By Quamrul Haider

Modern astronomy is a trip of the human intellect through two inseparable abstract entities: space and time. It is also a “history of receding horizons,” as noted by famous American astronomer Edwin Hubble. Indeed, the recently launched James Webb Space Telescope (JWST) has expanded the horizon by piercing through the hitherto “dark curtain” of the early universe and presented us with a treasure trove of information about the cosmos, both present and way back almost to the beginning of time.

So, what’s next for Webb? With more than 5,000 exoplanets – planets moving around other stars in our galaxy, the Milky Way – discovered so far, it is quite natural to ask the question: Can Webb find habitable exoplanets with alien life? The habitable zone is the area surrounding a star where it is neither too hot for life-giving liquid water to exist, nor too cold for the planet to be anything but a giant snowball.

Yes, the JWST can surely find habitable exoplanets, but it is not specifically designed to search for alien life. It already found the distinct signature of water, along with evidence for clouds and haze, in the atmosphere surrounding a hot, puffy, giant gaseous planet named WASP-96b, orbiting a distant Sun-like star 1,000 light years away from Earth.

An artist’s visualisation of the James Webb Space Telescope in space. Launched in December 2021, the telescope has already provided a treasure trove of information about the early stage of the universe. SOURCE: NASA

Keeping this in mind, astronomers are directing their attention on red dwarfs because the sheer number of these stars – at least 75 billion in the Milky Way – statistically increases the probability that there might exist habitable planets that are orbiting some of them. Although red dwarfs with masses that are about 0.08-0.60 times that of the Sun are important targets to search for alien life, they are difficult to observe because they are too faint, shining with only 0.05 percent the luminosity of the Sun. They are, however, relatively bright in the infrared region. This is where the JWST swings into action, because it is primarily designed to capture infrared light.

Webb has already started peering as a matter of priority at the most tantalising candidates for life elsewhere in the universe – seven rocky, Earth-sized planets orbiting about an ultra-cool red dwarf star known as TRAPPIST-1 (acronym for Transiting Planets and Planetesimals Small Telescope). Discovered in 2017 by the Spitzer Space Telescope, the star is roughly 40 light years away and is slightly larger than Planet Jupiter, with a mass of about nine percent of the Sun’s. Three of the planets orbit in their star’s habitable zone.

Analysis of data taken by Spitzer suggests, albeit not with certainty, that most of the planets orbiting TRAPPIST-1 may have atmospheres. Having an atmosphere is a prerequisite for life as we know it. Thus, a major focus of Webb’s observations of TRAPPIST-1 will be “atmospheric reconnaissance” – detecting subtleties in the atmosphere enveloping the star’s planets.

Measuring a planet’s atmosphere is not a daunting task for Webb. Whenever a planet passes in front of its star, its atmosphere will absorb some of the star’s light. Webb can capture the filtered starlight to observe the tell-tale signatures for the existence of an atmosphere.

What about alien life? The search for extraterrestrial life began in earnest in 1984 with the establishment of SETI Institute (acronym for Search for Extraterrestrial Intelligence) in Mountain View, California. Scientists at the institute monitor signals from outer space, using ground-based radio telescopes in California, Hawaii, Puerto Rico and Chile. To date, the SETI programme has scanned about 20,000 objects, mostly Sun-like stars. Unfortunately, nothing interesting has been discovered so far. The institute is still functioning, but after the Arecibo radio telescope in Puerto Rico collapsed in 2020, it has been limited in its ability to collect new data.

A recent study suggests that the JWST may be able to spot extraterrestrial life based on air pollution from their planets. The study claims that if the telescope can detect atmospheric pollution, particularly climate-warming gases, as well as industrial gases, such as chlorofluorocarbons and nitrogen dioxide, it could be a signature for an alien life – much in the same way we are identifiable here on Earth based on our environment-degrading emissions.

However, they caution that there are limitations to Webb’s ability to spot pollutants in the atmosphere. For example, if a planet’s host star is too bright, it will drown out the signal from its atmosphere. Or if a planet orbits close to its star, then violent outbursts of high-energy radiation from the star will blast the atmosphere away over time.

The strongest signal for life is free (unbonded) oxygen that much of life on Earth needs to survive. Besides, oxygen cannot be produced without some kind of life. It will be difficult for Webb to detect oxygen in an exoplanet’s atmosphere because it was not originally designed to scan distant planets for their oxygen concentrations. To put it simply, while Webb will be able to tell us if an exoplanet is potentially habitable for humans and/or contains the building blocks of life, it is by no means the smoking gun for the presence of life.

That being said, astronomers are hoping that out of the billions of stars in our galaxy alone, Webb might provide the first compelling evidence for life on one or more in their planetary system. But they don’t expect a “eureka” moment. Nevertheless, with an estimated lifespan of 20 years, Webb has plenty of time to look for alien life.

Finally, on the astronomical scale, the 4.55-billion-year-old Earth is in its early middle age. Hence, even if Webb cannot make a breakthrough discovery, we have another five billion years or so to find life somewhere in the unfathomably big cosmos before the Sun devours Earth and vaporises it. We should not give up hope of shaking hands with aliens – unless anthropogenic climate change or a nuclear war annihilates the human civilisation in the next few decades.

Dr Quamrul Haider is a professor of physics at Fordham University in New York, US.