Scientists have long speculated about the possibility of extraterrestrial life, and aliens have captured the public imagination to become a mainstay of popular culture and science fiction. Earth is, as of now, the only known home of life in the universe.
Many astronomers have suggested that life may have emerged on other bodies in our solar system, such as Mars and Europa, one of Jupiter’s four Galilean moons, due to the presence or possible presence of liquid water. However, verifying these claims is extremely difficult — with no readily apparent biosphere probes that would have to be sent across millions of miles to collect physical samples for further study.
Astronomers increasingly believe that the first signs of alien life will be found beyond the Solar System, on exoplanets orbiting other stars. Since the 1990s, scientists have discovered over 5,000 exoplanets, and theoretical models suggest that there are far, far more, with some estimates claiming that there could be at least 300 million habitable planets in our galaxy.
While sending probes to these planets will be impossible for the foreseeable future, observing light that has interacted with the planet could reveal telltale signs of complex life. Different substances absorb and reflect different wavelengths of light, which is why our eyes perceive them as being different colors. Chlorophyll in plants is green, for example, because it absorbs energy-intensive blue light and abundant red light emitted by our Sun, reflecting the less useful green light in the visible spectrum and maximizing the amount of energy available for photosynthesis.
The light reflected off planets is a minuscule fraction of the light emitted by their star, which makes extrasolar planets almost impossible to observe directly, however, when an exoplanet passes directly in front of its host star, the resulting “eclipse” blocks out a small fraction of the star’s rays. By closely observing a star over a sufficient period of time, astronomers can determine the orbital period of the planet, as well as its radius. When paired with the ‘radial velocity’ method, which observes the minute gravitational pull a planet exerts on its host star, astronomers can estimate the mass, density, and surface gravity of a planet with a high degree of confidence, and by observing the spectrum of light being absorbed and reflected by a planet during transit, they can get an idea about the chemical composition of its surface and atmosphere.
The James Webb Telescope, a successor to NASA’s famed Hubble Telescope that took three decades and $10 billion to make, was launched in December of 2021 and transmitted its first images in July of 2022. With a higher resolution and greater sensitivity than Hubble and other, older models, the Webb Telescope, which has already dazzled the world with its remarkably detailed images, represents a major step forward in the study of our universe. According to Christiaan Boersma, an astronomer at NASA’s Ames Research Center, “We’ll be able to see details — better details — on smaller scales than ever before.”
The presence of certain gasses is not definitive proof that a given planet is habitable; the Webb telescope has already taken a reading of the gas giant WASP-96b and found large amounts of water vapor, but scientists have good reason to believe the planet is inhospitable. Webb is also not built for discovering exoplanets and is unable to detect free oxygen — one of the strongest possible signs of life.
Despite these limitations, Webb has already proven that it can provide detailed information on many of the key questions surrounding the potential habitability of planets, and if all the stars align, scientists might find a match. The satellite is in high demand by researchers, but is already scheduled to turn its gaze toward TRAPPIST-1e, an Earth-like planet orbiting a star 39 light years away from our own. If all goes well, in the coming months scientists could discover extraterrestrial life less than 12 parsecs away.