Science

To find alien life, astronomers will look for clues in the atmospheres of distant planets – and the James Webb Space Telescope has just proven it’s possible.

This article was originally published in The Conversation. (will open in a new tab) The publication published an article in Expert Voices: Op-Ed & Insights on Space.com.

Chris Impey (will open in a new tab)Distinguished Professor of Astronomy at the University of Arizona
Daniel Apai (will open in a new tab)professor of astronomy and planetary sciences at the University of Arizona

The ingredients for life are scattered throughout the universe. Although the Earth is the only known place in the universe where life exists, the discovery of life beyond the Earth is the main goal. (will open in a new tab) modern astronomy (will open in a new tab) and planetary science (will open in a new tab).

We are two scientists who study exoplanets and astrobiology. (will open in a new tab). Thanks in large part to next-generation telescopes like the James Webb Space Telescope, researchers like us will soon be able to measure the chemical composition of the atmospheres of planets around other stars. It is hoped that one or more of these planets will have the chemical signature of life.

There are many known exoplanets in habitable zones — orbits not too close to the star for water to evaporate, but not far enough for the planet to freeze — as marked in green for both the Solar System and the Kepler-186 star system with its planets. labeled b, c, d, e, and f. (Image credit: NASA Ames/SETI Institute/JPL-Caltech/Wikimedia Commons)

habitable exoplanets

Life can exist in the solar system (will open in a new tab) where there is liquid water – for example, underground aquifers on Mars or in the oceans of Jupiter’s moon Europa. However, the search for life in these places is incredibly difficult, as they are difficult to reach, and discovering life would require sending a probe to return physical samples.

Many astronomers believe that there is a strong possibility that life exists on planets orbiting other stars. (will open in a new tab)and perhaps it is there that life will be found for the first time (will open in a new tab).

Theoretical calculations show that there are about 300 million potentially habitable planets. (will open in a new tab) only in the Milky Way galaxy and a few Earth-sized habitable planets (will open in a new tab) only 30 light years from Earth – in fact, the galactic neighbors of humanity. So far, astronomers have discovered more than 5,000 exoplanets. (will open in a new tab)including hundreds of potentially habitable, by indirect methods (will open in a new tab) which measures how a planet affects its nearest star. These measurements can give astronomers information about the exoplanet’s mass and size, but not much else.

Each material absorbs specific wavelengths of light, as shown in this chart depicting the wavelengths of light most readily absorbed by different types of chlorophyll. (Image credit: Daniele Pugliesi/Wikimedia Commons, CC BY-SA)

Searching for biosignatures

To detect life on a distant planet, astrobiologists will study starlight that has interacted with the planet’s surface or atmosphere. (will open in a new tab). If the atmosphere or surface has been transformed by life, the light may carry a clue called a “biosignature”.

During the first half of its existence, the Earth had an atmosphere without oxygen, although simple single-celled life existed on it. During this early era, the Earth’s biosignature was very weak. This changed dramatically 2.4 billion years ago. (will open in a new tab) when a new family of algae appeared. The algae used the process of photosynthesis to produce free oxygen—oxygen that is not chemically bonded to any other element. Since then, Earth’s oxygenated atmosphere has left a strong and easily detectable biosignature on light passing through it.

When light reflects off the surface of a material or travels through a gas, some wavelengths of light are more likely to remain in the gas or on the surface of the material than others. Selective capture of wavelengths of light explains why objects have different colors. The leaves are green because chlorophyll is particularly good at absorbing light in red and blue wavelengths. When light hits a leaf, the red and blue wavelengths are absorbed, leaving mostly green light reflected back into your eyes.

The nature of the transmitted light is determined by the specific composition of the material with which the light interacts. Because of this, astronomers can learn something about the composition of an exoplanet’s atmosphere or surface, essentially by measuring the specific color of light coming from the 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 traces in light. It can also be used to detect specific colors on the planet’s surface. On Earth, for example, chlorophyll and other pigments used by plants and algae for photosynthesis trap specific wavelengths of light. These pigments give the characteristic colors. (will open in a new tab) which can be detected with a sensitive infrared camera. If you were to see this color reflecting off the surface of a distant planet, it would potentially mean 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 changes in the light coming from a potentially habitable exoplanet. So far, the only telescope capable of such a feat is the new James Webb Space Telescope. How scientific activity began (will open in a new tab) In July 2022, James Webb measured the spectrum of the gas giant exoplanet WASP-96b. (will open in a new tab). The spectrum showed the presence of water and clouds, but a planet as large and hot as WASP-96b is unlikely to be home to life.

However, these early data show that James Webb is able to detect faint chemical signatures in the light coming from exoplanets. In the coming months, Webb will point its mirrors at TRAPPIST-1e, a potentially habitable Earth-sized planet just 39 light-years from Earth.

Webb can look for biosignatures by studying planets as they pass in front of their stars and by capturing starlight as it passes through the planet’s atmosphere. (will open in a new tab). But Webb was not designed to search for life, so the telescope is only able to carefully study a few nearby potentially habitable worlds. It can also only detect changes in atmospheric levels of carbon dioxide, methane, and water vapor. (will open in a new tab). While certain combinations of these gases may be indicative of life (will open in a new tab)Webb is unable to detect the presence of unbound oxygen, which is the strongest signal of life.

Leading concepts for future, even more powerful space telescopes include plans to block out the bright light of the planet’s host star to reveal starlight reflected off the planet. This idea is like using your hand to block out sunlight so you can see things better from a distance. Future space telescopes could use small inner masks or large outer spaceships that look like umbrellas to do this. Once the starlight is blocked, it becomes much easier to study the light bouncing off the planet.

Also currently under construction are three huge ground-based telescopes that will be able to search for biosignatures: the Giant Magellan Telescope. (will open in a new tab)Thirty meter telescope (will open in a new tab) and the European Extremely Large Telescope (will open in a new tab). Each of them is much more powerful than existing telescopes on Earth, and despite the fact that the Earth’s atmosphere distorts starlight, these telescopes can probe the atmospheres of nearby worlds for the presence of oxygen.

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

Is it biology or geology?

Even using the most powerful telescopes of the coming decades, astrobiologists will only be able to detect strong biosignatures created by worlds that have been completely transformed by life.

Unfortunately, most of the gases given off by terrestrial life can also be produced by non-biological processes—cows and volcanoes give off methane. Photosynthesis produces oxygen, but so does sunlight when it splits water molecules into oxygen and hydrogen. There is a good chance that astronomers will find several false positives. (will open in a new tab) while searching for a distant life. To rule out false positives, astronomers need to study the planet of interest well enough to understand whether its geological or atmospheric processes can mimic the biosignature. (will open in a new tab).

The next generation of exoplanet research may exceed the bar for extraordinary evidence. (will open in a new tab) needed to prove the existence of life. The first release of data from the James Webb Space Telescope gives us a sense of the exciting progress that is about to happen.

This article is reprinted from The Conversation (will open in a new tab) under a Creative Commons license. Read original article (will open in a new tab).

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