Two astrophysicists are exploring the possibilities of using the new telescope to analyze the atmosphere of extrasolar planets.
The ingredients of life are spread throughout the universe. Although Earth is the only known place in the universe where life exists, detecting life beyond 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 the James Webb, researchers like us will soon be able to measure the chemical composition of the atmospheres of planets around other stars. The hope is that one or more of these planets exhibit a chemical signature of life.
Life could exist in the solar system where there is liquid water, such as in the underground aquifers of Mars or the oceans of Europa, Jupiter’s moon. However, searching for life in these places is incredibly difficult, as they are hard to reach and detecting life would require sending a probe to bring back physical samples.
Many astronomers believe there is a good chance that life exists on planets orbiting other stars, and it is possible that this is where life is first found.
Theoretical calculations suggest that there are around 300 million potentially habitable planets in the Milky Way galaxy alone and several Earth-sized habitable planets just 30 light-years from Earth – essentially the galaxy’s galactic neighbors. ‘humanity.
So far, astronomers have discovered more than 5,000 exoplanets, including hundreds of potentially habitable ones, using indirect methods that measure how a planet affects its nearby star. These measurements can give astronomers information about an exoplanet’s mass and size, but not much more.
In search of biosignaturesTo detect life on a distant planet, astrobiologists study starlight that has interacted with the planet’s surface or atmosphere. If the atmosphere or surface has been transformed by life, the light can carry a clue, called a “biosignature.”
For the first half of its existence, the Earth possessed an oxygen-free atmosphere, although it supported simple, single-celled life. Earth’s biosignature was very weak during this early epoch. That changed abruptly 2.4 billion years ago when a new family of algae evolved.
These algae used a process of photosynthesis that produced free oxygen, that is, oxygen that was not chemically bound to another element. Since then, Earth’s oxygen-filled atmosphere has left a strong and easily detectable biosignature on the light that passes through it.
When light bounces off the surface of a material or passes through a gas, certain wavelengths of light are more likely to become trapped in the gas or material surface than others.
This selective trapping of light wavelengths explains why objects are different colors. Leaves are green because chlorophyll is particularly good at absorbing light in the red and blue wavelengths. When light hits a leaf, the red and blue wavelengths are absorbed, leaving mostly green light bouncing back into your eyes.
The pattern of missing light is determined by the specific composition of the material with which the light interacts. For this reason, astronomers can learn something about the composition of an exoplanet’s atmosphere or surface by measuring, essentially, the specific color of light that comes from a planet.
This method can be used to recognize the presence of certain atmospheric gases associated with life – such as oxygen or methane – because these gases leave very specific signatures in the light.
It could also be used to detect particular colors on the surface of a planet. On Earth, for example, chlorophyll and other pigments used by plants and algae for photosynthesis pick up specific wavelengths of light. These pigments produce characteristic colors that can be detected using a sensitive infrared camera. If you were to see this color reflected off the surface of a distant planet, it would potentially signify the presence of chlorophyll.
Telescopes in space and on Earth
It takes an incredibly powerful telescope to detect these subtle changes in the light coming from a potentially habitable exoplanet. For now, the only telescope capable of such a feat is the new James Webb Space Telescope.
When he began his scientific operations in July 2022, James Webb noted 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 support life.
However, these early data show that James Webb is able to detect faint chemical signatures in light from exoplanets. In the coming months, Webb is expected to turn his mirrors toward TRAPPIST-1e, a potentially habitable Earth-sized planet located just 39 light-years from Earth.
Webb can search for biosignatures by studying planets as they pass in front of their host star and capturing starlight that filters through the planet’s atmosphere. But Webb was not designed to search for life, so the telescope can only examine a few of the nearest potentially habitable worlds.
It can also only detect changes in atmospheric levels of carbon dioxide, methane and water vapour. While certain combinations of these gases may suggest life, Webb is unable to detect the presence of unbound oxygen, which is the strongest signal for life.
Cutting-edge concepts for future, even more powerful space telescopes include blocking bright light from a planet’s host star to reveal starlight reflected from the planet. This idea is similar to using your hand to block sunlight to better see something in the distance. Future space telescopes could use small inner masks or large umbrella-shaped outer spacecraft to do this. Once starlight is blocked, it becomes much easier to study light bouncing off a planet.
There are also three huge ground-based telescopes being built that will be able to search for biosignatures: the Giant Magellan Telescope, the Thirty Meter Telescope, and the European Super-Large Telescope. Each of these telescopes is far more powerful than existing telescopes on Earth, and despite the handicap of Earth’s atmosphere which distorts starlight, these telescopes might be able to probe the atmosphere of the nearest worlds in search of of oxygen.
Is it biology or geology?
Even using the most powerful telescopes for decades to come, astrobiologists won’t be able to detect strong biosignatures produced by worlds that have been completely transformed by life.
Unfortunately, most gases released by life on Earth can also be produced by non-biological processes – cows and volcanoes both release methane. Photosynthesis produces oxygen, but sunlight also produces oxygen when it splits water molecules into oxygen and hydrogen.
It is highly likely that astronomers will detect a few false positives when searching for life at a distance. To rule out false positives, astronomers will need to understand a planet of interest well enough to know whether its geological or atmospheric processes could mimic a biosignature.
The next generation of exoplanet studies has the potential to pass the bar of extraordinary evidence needed to prove the existence of life. The first data released by the James Webb Space Telescope gives us an idea of the exciting progress that will soon be made.
Chris Impey, Distinguished Professor of Astronomy at the University of Arizona, and Daniel Apai, Professor of Astronomy and Planetary Sciences at the University of Arizona.