An artist’s impression of the James Webb Space Telescope, fully deployed. The James Webb Space Telescope is expected to be fully operational this summer. Credit: NASA
The James Webb Space Telescope (JWST), launched in December of last year, has been slowly powering up its instruments and deploying its sunshield, and is now in the process of aligning its mirrors in preparation for operation. In a few months, the most powerful space telescope ever built will set its sights on the stars. Astronomers hope that what JWST sees will change the way we understand our universe, just as the Hubble Space Telescope did decades before.
One tantalizing possibility that JWST offers, and that Hubble couldn’t, is the opportunity to directly image planets orbiting distant stars and maybe, just maybe, detect signs of life.
The possibility of detecting biosignatures remotely has been a hot topic in recent years. In our own solar system, the recent discovery of phosphine in the atmosphere of Venus prompted speculation that the chemical could be created by a microbial life form. Similarly, remote sensing experts have proposed that plant life, which uses photosynthesis for energy, could be detected at infrared wavelengths, since chlorophyll absorbs visible light but appears bright in infrared and would give it foliage-covered planets a distinctive “red border”.
A single-pixel photo of a distant planet could contain enough information to tell us whether biological life is there, based on the information stored in the wavelengths of light reaching the telescope lens.
But what about intelligent life? Could JWST detect civilizations similar to ours? How would we look for them? The best answers come from understanding what humanity’s presence on Earth looks like from outer space. We emit waste heat (from industry, homes, etc.) and artificial light at night, but perhaps more significantly, we produce chemicals that fill our atmosphere with compounds that would not otherwise be present. These man-made atmospheric constituents could be the giveaway to a distant alien species scanning the galaxy with its own powerful telescope.
A recent article, available in preprint on ArXiv, examined the possibility of using the Webb telescope to search for industrial pollutants in the atmospheres of exoplanets. The paper specifically focused on chlorofluorocarbons (CFCs), which, on Earth, are produced industrially as refrigerants and cleaning agents. CFCs created a massive hole in the Earth’s ozone layer in the 1980s, before an international ban on their use in 1987 helped reduce the level of CFCs to less harmful levels. These “potent greenhouse agents with long residence times in the atmosphere”, if found in other parts of the galaxy, are almost certainly the result of a civilization capable of rampant industrialization.
There are some limitations to the Webb telescope’s search capabilities for CFCs. If a planet’s star is too bright, it will drown out the signal. Therefore, the telescope will have more success observing M-class stars, which are dim, long-lived red dwarfs. A close example is TRAPPIST-1
, a red dwarf 40 light-years away, with several Earth-size planets orbiting within its habitable zone. The James Webb Space Telescope could see CFCs on the planets of TRAPPIST-1, because the dim star would not smother the CFC signature in the same way that a bright star, like our Sun (a G-type star), would.
By contrast, a telescope similar to JWST on TRAPPIST-1 would not be able to see Earth’s CFCs: our Sun is too bright.
Unfortunately, M-class stars are not usually conducive to life, because when they are young, they are unstable and emit powerful solar flares that could wipe out any nascent life forms on nearby planets. However, they tend to calm down as they age, so it’s not impossible. It simply means that we should moderate our expectations a bit.