The quest to find life beyond Earth has long captivated humanity. Today, this search is no longer confined to speculation but stands as a vibrant frontier of modern science. Astronomers are actively exploring exoplanets—planets orbiting stars outside our solar system—in hopes of uncovering environments that could harbor life. This pursuit involves understanding conditions necessary for life and developing methods to detect its subtle traces across vast cosmic distances. The ongoing exploration of these distant worlds promises to reshape our understanding of life’s prevalence in the universe.
Defining Habitable Worlds
The search for life on exoplanets begins with identifying “habitable worlds,” environments that could support life as we know it. The “habitable zone,” often called the “Goldilocks zone,” is the specific orbital range around a star where temperatures are suitable for liquid water to exist on a planet’s surface, a substance essential for Earth-like life.
Beyond liquid water, a planet’s ability to host life depends on other factors. A stable star without frequent harmful flares is beneficial, as is a suitable atmosphere to regulate temperature and protect against radiation. Geological activity, such as plate tectonics, also cycles nutrients and regulates atmospheric composition. Different types of stars influence the location and characteristics of their habitable zones; for instance, smaller, cooler M-dwarf stars have habitable zones much closer to the star compared to Sun-like stars. Planets orbiting M-dwarfs may also experience intense X-ray and ultraviolet radiation, potentially impacting their atmospheres.
Detecting Signs of Life
Scientists primarily search for “biosignatures,” chemical compounds or features indicating biological activity on exoplanets. A leading method involves analyzing starlight as an exoplanet transits. During a transit, some starlight filters through the exoplanet’s atmosphere, carrying clues about its composition.
This light is broken down into wavelengths using spectroscopy. Studying the resulting spectrum identifies specific gases that absorb certain wavelengths. The presence of gases like oxygen, methane, and water vapor in unexpected proportions, or non-equilibrium concentrations, could indicate biological processes. Direct imaging of exoplanets is challenging due to their host stars’ overwhelming brightness. However, transmission spectroscopy during transits allows for atmospheric analysis. Other methods, like the radial velocity method, detect planets by observing their host star’s slight wobble, primarily determining mass rather than atmospheric composition for biosignatures.
Promising Exoplanet Candidates
Several exoplanet systems have emerged as compelling candidates for habitability, based on their characteristics. The TRAPPIST-1 system, located about 40 light-years away, hosts seven Earth-sized planets, with three—TRAPPIST-1e, f, and g—residing within their ultracool dwarf star’s habitable zone. While these planets are remarkably close to their star, their star’s dimness allows for liquid water at these distances. However, the exact surface conditions and the presence of atmospheres on these worlds are still subjects of ongoing investigation.
Proxima Centauri b, the closest known exoplanet at 4.2 light-years, orbits within its red dwarf star’s habitable zone. Despite receiving significantly more X-ray and ultraviolet radiation than Earth, models suggest it could potentially retain enough volatiles, including water, to sustain surface habitability.
Kepler-186f, discovered in 2014, was the first Earth-sized planet confirmed within its star’s habitable zone. This planet, approximately 500 light-years away, receives about one-third of the energy Earth gets from the Sun, placing it near the outer edge of its habitable zone.
TOI 700 d, found by NASA’s TESS, is an Earth-sized planet orbiting a quiet M-dwarf star about 101 light-years away. It receives about 86% of the energy Earth receives from the Sun, making it a robust candidate for a habitable world, though atmospheric characterization remains a challenge. While these planets are promising, definitive evidence of life has not been found on any of them.
The Search Continues
The pursuit of exoplanet life is an evolving field, driven by advancements in observational capabilities and scientific understanding. The James Webb Space Telescope (JWST), launched in 2021, is a major advancement, offering unprecedented sensitivity to study exoplanet atmospheric composition. JWST has already provided initial insights into exoplanet atmospheres, and while challenging, it may identify biosignature gas candidates. For instance, observations of K2-18b, a super-Earth in its star’s habitable zone, have revealed methane, carbon dioxide, and possible traces of dimethyl sulfide, a gas associated with marine life on Earth.
Challenges persist in confirming biosignatures, including distinguishing biological signals from mimicking non-biological processes. Vast distances to exoplanets and the extreme faintness of their light also pose considerable hurdles. Future mission concepts like the Habitable Exoplanet Observatory (HabEx) and the Large UV/Optical/IR Surveyor (LUVOIR) are being developed. These ambitious observatories aim to directly image Earth-like exoplanets and conduct more detailed atmospheric analyses, including searching for combinations of gases like methane and oxygen, which strongly suggest a biological origin. This ongoing scientific endeavor continues to expand humanity’s perspective on its place in the cosmos.