasible for known transiting targets within 12 parsecs and closer. Although this technique still faces challenges, these large telescopes will provide opportunities that may not be achievable with the JWST.

Conclusion

These molecules can provide valuable information about the presence of life, habitability, and the possibility of false-positive biosignatures. Ground-based searches for O2 offer the most effective means of detecting this crucial molecule, as it may not be accessible with space-based telescopes. The detectability of these molecules depends not only on photochemistry and instrument sensitivity but also on the physical characteristics of the host star. The study proposes an observing protocol to discriminate between different terrestrial planet environments. The study suggests that ELTs have the potential to search for two biosignature pairs: O2/CH4 and CO2/CH4. Overall, the study provides insights into the detectability of key molecular species using ELTs and their potential implications for understanding terrestrial planet atmospheres and searching for signs of life.

References:

Currie, M. H., Meadows, V. S., & Rasmussen, K. C. (2023). There’s more to life than O2: Simulating the detectability of a range of molecules for ground-based, high-resolution spectroscopy of transiting terrestrial exoplanets. The Planetary Science Journal, 4(5), 83. https://doi.org/10.3847/psj/accf86