5

This is not the first direct image of an exoplanet ever taken from space, the Hubble Space Telescope had previously captured a direct image of a planet in orbit around the star Fomalhaut, but it is a proof of concept that the JWST can do this at infrared wavelengths.

But when it comes to exoplanet research, the JWST’s biggest contribution is undoubtedly its ability to break down the light it receives into spectra. Spectra are a measure of how much light at each wavelength is being received.

A lot of science can be extracted from spectra, because atoms and molecules each like to interact with different wavelengths. This creates a pattern of dark lines in the spectra that are effectively like fingerprints, each one unique to a specific atom or molecule. The JWST is so important in this regard because molecules really like to interact with infrared wavelengths. Hence, an infrared spectrum of a celestial object can reveal its chemical composition.

This is exactly what astronomers did with the JWST’s NIRISS instrument on the exoplanet WASP-96 b. The resulting graph showed the distribution of infrared light from 0.6 to 2.8 micrometres. WASP-96 b is notable because it often passes in front of its parent star.

A small proportion of the star’s light therefore passes through the exoplanet’s atmosphere, where the constituent atoms and molecules absorb their preferred wavelengths. This shows up as a drop in the intensity at those wavelengths. In this particular case, the JWST showed that WASP-96 b contained water vapour in its atmosphere.

The planet is a ‘hot Jupiter’, so-called because it has a mass of around half that of Jupiter in our own Solar System, yet orbits so close to its star that a year lasts just 3.4 days. The results themselves are still preliminary because a computer model of the planet’s atmosphere must be constructed. The model includes things like the abundance of various gases in the planet’s atmosphere, and the height and thickness of any clouds in the exoplanet’s atmosphere.

The next phase of this research is to extend this work to smaller and smaller exoplanets, eventually analysing Earth-sized worlds. This is more difficult, because smaller worlds have less dense atmospheres, but astronomers are optimistic.

“The JWST opens the door to smaller planets and cooler planets, more similar to our own Earth. And it will allow us to study giant planets in much more detail than we’ve ever had access to before,” says Laura Kreidberg, an exoplanet expert from the Max Planck Institute for Astronomy, Germany. “I feel like we’re at the very, very beginning of a really exciting journey.”

Planetary systems

It is not just planets around other stars that the JWST has been looking at. It has also been targeting some of the planets in our own Solar System. In its first released image of Jupiter, different wavelengths from the NIRCam instrument were combined to create an image where brightness represented altitude in the Jovian atmosphere. The higher a feature is, the more infrared light it reflects, so the brighter it appears.

Jupiter’s Great Red Spot, for example, a storm system so large that it could engulf the entire planet Earth, is so high in the planet’s atmosphere that it appears extremely bright at infrared wavelengths. The deeper cloud layers and hazes appear much darker by contrast. The auroras show up at the northern and southern poles of the planet in this image too. They are created when particles trapped in Jupiter’s magnetic field are funnelled into the giant world’s atmosphere, where they strike atoms and molecules and cause them to fluoresce.