The elements of life are spread all over the universe. Although Earth is the only known place in the universe with life, finding life outside of Earth is a major goal of modern astronomy and planetary science.
We are two scientists studying exoplanets and astrobiology. Thanks in large part to next-generation telescopes like James Webb’s, researchers like us will soon be able to measure the chemical composition of the planet’s atmosphere around other stars.
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The hope is that one or more of these planets will have chemical signatures of life.
Habitable exoplanets
There may be life in the solar system where there is liquid water – such as on Mars or in the oceans of Jupiter’s moon Europa.
However, finding life in these places is incredibly difficult, as they are difficult to reach, and investigations will be needed to return physical specimens to find life.
Many astronomers believe that there is a good chance that life exists on planets orbiting other stars, and it is possible that life will be found there.
Theoretical calculations show that the Milky Way galaxy alone has about 300 million potentially habitable planets and Earth-sized planets capable of living within just 30 light-years of Earth – essentially the neighbor of humanity’s galaxy.
So far, astronomers have discovered more than 5,000 explosions, hundreds of which are potentially habitable, using indirect methods to measure how a planet affects its nearest star.
These measurements can provide astronomers with information about the mass and size of a planet, but not much.
Looking for biosignatures
To find life on a distant planet, astronomers will study the light of a star that has interacted with the surface or atmosphere of a planet.
If the environment or the surface of life were changed, the light could carry a signal, called a “bio-signature”.
In the first half of its existence, the Earth played with the atmosphere without oxygen, even though it hosted simple, single-celled life.
Earth’s bio-signature was very weak in this early period. That changed abruptly 2.4 billion years ago when a new algae family emerged.
Algae used a process of photosynthesis that produces free oxygen – oxygen that is not chemically bound to any other element. Since then, the Earth’s oxygen-rich atmosphere has left a strong and easily identifiable bio-signature on the light that passes through it.
When light bounces off the surface of a substance or passes through a gas, some wavelengths of light are more likely to be trapped in the surface of the gas or material than others.
This selective trapping of light wavelengths is why objects are of different colors. The leaves are green because chlorophyll is good at absorbing light, especially in red and blue wavelengths.
As light hits a leaf, red and blue wavelengths are absorbed, allowing most of the green light to return to your eyes.
The pattern of absent light is determined by the specific structure of the material with which the light interacts. Because of this, astronomers can learn something about a planet’s atmosphere or surface structure by measuring the specific color of light coming from a planet.
This method can be used to detect the presence of certain atmospheric gases that are associated with life – such as oxygen or methane – because these gases emit very specific signatures in light.
It can also be used to detect specific colors on the surface of a planet. On Earth, for example, chlorophyll and other oil plants and algae use specific wavelengths of light to synthesize light.
These pigments produce characteristic colors that can be detected using a sensitive infrared camera. If you see this color reflected from the surface of a distant planet, it will probably indicate the presence of chlorophyll.
Telescopes in space and on Earth
An incredibly powerful telescope is needed to detect these subtle changes in light from a potentially habitable exoplanet. Currently, the only telescope capable of such a feat is the new James Webb Space Telescope.
As he began scientific operations in July 2022, James Webb studied the spectrum of the gas’s largest exoplanet, WASP-96b. The spectrum showed the presence of water and clouds, but a large and warm planet like WASP-96b is unlikely to host life.
However, preliminary data show that James Webb is able to detect faint chemical signatures in the light coming from expulsions.
In the coming months, the Web is set to turn its mirror to TRAPPIST-1e, a potentially habitable planet about 39 light-years from Earth.
The web can search for bio signatures by studying the planets passing in front of their host stars and capturing the light of stars filtered from the planet’s atmosphere.
But the web was not designed to search for life, so the telescope is only able to examine a few of the closest possible habitable worlds.
It can also detect changes in the environmental levels of carbon dioxide, methane and water vapor. Although some combinations of these gases may suggest life, the web is unable to detect the presence of untreated oxygen, which is the strongest indication for life.
Key concepts for the future, even more powerful, are the use of space telescopes to block the bright light of the planet’s host star in order to reflect the light of the star reflected back from the planet. The idea is to use your hand to block out sunlight so that you can see something better at a distance.
Future space telescopes can use small, internal masks or large, outer, umbrella-like spacecraft to do this. Once the starlight is blocked, it becomes much easier to study the light bouncing off a planet.
There are also three large, ground-based telescopes currently under construction that will be able to locate bioscientists: The Giant Magellan Telescope, the Thirty Meter Telescope and the European Largest Telescope.
Each is far more powerful than the current telescopes on Earth, and despite distorting the Earth’s atmosphere and distorting the light of the stars, these telescopes may be able to test the nearest Earth’s atmosphere for oxygen.
Is it biology or geology?
Even using the most powerful telescopes of decades to come, astronomers will be able to detect only the strongest biosciences produced by worlds that have been completely transformed by life.
Unfortunately, most of the gases released by terrestrial life can also be produced by non-biological processes – both cows and volcanoes release methane. Photosynthesis produces oxygen, but also sunlight, when it divides water molecules into oxygen and hydrogen.
There is a good chance that astronomers will find some false positives in their search for longevity. To help dispel false positives, astronomers will need to better understand the planet of interest in order to understand whether its geological or environmental processes can mimic the bio-signature.
Exoplanet studies have the potential to transcend the bar of extraordinary evidence needed to prove the existence of life in the next generation. The first data release from the James Webb Space Telescope gives us an idea of this exciting development that is coming soon.
