From the TRAPPIST-1 System in Search of Life

演化之聲
Mar 20
2 min read

Updated: Apr 9

TRAPPIST-1 system artist's impression（Courtesy of NASA/JPL-Caltech）

TRAPPIST-1 is a stellar system located in the constellation Aquarius, approximately 39 light-years from Earth. Its host star is a cool red dwarf with a mass of only about 8% that of the Sun. Seven planets have been discovered orbiting this star, designated b, c, d, e, f, g, and h, with orbital periods ranging from 1.5 to 18.7 Earth days. Due to their close proximity to the star, these planets are likely tidally locked, meaning that one hemisphere permanently faces the star while the opposite side remains in perpetual darkness.

Size comparison of TRAPPIST-1 planets and Earth（Courtesy of NASA/JPL-Caltech）

Among them, planets e, f, and g lie within the habitable zone. Their masses are approximately 0.69, 1.04, and 1.32 times that of Earth, respectively. They are thought to possess relatively dense atmospheres and potentially harbor oceans, making the TRAPPIST-1 system a major focus of interest for astronomers and astrobiologists.

However, given the vast distance, the existence of life on these planets can only be inferred indirectly through space telescopes, ground-based observatories, and other signal-detection instruments. Primitive life forms are effectively undetectable using current methods. As a result, detection efforts rely on the assumption that technologically advanced civilizations may exist, allowing us to search for artificial radio signals originating from the TRAPPIST-1 system.

The Search for Extraterrestrial Intelligence (SETI) is a long-running scientific effort that began in the 1960s, aiming to detect intelligent life beyond Earth. One of its most famous events is the Wow! signal detected in 1977. In a recent study published in the Astronomical Journal, researchers from Pennsylvania State University and the SETI Institute used the Allen Telescope Array to search for and analyze radio signals from TRAPPIST-1.

Allen Telescope Array（Courtesy of Seth Shostak / The SETI Institute）

Their approach utilized the concept of planet–planet occultations (PPOs), in which one exoplanet passes in front of another from Earth’s perspective. In such a configuration, if a civilization on one planet transmits a radio signal toward another planet, part of that signal may extend beyond its intended target—similar to how a flashlight beam spreads. If Earth happens to lie within this spillover region, it becomes possible to detect the signal. By focusing observations during these alignment events, the probability of detecting extraterrestrial radio emissions can be increased.

The research team conducted a 28-hour search for narrowband radio technosignatures across a frequency range of 0.9 to 9.3 GHz. From an initial dataset containing tens of millions of signals, filtering processes reduced the number to 11,127 candidate signals, of which 2,264 were associated with PPO observation windows.

Despite these efforts, no radio signals of non-human origin were detected. Nevertheless, the use of PPO-based observational strategies represents a novel and promising method for improving the efficiency of future searches for extraterrestrial intelligence.