JWST has shown it can detect signatures of life on exoplanets

The ingredients of life are spread throughout the universe. While Earth is the only known place in the universe with life, detecting life beyond Earth is a major goal of modern astronomy and planetary science.

We are two scientists who study exoplanets and astrobiology. Thanks in large part to next-generation telescopes like 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 will have a chemical signature of life.

There are many known exoplanets in the habitable zones – orbits not too close to a star where water is boiling but not too far for the planet to be frozen – as shown in green for the Solar System and the Kepler Star System – 186 with its planets labeled b, c, d, e and f. Image credit: NASA Ames/SETI Institute/JPL-Caltech/Wikimedia Commons

Habitable exoplanets

Life could exist in the solar system where there is liquid water – like the underground aquifers on Mars or in the oceans of Jupiter’s moon Europa. 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 return physical samples.

Many astronomers believe there’s a good chance that life exists on planets orbiting other stars, and it’s possible that’s where life was 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 exoplanets, 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 else.

Each material absorbs certain wavelengths of light, as shown in this diagram illustrating which wavelengths of light are most readily absorbed by different types of chlorophyll. Image credit: Daniele Pugliesi/Wikimedia Commons, CC BY-SA

In search of biosignatures

To detect life on a distant planet, astrobiologists will study starlight that has interacted with a 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 sported an oxygen-free atmosphere, even though it supported simple, single-celled life. Earth’s biosignature was very weak during this first era. That changed abruptly 2.4 billion years ago when a new family of algae evolved. Algae have used a process of photosynthesis that produces free oxygen – oxygen that is not chemically bound to anything else. 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, red and blue wavelengths are absorbed, leaving mostly green light to bounce back into your eyes.

The missing light pattern 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 essentially measuring 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 a planet’s surface. On Earth, for example, chlorophyll and other pigments that plants and algae use for photosynthesis capture specific wavelengths of light. These pigments produce characteristic colors that can be detected using a sensitive infrared camera. If you saw this color reflecting off the surface of a distant planet, it would potentially signify the presence of chlorophyll.

Telescopes in space and on Earth

The James Webb Space Telescope is the first telescope capable of detecting the chemical signatures of exoplanets, but its capabilities are limited. Image credit: NASA/Wikimedia Commons

It takes an incredibly powerful telescope to detect these subtle shifts in light coming from a potentially habitable exoplanet. For now, the only telescope capable of such a feat is the new James Webb Space Telescope. At the start of science operations in July 2022, James Webb took a reading of 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 just 39 light-years from Earth.

Webb can search for biosignatures by studying planets as they pass in front of their host stars and capturing starlight that filters through the planet’s atmosphere. But Webb was not designed to search for life, so the telescope can only scan 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 can suggest life, Webb is unable to detect the presence of unbound oxygen, which is the strongest signal for life.

Key concepts for future, even more powerful space telescopes include plans to block 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 internal masks or large external umbrella-shaped spacecraft to do this. Once starlight is blocked, it becomes much easier to study light bouncing off a planet.

There are also three massive ground-based telescopes currently under construction that will be able to search for biosignatures: the Giant Magellen Telescope, the Thirty Meter Telescope, and the European Extremely Large Telescope. Each is far more powerful than existing telescopes on Earth, and despite the handicap of Earth’s atmosphere distorting starlight, these telescopes might be able to probe the atmospheres of the nearest worlds for oxygen.

Animals, including cows, produce methane, but so do many geological processes. Image credit: Jernej Furman/Wikimedia Commons, CC BY

Is it biology or geology?

Even using the most powerful telescopes for decades to come, astrobiologists will only be able to detect 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 both release methane. Photosynthesis produces oxygen, but so does sunlight when it splits water molecules into oxygen and hydrogen. Chances are astronomers will pick up false positives when searching for distant life. To help eliminate false positives, astronomers will need to understand a planet of interest well enough to understand 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 release of data from the James Webb Space Telescope gives us an idea of ​​the exciting progress to come.The conversation

Chris Impey, Emeritus University Professor of Astronomy, University of Arizona and Daniel Apai, professor of astronomy and planetary sciences, University of Arizona

This article is republished from The Conversation under a Creative Commons license. Read the original article.

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