The building material of life is spread throughout the universe. However, at this point, Earth is the only known place that hosts life. The discovery of life beyond our planet is a major goal of modern astronomy and planetary science.
In this regard, scientists are optimistic. They say that thanks to next-generation telescopes like James Webb’s, they will soon be able to accurately measure the chemical composition of the atmospheres of planets around other stars. There is hope that at least one of these planets will have the chemical signature of life.
Life can exist in the solar system where there is liquid water — like in the subsurface on Mars or Jupiter’s moon Europa. However, the search for life in these places is incredibly difficult because they are difficult to access. Finding life here would require sending a probe to return physical samples.
Many astronomers believe that there is a good chance that life could exist on planets orbiting other stars. Therefore, we may discover life for the first time on exoplanets.
It is estimated that there are around 300 million potentially habitable planets in the Milky Way galaxy alone. So far, astronomers have discovered more than 5,000 exoplanets, including hundreds of potentially habitable ones. These measurements can give us information about the exoplanet’s mass and size, but not much more.
In search of a Biosignature
To detect life on a distant planet, astrobiologists study starlight that interacts with the planet’s surface or atmosphere. If the atmosphere or surface is teeming with life, the light emitted from it may have a signature called a “biosignature.”
For the first half of its existence, Earth had an oxygen-free atmosphere, but the planet still had simple single-celled life. During this early era, Earth’s bioprint was very weak. This changed dramatically 2.4 billion years ago when a new family of algae arose. These algae used the process of photosynthesis to produce oxygen—oxygen that is not chemically bound to other elements. Since then, Earth’s oxygen-rich atmosphere has left a strong biosignature in the light passing through it.
That is why astronomers can get some information about the composition of the exoplanet’s atmosphere or surface. This is done by measuring the colors of the light coming from this planet. All this can be recorded with a sensitive infrared camera.
Telescopes in space and on Earth
It takes an incredibly powerful telescope to detect these small changes in light from a potentially habitable exoplanet. At the moment, the only telescope that can do this is the James Webb Space Telescope.
At the start of science operations in July 2022, James Webb observed the spectrum of the gas giant exoplanet WASP-96b. The spectrum showed the presence of water and clouds, but a planet as large and hot as WASP-96b is unlikely to host life.
However, this early data suggests that James Webb can detect faint chemical signatures in light from exoplanets. In the coming months, Webb will turn its mirrors to TRAPPIST-1e, a potentially habitable Earth-sized planet. This planet is only 39 light years away from us.
A web can do this when a planet transits in front of its host star. At this time, the telescope captures the starlight that will pass through the planet’s atmosphere. However, the web was not created just to look for life, and it only looks for signs of life on nearby planets.
It can also only detect changes in atmospheric levels of carbon dioxide, methane, and water vapor.
Is it biology or geology?
Even with the most powerful telescopes of the coming decades, astrobiologists will only be able to detect the strong biosignatures produced by worlds completely transformed by life.
Unfortunately, most of the gases released by life on Earth can also be produced by non-biological processes—cows and volcanoes release methane. Photosynthesis produces oxygen, but sunlight also splits water molecules into oxygen and hydrogen.
There is a chance that astronomers will find false positives for extraterrestrial life. To rule out such errors, astronomers would need to understand the planet’s geological and atmospheric processes well enough.
The next generation of exoplanet telescopes has the potential to overcome such a barrier. As for James Webb, his first data gives us hope that he will be a pioneer in this regard and start to follow the path.