The quest for life beyond Earth has become a central focus in modern astronomy and planetary science. In an effort to uncover the mysteries of extraterrestrial life, the United States is investing in the construction of multiple major telescopes and planetary probes. However, identifying potential signs of life—known as biosignatures—poses significant interpretative challenges. Furthermore, determining the most promising locations to search for these biosignatures remains a complex task.
As an astrophysicist and astrobiologist with over 20 years of experience in studying extrasolar planets—those located outside our solar system—I, along with my colleagues, have developed a novel approach that aims to identify the most intriguing celestial bodies for potential life searches. This method, recently shared on the arXiv preprint server, models how various organisms might thrive in diverse environments, drawing insights from studies on the limits of life on Earth.
To enhance our understanding of alien worlds, astronomers are actively developing advanced technology and plans for powerful space telescopes. A notable example is NASA's proposed Habitable Worlds Observatory, which aims to capture ultrasharp images of planets orbiting nearby stars. In addition, my team is working on the Nautilus space telescope constellation, designed to examine hundreds of potentially Earthlike planets as they transit in front of their host stars. These telescopes are expected to provide more sensitive studies of extraterrestrial environments, prompting crucial questions: Where should we look for life? Are the environments where we observe potential biosignatures truly habitable?
The recent, contentious claims regarding potential biosignatures on the exoplanet K2-18b, announced in April 2025, alongside earlier claims regarding Venus, illustrate the complexities involved in conclusively identifying life through remote-sensing data. The Oxford Languages definition of habitable refers to being suitable for life, but how can scientists ascertain what is “good enough” for extraterrestrial organisms? Could alien microbes survive in extreme conditions, such as boiling acid lakes or frigid liquid methane? NASA has traditionally adhered to the principle of "follow the water," given that liquid water is essential for all known Earth life and typically indicates a temperate environment conducive to life.
As the capabilities of astronomers in characterizing alien worlds expand, astrobiologists require a more quantitative and nuanced framework than the binary water/no-water classification. In collaboration with a team of experts in a NASA-funded project called Alien Earths, we developed a new methodology to tackle the challenges of identifying habitable environments. This project involved astrobiologists, planetary scientists, exoplanet specialists, ecologists, biologists, and chemists from the renowned Nexus for Exoplanet System Science (NExSS). Our research highlighted two primary questions: How do we determine what life requires if we lack a comprehensive understanding of extraterrestrial life? And how can we effectively work with incomplete data?
Our innovative approach, termed the quantitative habitability framework, features two key aspects. Firstly, we shifted from the vague question of whether a location is habitable to a more concrete inquiry: Would the known or unknown conditions in a given habitat allow a specific species or ecosystem to survive? This shift allows us to focus on specific organisms, making the question more actionable. Secondly, our framework embraces uncertainty, offering probabilistic answers rather than strict yes/no conclusions. By comparing computer models of the requirements of various organisms (organism models) with the conditions of different habitats (habitat models), we can assess the compatibility of life and environment, despite inherent uncertainties.
To explore the boundaries of life, our team analyzed literature on extreme organisms—from insects thriving in the Himalayas to microorganisms in hydrothermal ocean vents. We applied our models to assess their potential survival in environments such as the Martian subsurface or the oceans of Europa. Additionally, we investigated whether marine bacteria capable of producing oxygen might survive on known extrasolar planets. Although our approach is comprehensive, it does make important simplifications; for instance, it does not consider how life might influence its planetary environment or the full range of nutrients organisms may need.
The quantitative habitability framework equips us to address critical questions regarding potential sites for life beyond Earth. This open-source model allows other astrobiologists to utilize and enhance our work for ongoing and future projects. If scientists successfully detect a potential biosignature of life, our framework can help evaluate whether the environment can support the type of life that produced the detected signature.
Looking ahead, our next steps involve compiling a database of terrestrial organisms that inhabit extreme environments, representing the limits of life. By incorporating models for hypothetical alien life into our quantitative habitability framework, we aim to refine our understanding, interpret new data from other worlds, and guide the search for signatures of life beyond our planet.
This article is republished from The Conversation under a Creative Commons license. Read the original article for further insights.
