We live in very exciting times. The answers to some of humanity’s oldest questions are within our reach. One question is: Is Earth the only place where life exists?

Over the past 30 years, the question of whether the Sun is the sole host of planetary systems has been successfully answered. We now know that there are thousands of exoplanets orbiting other stars.

But can we use telescopes to detect whether life exists on these distant worlds? A promising method is to analyze the gases present in the atmospheres of these planets.

Currently, more than 6,000 exoplanets are known. With so many things cataloged today, there are many ways to narrow down which worlds are most promising for biology. For example, astronomers can use a planet’s distance from its host star to determine the planet’s estimated temperature.

Earth is the only planet in the solar system with an ocean of liquid water on its surface, so warm temperatures are considered necessary for a habitable planet. Whether a planet has a suitable temperature for liquid water is strongly influenced by the presence and nature of the planet’s atmosphere.

Remarkably, it is possible to identify molecules present in the atmospheres of exoplanets. Thanks to quantum mechanics, each chemical in the atmosphere has a unique barcode-like pattern that remains in the light that passes through it. By collecting starlight filtered through an exoplanet’s atmosphere, telescopes can see the barcode of the molecules that make up that atmosphere.

To take advantage of this, the planet must pass in front of the star from our perspective. This means it only works for a small fraction of known exoplanets.

The strength of the signal depends on the abundance of molecules in the atmosphere, being stronger for the most abundant molecules and gradually weakening as the abundance decreases. This means that the main molecule is generally the easiest to detect, but this is not always the case. Some barcodes are inherently strong, while others are weak.

For example, Earth’s atmosphere is dominated by diatomic nitrogen (N₂), but this molecular barcode is weak compared to the much less abundant diatomic oxygen (O₂), ozone (O₃), carbon dioxide (CO₂), and water (H₂O).

Detection of molecules

The James Webb Space Telescope (JWST) is a large space telescope that collects light at infrared wavelengths. It has been used to investigate the atmospheres of various exoplanets.

Detecting molecular signatures in the atmospheres of exoplanets is not entirely straightforward. Different teams of workers can make slightly different choices in how to process the same data, resulting in different results. However, despite these difficulties, reproducible and robust detection of molecules has been achieved. Simple molecules with strong barcodes have been detected, including methane, carbon dioxide, and water.

Planets larger than Earth and smaller than Neptune, so-called subneptunes, are the most common type of exoplanet known. It was on one of these planets, K2-18b, that bold claims were made that a biosignature was detected in 2025. The analysis detected dimethyl sulfide and claims the chance of this detection being false is less than once in 1,000 years.

On Earth, dimethyl sulfide is produced by oceanic phytoplankton, but it is rapidly broken down in seawater exposed to sunlight. Since K2-18b may be a planet entirely covered by oceans of water, the detection of dimethyl sulfide in its atmosphere could indicate a continuous supply of dimethyl sulfide from its microbial marine life.

Reexamination of K2-18b dimethyl sulfide detection by other researchers casts doubt on this claim. Most importantly, a 2025 demonstration by Lewis Wellbanks and colleagues at Arizona State University showed that the choice of molecular barcodes to include in an analysis fundamentally influences the results.

They found that a number of alternatives not considered in the original paper provided equivalent or better fits to the measured data.

For an Earth-sized planet that is probably rocky, it would be very difficult to detect an atmosphere with JWST. But the future is promising, with a number of planned missions allowing us to learn more about potentially Earth-like planets.

Future missions

The European Space Agency’s Plato Telescope, scheduled to launch in 2026, will identify planets that are much more similar to Earth than currently known planets and are therefore amenable to transmission spectroscopy.

NASA’s Nancy Grace Roman Space Telescope, scheduled to launch in 2029, will pioneer coronagraphy techniques that can cancel out starlight and directly study very faint planets orbiting nearby stars.

The European Space Agency’s Ariel telescope, scheduled to launch in 2029, is a dedicated transmission spectroscopy mission designed to determine the composition of exoplanets’ atmospheres.

NASA’s Habitable World Observatory (HWO) is currently in the planning stages. The mission will use coronagraphs to study about 25 Earth-like planets, looking for various characteristics of habitability.

HWO covers a wide range of wavelengths from ultraviolet to near-infrared. If Earth’s twin orbits one of the HWO’s nearby target stars, the telescope will collect the starlight reflected from the planet. This reflected starlight contains the barcode signature of diatomic oxygen (O₂) and other gases that are characteristic of Earth’s atmosphere. It will also reveal traces of starlight absorbed by photosynthesizing plants, the so-called “vegetation red edge.”

The Earth’s surface is divided into land and ocean, which reflect light differently. HWO will be able to reconstruct low-resolution maps of the Earth’s surface from changes in reflected light as continents and oceans rotate in and out of view.

Therefore, the future looks very promising. A spacecraft scheduled to launch in the next few years may move us closer to the question of whether Earth is unique in hosting life.