While Earth is the only known place in the universe with life, detecting life beyond it is a major goal of modern astronomy and planetary science.
Chris Imbe, University Distinguished Professor of Astronomy, and Daniel Abay, Professor of Astronomy and Planetary Sciences, University of Arizona, study exoplanets and astrobiology. Thanks in large part to next-generation telescopes like James Webb, researchers like him will soon be able to measure the chemical composition of planetary atmospheres around other stars.
Life may exist in the solar system where there is liquid water - such as the aquifers on Mars or in the oceans of Jupiter's moon Europa. However, the search for life in these places is very difficult, as they are difficult to reach, and detecting life requires sending a probe to return physical samples.
Many astronomers believe that there is a good chance that life exists on planets orbiting other stars, and that this is likely to be where life first exists.
Theoretical calculations indicate that there are about 300 million potentially habitable planets in the Milky Way alone, and many more Earth-sized habitable planets within just 30 light-years of Earth - humanity's neighboring galaxies.
To date, astronomers have discovered more than 5,000 exoplanets, including hundreds of potentially habitable planets, using indirect methods that measure how a planet affects its nearby star. These measurements can give astronomers information about the mass and size of an exoplanet, but nothing more than that.
To detect life on a distant planet, astrobiologists will study starlight that interacts with the planet's surface or atmosphere. If the atmosphere or surface has been altered by life, the light may hold evidence called a "biosignature".
During the first half of its existence, Earth had an atmosphere without oxygen, although it hosted simple single-celled life. Earth's biosignature was very faint during this early era. That changed abruptly 2.4 billion years ago when a new family of algae evolved.
The algae used photosynthesis, which produces free oxygen - oxygen not chemically bound to any other element. From that time on, Earth's oxygen-rich atmosphere left a strong and easily detectable biosignature on the light passing through it.
When light bounces off the surface of a material or passes through a gas, certain wavelengths are more likely to remain trapped in the gas or surface of the material than others. This selective fit of the wavelengths of light is the reason for the different colors of objects. And the leaves are green because chlorophyll is particularly good at absorbing light in the red and blue wavelengths. When light hits the paper, the red and blue wavelengths are absorbed, leaving mostly green light to bounce back into your eyes.
The pattern of lost light is determined by the specific composition of the material the light interacts with. For this reason, astronomers can learn something about the composition of an exoplanet's atmosphere or surface by measuring the specific color of the light that comes from a planet.
This method can be used to identify the presence of certain gases in the atmosphere associated with life - such as oxygen or methane - because these gases leave very specific signatures in the light. It can also be used to detect strange colors on a planet's surface.
On Earth, for example, plants use chlorophyll and other pigments, and algae for photosynthesis, use specific wavelengths of light.
These pigments produce distinct colors that can be detected with a sensitive infrared camera. And if you see this color reflected off the surface of a distant planet, it probably indicates the presence of chlorophyll.
It takes an incredibly powerful telescope to detect these subtle changes in the light coming from a potentially habitable exoplanet. Currently, the only telescope capable of such a feat is the new James Webb Space Telescope.
When it began its science operations in July 2022, James Webb took a spectrogram 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 shows that James Webb is able to detect faint chemical signals in the light from exoplanets.
In the coming months, the telescope is set to turn its mirrors toward TRAPPIST-1e, a potentially habitable Earth-sized planet just 39 light-years from Earth.
James Webb can search for biosignatures by studying planets as they pass in front of their host stars and capturing starlight streaming through a planet's atmosphere. But Webb wasn't designed to search for life, so the telescope is only able to probe a few of the nearest potentially habitable worlds.
It can also detect changes in the levels of carbon dioxide, methane and water vapor in the atmosphere. While certain groups of these gases may suggest life, the James Webb telescope is unable to detect the presence of unbound oxygen, which is the strongest indication of life.
Pioneering concepts for future, and more powerful, space telescopes include plans to block out the bright light of the planet's host star to detect starlight reflected from the planet. The idea is similar to using your hand to block sunlight to better see something in the distance.
And future space telescopes could use small inner visors or large outer, parachute-like spacecraft to do so. Once the starlight is blocked, it becomes much easier to study the light bouncing back from a planet.
There are also three large ground-based telescopes currently under construction that will be able to search for biosignatures: the Giant Magellan Telescope, the Thirty Meter Telescope and the European Very Large Telescope. Each is much more powerful than telescopes on Earth, and although hampered by Earth's atmosphere that distorts starlight, these telescopes may be able to probe the atmospheres of the nearest worlds in search of oxygen.
Even with the most powerful telescopes of the coming decades, astrobiologists will only be able to detect the powerful biosignatures produced by worlds completely transformed by life.
Unfortunately, most of the gases that terrestrial life gives off can also be produced through non-biological processes - cows and volcanoes release methane. Photosynthesis produces oxygen, but sunlight also produces it when it splits water molecules into oxygen and hydrogen.
And there's a good chance that astronomers will spot some false positives when searching for distant life. To help rule out false positives, astronomers will need to understand an interesting planet well enough to understand whether its geological processes or atmosphere can mimic the biosignature.
The next generation of exoplanet studies has the potential to push through the extraordinary barrier of evidence needed to prove the existence of life. And the first release of data from the James Webb Space Telescope gives us a sense of the exciting progress that's coming soon.