Establishing a New Habitability Metric for Future Astrobiology Surveys

The search for exoplanets has grown immensely in recent decades thanks to next-generation observatories and instruments. The current census is 5,766 confirmed exoplanets in 4,310 systems, with thousands more awaiting confirmation. With so many planets available for study, exoplanet studies and astrobiology are transitioning from the discovery process to characterization. Essentially, this means that astronomers are reaching the point where they can directly image exoplanets and determine the chemical composition of their atmospheres.

As always, the ultimate goal is to find terrestrial (rocky) exoplanets that are “habitable,” meaning they could support life. However, our notions of habitability have been primarily focused on comparisons to modern-day Earth (i.e., “Earth-like“), which has come to be challenged in recent years. In a recent study, a team of astrobiologists considered how Earth has changed over time, giving rise to different biosignatures. Their findings could inform future exoplanet searches using next-generation telescopes like the Habitable Worlds Observatory (HWO), destined for space by the 2040s.

The study was led by Kenneth Goodis Gordon, a graduate student with the University of Central Florida’s (UCF) Planetary Sciences Group. He was joined by researchers from the SETI Institute, the Virtual Planetary Laboratory Team at the University of Washington, NASA’s Nexus for Exoplanet System Science (NExSS), the Space Science Division and Astrobiology Division at the NASA Ames Research Center, the Sellers Exoplanet Environments Collaboration (SEEC) at the NASA Goddard Space Flight Center, and NASA’s Jet Propulsion Laboratory. The paper that describes their findings is being considered for publication in The Astrophysical Journal.

As the team indicates in their paper, the current census of exoplanets includes more than 200 terrestrial planets, dozens of which have been observed in their parent stars’ habitable zone (HZ). Many more are expected in the coming years, thanks to next-generation instruments like the James Webb Space Telescope (JWST) and the ESO’s Extremely Large Telescope (ELT). Equipped with cutting-edge spectrometers, adaptive optics, and coronographs, these and other telescopes will enable the characterization of exoplanets, identify biosignatures, and determine their habitability.

This is a complex problem since a range of different planetary, orbital, and stellar parameters must be considered. To date, Earth is the only planet known to harbor life, which limits our perspective. But as Goodis Gordon told Universe Today via email, this is not the only way in which habitability studies have been constrained:

“Currently, there is only one example of a planet known to harbor life: our own Earth. However, when we think of habitability, most of the time, people will only relate that term to modern-day Earth-like conditions: large-scale vegetation, animals, humans, etc. This can severely limit our approach to finding habitable exoplanets because it only provides us with one data point to compare against.

“But we know from biogeochemical analyses that the Earth is not just one data point and that our planet has actually been habitable for eons. So better understanding the signatures of the Earth throughout its evolution provides us with more comparison points when searching for habitable worlds elsewhere.”

For instance, life emerged on Earth during the Archeon Eon (ca. 4 billion years ago), when the atmosphere was predominantly composed of nitrogen, carbon dioxide, methane, and inert gases. By the late Paleoproterozoic Era (ca. 2.5 to 1.6 billion years ago), the Great Oxygenation Event occurred after a billion years of cyanobacterial photosynthesis. This period lasted from 2.46 to 2.06 billion years ago and caused Earth’s atmosphere to transition from a reducing atmosphere to an oxidizing atmosphere, which led to the emergence of more complex life forms.

During this same period, the Sun underwent evolutionary changes over the past 4.5 billion years. At this time, the Sun was 30% dimmer than it is today and has gradually grown brighter and hotter since. Despite this, Earth maintained liquid water on its surface, and life continued to survive and evolve. The complex interrelationship between Earth’s evolving atmosphere and our Sun’s evolution is key to maintaining habitability for billions of years. As Goodis Gordon explained:

“In addition to that, current exoplanet characterization strategies tend to rely solely on the unpolarized light received from these worlds, which studies have shown can result in errors in the retrieved fluxes and degeneracies in the calculated planetary parameters. For example, if an exoplanet has really thick clouds or hazes in its atmosphere, the observed flux spectrum can be flat with almost no spectral features. This makes it extremely difficult to detect what gases are in the atmosphere or even what those clouds or hazes that blocked the light are made of.”

In recent years, several studies have begun to examine the signatures of light reflected by an early Earth. Others have simulated different scenarios throughout the Archean, Proterozoic (2.5 billion to 541 million years ago), and Phanerozoic Eons (538.8 million years ago to the present). Lastly, some studies analyzed how the signatures of these early-Earth analogs would change if they orbited different types of stars. But as Goodis Gordon pointed out, nearly all of these studies focused on the unpolarized flux from these worlds, so they missed some of the information available in the light:

“Polarization is a more sensitive tool than flux-only observations and can enhance exoplanet characterizations. Polarimetry is extremely sensitive to the physical mechanism scattering the light, thereby allowing for accurate characterizations of the properties of a planetary atmosphere and surface. Also, since polarization measures light as a vector, it is sensitive to the locations of features on the planet, such as cloud and land distributions, as well as diurnal rotation and seasonal variability.

“Within the Solar System, polarimetric observations helped characterize the clouds of Titan, Venus, and the gas giants, while outside of it, polarimetry has been used to characterize the cloud properties of brown dwarfs. In most of these cases, the characterizing discovery was possible only with polarimetry!”

This could have profound implications for the study and characterization of exoplanets in the near future. Using an expanded concept of habitability that takes into account how Earth has evolved over time and benefits from the study of polarized light, astronomers will likely identify far more habitable planets when next-generation observatories like the HWO become available. The plans for this observatory build upon two earlier mission concepts – the Large Ultraviolet Optical Infrared Surveyor (LUVOIR) and the Habitable Exoplanets Observatory (HabEx).

Based on these previous studies and the experience astronomers have accrued by working with previous exoplanet-hunting missions—i.e., Hubble, Kepler, the Transiting Exoplanet Survey Satellite (TESS), and the JWST—the HWO will be designed specifically to examine the “atmospheres of exoplanets for potential indications of life” (aka “biosignatures”) and determine if they are potentially habitable planets. As Goodis Gordon indicated, his team’s research could help inform future surveys using the HWO and other next-generation observatories:

“Our models provide more data points to compare observations of terrestrial exoplanets against and therefore help to inform habitability studies of these worlds. Additionally, there has been a push in the exoplanet community in recent years to include polarimetry in near-future observatories like the Extremely Large Telescopes on the ground or the Habitable Worlds Observatory in space. Our hope is that our models will help prove the power of polarimetry in characterizing and distinguishing between different habitable exoplanet scenarios in ways that unpolarized flux observations cannot.”

Further Reading: arXiv