Space

JWST: A new era of planetary science

In case you missed part one of the JWST series, check it out here for explanations of more of the first images, as well as an insight into how the telescope works!

But now, its time to move on to the JWST targets I find most intriguing- planets!

WASP 96b: A distant world of discovery

Although this image may not be as immediately awe-inspiring as the others, it is actually showing the composition of the atmosphere, and subsequent discovery of water, on the exoplanet WASP 96b.

How? Well the graph is showing the how much of the light detected at each wavelength is ‘missing’, ie. how much has been absorbed by the exoplanet’s atmosphere as light from its star passed through on its way to JWST.

This occurs during a primary transit, when the planet passes between the earth and the star, blocking some of the star’s light, and filtering an even more minute fraction through its atmosphere, which is detected by JWST. More specifically, it is detected by NIRISS, a specialised instrument aboard the telescope which can split the light it receives into its component wavelengths (like a prism making a rainbow). As the light passed through the exoplanet atmosphere, the molecules in the atmosphere absorbed some of the light. Different molecules will absorb different combinations of wavelengths, so interpreting the absorption lines detected can reveal the composition of the target (in this case, the exoplanet’s atmosphere.


Welcome to WASP 96b

WASP 96b is a hot Jupiter, ie. it is a massive, gaseous planet (in this case about half the mass of Jupiter, so its really more of a hot Saturn but that doesn’t quite have the same ring to it), orbiting very close to its star- within 0.043 Astronomical Units. Because of this (this being its toasty temperature of 723°C, nearly double that of Venus), the planet has a lower density than Jupiter, so is 1.2x the size of our largest planet, despite having less mass.

For reference, an astronomical unit is the distance between the earth and the sun, and tends to be the standard unit for measurements within planetary systems- in our solar system, Mercury orbits at 0.387AU, so WASP 96b is almost 10 times closer, orbiting once every 3.4 days! A lot of this characterisation data was already known before JWST, but its always reassuring when the numbers stand up to scrutiny by the next generation telescope!

WASP 96 may be a sun-like star, but that may be where the similarities end for this planetary system. And although alien to us, hot Jupiters actually appear to be very common, making up 20% of exoplanets discovered by 2011, but this is probably because they are the easiest to detect and confirm due to their large sizes (so block more light in a transit), large masses (for detection by radial velocity) and short orbital periods, so the actual proportion is probably closer to 1-2%.


The simple fact we have found water using the JWST spectrograph isn’t that surprising, as water is one of the most common compounds in the universe, simply due to the fact its constituents- hydrogen and oxygen- are some of the most common elements in the universe (1st & 3rd, respectively, just separated by the unreactive Helium).

What is surprising is where we found it:

We’ve found water on exoplanets before, but only on worlds within a few hundred light years, limited by the resolution of existing telescopes. WASP 96b? Over a thousand light years away, meaning the light Webb detected was emitted in 902AD, the time of the Vikings, Eastern Roman/Byzantine Empire and the end of the Tang Dynasty in China.

So just to detect the light from the star, and isolate the tiny fragment of this light which has passed through the atmosphere of this planet, and collect enough of it to be able to observe its spectrum, is truly a technological feat.

Spectral features change depending on the atmosphere’s structure (via astronomy.com, N. Nikolov/E. de Mooij)

Additionally, before this new data, WASP 96b was thought to have a transparent, waterless atmosphere. This is because for most hot jupiters, if sodium is detected in their atmospheres, they tend to be cloudy, as shown by having missing or truncated spectral lines, but 2018 data showed full sodium spectral lines, suggesting it was cloudless, and no water signature was detected. However, the study also concluded with the following thoughts:

“Though there is a hint of a slight slope blue-ward of 550 nm, there was not substantial evidence supporting the need for a scattering slope. However, bluer, space-borne observations of the planet could discern if the hint of a blue-ward slope is indeed a true feature”

And here we are, 4 years later, with a spectrum from a more powerful space telescope covering a wider range of wavelengths (600nm to 28300nm compared to just 400-835nm) that it indeed has a downward slope towards shorter wavelengths (‘blue-ward’), which suggests a hazy atmosphere!

The paper also finished with recommending more “in-depth analysis of a higher sample of planets in order to find such a correlation” [to identify what determines the formation of clouds on exoplanets], so perhaps this will be one of the medium-long term outcomes of JWST’s future observations.

However, what will be interesting to see (and will hopefully be determined following greater analysis of the spectrum), is just how much water is in WASP 96b’s atmosphere, as the incredible resolution of JWST could mean it is able to pick up much smaller quantities than previously. It is particularly interesting as a 2019 study suggested that although water is frequently present in exoplanet atmospheres, it is often not as abundant as one might expect, especially for hot Jupiters, and JWST may help to investigate this further.


Why is JWST so cool for planetary science?

In particular, it is useful for studying the atmospheres of planets, due to both the ground-breaking level of detail and resolution possible, but also because of the range of wavelengths it is able to cover can include absorption lines from key molecules like oxygen, methane and carbon dioxide, as well as water!

And at risk of sounding like a broken record, again its the level of detail possible, in such a short period of time- this data comes from a single 2.5 hour transit, although the telescope also observed slightly before and after, but still only taking a total 6 hours 23 minutes for the entire observation!


The Bonus Images: Jupiter in a new light

Not included in the official release of first images, this spooky view of Jupiter was actually taken in the commissioning phase, when engineers were checking how well JWST was able to track the (relatively) fast moving objects like planets and other close targets it will attempt to image in the future. Meaning this image is nowhere near the best one of the gas giant we might be getting at some point!

Jupiter and some of its moons in the Infrared (L- 2.12 micron filter, R- 3.23 micron filter)

The data comes from the telescope’s commissioning period– the time after launch when the telescope was being deployed, aligned and tested to ensure it is ready for the first round of science observations.

The Solar System objects observed (Jupiter and some asteroids such as the one depicted below) were used to assess the speed at which the telescope is able to track objects across the sky, and luckily, it exceeded expectations and is able to track at speeds twice as fast as planned!

What would you like to see JWST image? Stay tuned for my next post on the last of the first photos!

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