JWST’s carbon dioxide discovery is good news for Earth-like worlds

NASA’s James Webb Space Telescope (JWST) made a big splash when it revealed its first science images in July; since then, the fresh, exciting science results haven’t stopped coming. The telescope’s new insight into exoplanets is among its most fascinating findings.

Just a month after the observatory’s first science images were revealed, an international collaboration of over 300 scientists led by UC Santa Cruz Professor Natalie Batalha has announced the discovery of carbon dioxide in the atmosphere of the hot Jupiter WASP-39b, and another international collaboration has revealed the first direct image of the giant exoplanet HIP 65426b from the observatory.

The discovery of carbon dioxide in another Solar System is massive news, with exciting implications for future JWST observations of Earth-like worlds. Carbon dioxide — along with oxygen and ozone — are key molecules that will help astronomers understand the potential habitability of Earth-like exoplanets. Learning more about WASP-39b is an important part of this.

What is WASP-39b?

WASP-39b is a hot Jupiter discovered in 2011 as part of the Wide Angle Search for Planets (WASP) survey. This planet has a low density, and therefore a puffy, extended atmosphere making it a good target for transmission spectroscopy: it’s easier to measure an atmosphere when it blocks out more starlight.

This is why WASP-39b was selected by the leaders of the JWST Transiting Exoplanet Community Early Release Science (ERS) program, who sought to measure the transmission spectrum of a planet with many different JWST instruments and test their performance.

The transmission spectrum of WASP-39b had been previously measured by the Hubble Space Telescope at shorter wavelengths of light, but because JWST can measure longer infrared radiation, newer molecules could be detected with its suite of instruments. Plus, the star WASP-39 is relatively stable in its brightness over time. This combination of quiet star, puffy planet, and plenty of archival observations from HST made the WASP 39 system a safe place to start JWST’s exoplanetary journey.

In their publication (accepted to the journal Nature), the Transit ERS team used JWST’s Near Infrared Spectrograph (NIRSPEC), capturing a never-before-seen look at a planet’s atmosphere between 0.5 and 5.5 microns. The result was a clear detection of absorption caused by carbon dioxide in the planet’s atmosphere.

“We expected to see carbon dioxide in WASP-39b's atmosphere, but the quality of the data was like looking at a model,” said Lili Alderson, a PhD student at Bristol University and member of the ERS team.

This result is truly groundbreaking, as it proves that JWST will be able to make measurements of exoplanet atmospheres that were previously impossible. But, as Alderson explains, measurements of Earth-like worlds will be much harder, and will require multiple observations that can build up the strength of the signal.

With that in mind, what does this result mean for future studies of rocky, terrestrial planets? And how do you measure the molecules in an exoplanet’s sky in the first place? To answer these questions, we need to understand the observational technique called “transmission spectroscopy.”

Demystifying exoplanet atmospheres

One method astronomers use to observe an exoplanet’s atmosphere is called “transmission spectroscopy.” Planets that pass between their host stars and our line of sight from the Earth “transit” and block out a portion of their star’s light periodically. This chance alignment has enabled surveys using the “transit method” to detect thousands of planets; space-based observatories like Kepler and TESS have become highly successful with this technique, discovering hundreds of fascinating planetary systems.

As a planet transits its host star, different molecules in the planet’s atmosphere can absorb more or less starlight of a particular color (that is, of a particular wavelength). This was demonstrated by one of JWST’s first release images, the transmission spectrum of another hot Jupiter WASP-96b. The water molecules in this planet’s atmosphere absorb light, creating lumpy absorption signals, like the bump you can see between 1.3 and 1.5 microns.

By measuring the spectrum of a planet-hosting star during a transit event, astronomers can measure how much light is blocked at a given wavelength, and use this “transmission spectrum” to construct different models that explain what kinds of molecules exist in the planet’s atmosphere. This is great for astrobiologists, who’ve spent years studying what kinds of molecules might indicate the most habitable exoplanets or even the presence of life itself.

Why is finding carbon dioxide important?

Astronomers haven’t yet been able to observe molecules (like, any) in the atmospheres of terrestrial planets. Using Hubble, the most some teams could do was rule out large, extended hydrogen atmospheres on terrestrial planets.

Even before its launch, JWST has been expected to change that. The ERS programs were intended to establish the capabilities of this flagship observatory — so what’s the verdict?

“It is clear that the NIR instruments are performing well, which bodes well for future observations,” said Zafar Rustamkulov, a PhD student at Johns Hopkins University and member of the ERS team.

“As an observer, I definitely breathed a sigh of relief that, yes, we are going to see some really awesome things with this telescope,” said Alderson.

There are still some major hurdles to overcome before astronomers can enjoy the sunsets of Earth-like planets, but the instrumentation itself doesn’t seem to be one of them.

Rustamkulov explains that the performance of the telescope should allow astronomers to see even thin atmospheres, akin to Mars’ sparse, carbon dioxide-dominated atmosphere. The problem is that many rocky, transiting planets might not have atmospheres at all.

Many terrestrial planets being targeted by JWST orbit M dwarfs — small but notoriously magnetically active stars. There are many reasons why M dwarfs are promising targets for transmission spectroscopy, but their bright flares and strong magnetic activity produce a lot more high-energy radiation than quiet, Sun-like stars. This excess radiation, combined with their blisteringly close orbits, might have stripped their atmospheres away, long before JWST could observe them.

“These planets are bathing in many times the x-ray and ultraviolet radiation we get on Earth,” Rustamkulov explained.

Additionally, unlike WASP-39, many M dwarfs are highly variable in their brightness, with many starspots. Uneven features on the surface of a star can complicate the transmission spectrum measured for a crossing planet, leading to misidentification of molecules or extra noise.

“We will need to be really careful as we analyze the data from terrestrial planets,” Alderson remarked.

Despite these issues, this first carbon dioxide detection has left astronomers feeling optimistic. After the result was published, ERS Principle Investigator Natalie Batalha said on Twitter: “We really do have a chance to detect the atmospheres of terrestrial-size planets like those orbiting TRAPPIST-1.”

Until those observations are thoroughly processed and analyzed, however, we can simply enjoy the infrared colors of WASP-39b’s sunset.