Navigation map condensed from 6000 exoplanets to 45

Astronomers have narrowed the search for extraterrestrial life from thousands of distant worlds to just a few dozen promising candidate planets. This study presents a clearer roadmap for the biggest question in science: ‘Are we alone in the universe?’

A research team led by Professor Lisa Kaltenegger of the Carl Sagan Institute at Cornell University selected 45 rocky exoplanets located within the habitable zone of their star. The habitable zone refers to an orbital region with temperature conditions that allow liquid water to exist on the surface. Among the more than 6,000 exoplanets discovered to date, these planets are considered the most likely candidates for the search for extraterrestrial life.

The research team used data from the European Space Agency (ESA)'s Gaia mission and NASA's exoplanet archive to precisely re-estimate the amount of stellar energy each planet receives. This is a key factor in determining whether a particular world is too hot, too cold, or suitable for life.

Asking the limits of habitability

The core of this research is not simply to ask which planets can support life, but to determine where the limits of habitability are. The research team intentionally included borderline planets in the list.

“We know that Earth is habitable and Venus and Mars are not,” explains co-author Abigail Bohl.

“Observing these planets will help us understand when habitability is lost, how much energy is excessive, and which planets are still habitable or were never habitable in the first place,” Ball added.

The research team also included planets with elliptical orbits into consideration. Future research is expected to determine how large an orbital eccentricity a planet can have before it loses habitability.

Historical context of the search for extraterrestrial life

The history of exoplanet search dates back to the discovery of the first exoplanet in 1995. NASA's Kepler space telescope, launched in 2009, revolutionized exploration by discovering thousands of exoplanets. In 2018, the Transiting Exoplanet Survey Satellite (TESS) followed Kepler's lead and began investigating planets in closer star systems.

In particular, the TRAPPIST-1 star system discovered in 2017 became a new milestone in the search for extraterrestrial life in that several Earth-sized rocky planets exist within the habitable zone around a single star. With the advent of the James Webb Space Telescope (JWST), launched in 2021, an era has opened in which we can directly analyze the atmospheric components of these planets.

Based on this accumulated data and technology, this study presents strategic guidelines for the most efficient distribution of limited observation resources.

Notable candidate planets

The object of most attention is the TRAPPIST-1 star system, located about 40 light-years away from Earth. This system contains several potentially habitable Earth-sized rocky planets. Interestingly, these planets are likely tidal locked, with perpetual day on one side and perpetual night on the other. Nevertheless, TRAPPIST-1 e is considered one of the most likely candidates for holding liquid water.

Another notable object is LHS 1140 b, about 48 light-years away. There is a possibility that this 'Super-Earth', which is denser and has a mass more than five times that of Earth, may be an ocean planet with abundant water.

The closest candidate is Proxima Centauri b, located just 4.25 light-years away in our solar system. Astronomers are interested in it because it is the closest planet to its star, but there are also concerns that intense stellar flares could strip it of its atmosphere.

Future outlook [AI analysis]

This list is not intended to be a compendium of all potentially habitable worlds, but rather a strategic guide for future research. By sorting out which planets are best suited to which observation techniques, the research team provides direction so that powerful instruments like the James Webb Space Telescope can focus on searching for biological signatures, or signs of life.

“It’s difficult to say for sure what makes life more likely, but knowing where to look is a key first step,” said co-author Gillis Lowry, now a graduate student at San Francisco State University. “The goal of this project is to suggest ‘here are the best targets for observations.’”

As the next generation of observatories comes online, future observations of these 45 worlds are likely to bring scientists closer than ever to determining whether life exists beyond Earth. In particular, when the James Webb Space Telescope begins to publish the atmospheric analysis results of the TRAPPIST-1 planets in earnest, the discussion on extraterrestrial life is expected to shift from the realm of speculation to the realm of verification.