Could there be life on faraway planets? A new discovery by NASA has taken researchers one step closer to finding out. 

NASA’s James Webb Space Telescope has captured the first clear evidence of carbon dioxide in the atmosphere of a planet outside the Earth's solar system.

The finding was on WASP-39 b, a gas giant discovered in 2011 which has a mass about the same as Saturn, a diameter 1.3 times greater than Jupiter and is orbiting a Sun-like star 700 light-years away.

Its extreme puffiness is related in part to its high temperature — 900 degrees Celsius.

Unlike the cooler, more compact gas giants in the Earth's solar system, WASP-39 b orbits very close to its star – about one-eighth the distance between the Sun and Mercury – completing one circuit in slightly more than four Earth days.

Previous observations from other telescopes, including NASA's Hubble and Spitzer space telescopes, revealed the presence of water vapour, sodium, and potassium in the planet's atmosphere.

However, James Webb's infrared sensitivity has confirmed the presence of carbon dioxide.

James Webb Space Telescope (JWST) Transiting Exoplanet Community Early Release Science team member Zafar Rustamkulov said as soon as the data hit their screen a "whopping carbon dioxide feature"  grabbed him. 

How do they do it? 

The research team use a method called spectroscopy. 

The basic premise of spectroscopy is that different materials emit and interact with different wavelengths or colours of light in different ways, depending on properties like temperature and composition.

From there researches can use spectra — the detailed patterns of colours — to figure out things like exactly how hot something is and exactly what elements and compounds it is made of, without ever sampling it directly.

The research team used James Webb's Near-Infrared Spectrograph (NIRSpec) for its observations of WASP-39 b. 

Transiting planets like WASP-39 b, whose orbits are observed edge-on rather than from above, can provide researchers with ideal opportunities to probe planetary atmospheres.

During a transit, some of the starlight is eclipsed by the planet completely, causing the overall dimming, and some is transmitted through the planet's atmosphere.

Because different gases absorb different combinations of colours, researchers can analyse small differences in brightness of the transmitted light across a spectrum of wavelengths to determine exactly what an atmosphere is made of.

In the resulting spectrum of the exoplanet's atmosphere, a small hill between 4.1 and 4.6 micrometres (one one-thousandth of a millimetre) presented the first clear, detailed evidence for carbon dioxide ever detected in a planet outside our solar system.

What does this mean? 

This is the first time an observatory has measured such subtle differences in brightness of so many individual colours across the 3 to 5.5 micrometre range in an exoplanet transmission spectrum.

According to NASA, access to this part of the spectrum is crucial for measuring abundances of gases like water and methane, as well as carbon dioxide, which are thought to exist in many types of exoplanets.

"Detecting such a clear signal of carbon dioxide on WASP-39 b bodes well for the detection of atmospheres on smaller, terrestrial-sized planets," JWST team leader Natalie Batalha said. 

NASA says understanding the composition of a planet's atmosphere is important because it tells us something about the origin of the planet and how it evolved.

"Carbon dioxide molecules are sensitive tracers of the story of planet formation," another researcher Mike Line said.

"By measuring this carbon dioxide feature, we can determine how much solid versus how much gaseous material was used to form this gas giant planet.

The finding, accepted for publication in Nature, offers evidence that in the future Webb may be able to detect and measure carbon dioxide in the thinner atmospheres of smaller rocky planets.