That’s hot! A glimpse at the atmosphere of WASP-33b

3000K in daylight and 1500K overnight. These are the temperature conditions on WASP-33b, a tidally-locked ultra-hot Jupiter exoplanet, where one half of the planet is perpetually bathed in starlight, and the other half remains timelessly in the dark. Such extreme conditions trigger exceptional chemistry on the planet and, as always, spectroscopy can help us study that chemistry and how it shapes the planet’s atmosphere. Join us in today’s astrobito to learn more about the atmospheric structure of WASP-33b and how ground-based observations have helped improve our understanding of this blistering planet.

Paper details:

Title: Out of the Darkness: High-resolution detection of CO absorption on the nightside of WASP-33b

Authors: Georgia Mraz, Antioine Darveau-Bernier, Anne Boucher, Nicolas B. Cowan, David Lafreniere, Charles Cadieux

First-author Institution: Department of Physics and Trottier Space Institute, McGill University, Quebec, Canada

Status: Accepted for publication in ApJ Letters.

Imagine a world where, depending on where you stand, you could experience an entire year of endless daylight or perpetual darkness. This is the reality on WASP-33b, an ultra-hot Jupiter exoplanet situated about 380 light-years away in the constellation Andromeda.

With an orbital period of roughly 1.22 Earth days, WASP-33b is tidally locked to its host star. This means that the time it takes to rotate on its axis is about the same as the time it takes to complete an orbit around the star. As a result, one side of the planet is constantly bathed in starlight, while the other side remains in the dark. This unique environment leads to fascinating chemical and physical characteristics on the planet, and in today’s astrobito, we’ll explore how a team of researchers used ground-based telescope observations to investigate this intriguing exoplanet!

Ultra hot Jupiters

You may have heard of hot Jupiters—massive, gaseous exoplanets about the size of Jupiter that orbit very close to their host stars, almost like placing Jupiter in the orbit of Mercury in our solar system. Ultra-hot Jupiters are similar, but they orbit much hotter and larger stars, usually F- and A-type stars, rather than stars similar to our Sun (G- and K-type stars). This characteristic, together with their very short star-planet distance, creates extreme temperature conditions on ultra-hot Jupiter exoplanets.

Main star sequence. Our sun (K-type star) has a temperature of ~6000K. A and F-type stars have temperatures greater than 6000K. This figure has been adataped from https://www.skyatnightmagazine.com/space-science/main-sequence-stars

Indeed, ultra-hot Jupiters can reach astonishing temperatures typically exceeding 2000 K; that is hot enough to melt iron! But what’s even more fascinating is that, as these planets are tidally locked with one half always facing their star and the other looking in the opposite direction at all times, there is a huge temperature difference of up to 1000 K between the planet’s dayside and nightside. This dramatic temperature variation has significant effects on the chemistry that these planets experience.

During the scorching daytime, the intense heat can break down molecules into their basic atomic components, letting loose different atoms and even vaporising elements like sodium (Na) and potassium (K). Meanwhile, the “cooler” nighttime conditions allow these atoms to recombine into new molecules.

The presence of these vaporised elements in the dayside atmosphere leads to another intriguing feature of ultra-hot Jupiters: thermal inversions during the day. On Earth, temperatures usually decrease with altitude; for example, it’s warmer at sea level than on the summit of Everest. However, on the dayside of ultra-hot Jupiters the opposite occurs. As you rise in the atmosphere, the temperature actually increases. The reason for this? Well, it is all about spectroscopy.

Absorption and emission features

In the scorching dayside atmosphere of ultra-hot Jupiters, free atoms like sodium and potassium roam freely, allowing them to absorb high-energy radiation from the star. When these atoms absorb this radiation—primarily ultraviolet light—they become excited and can later release that energy back into the atmosphere, typically as infrared light. This mechanism helps the planet retain heat in its upper atmosphere, leading to increasing temperatures as you move upwards.

The story is different on the nightside. The much “cooler” temperatures in the atmosphere encourage the recombination of atoms into molecules, directly affecting how much radiation is emitted back into the atmosphere. Molecules will have different emission requirements than free atoms that might not be met by the amount of radiation available on the nightside. This means that, while some radiation is emitted back into the atmosphere, it is not enough to create a thermally inverted atmosphere.

During daytime, the molecules in the atmosphere of ultra hot-Jupiters dissociate and get spectroscopically excited, emitting radiation back into the atmosphere. At night time, the “cooler” temperatures allow recombination of molecules, leading to less intense emissions.

These spectroscopic differences can tell us a lot about the planet’s underlying chemistry and physics. Non-inverted atmospheres will usually be characterised by strong absorption features, whereas inverted atmospheres will have strong emission features.

Using these criterion, the team behind today’s paper was able to confirm the atmospheric structure of WASP-33b: a temperature inversion on the dayside, characterised by strong emission features from chemical species like CO, Fe, OH and H2O, and a non-inverted atmosphere on the nightside, as evidenced by the detection of CO in absorption but not in emission.

Ground-based observations

With telescopes like the James Webb Space Telescope currently rewriting our approach for studying exoplanets, it is normal to think that ground-based telescopes might become obsolete at some point. This is not the reality at all though. Ground-based telescopes can have an incredible resolution and overcome some of the limitations that space-based telescopes face. This is exactly what this research group proved with their atmospheric study of WASP-33b.

To study both the dayside and nightside atmosphere of WASP-33b, or any other hot or ultra-hot Jupiter, using space-based telescopes, astronomers need to analyse how the light from the planet changes as the planet moves around its full orbit. Performing these observations is complex and requires significant resources to carry out.

Ground-based observations, on the other hand, can help ease some of this complexity. By observing the planet just before and after it transits in front of its host star (as seen from Earth), we can have a look at the light directly emitted by the planet and distinguish it from that of the star. This allows us to glimpse into what happens on the nightside of the planet, without performing full orbit observations.

Orbital phases of WASP-33b. The nightside of the exoplanet can be observed using ground-based telescopes when it transits in front of its host star. Figure adapted from Figure 1 in the main paper.

The team behind this scientific paper took advantage of this by observing the nightside of WASP-33b over five Earth-nights using the Spectro-Polarimètre Infra-Rouge on the Canada-France-Hawaii Telescope. Ultimately, these observations allowed them to detect the CO molecular absorption on the nightside of WASP-33b, confirming a non-thermally inverted atmosphere. To date, this is the strongest ground-based detection of nightside thermal emission from an exoplanet.

A story forged in extreme conditions

Ultra-hot Jupiters showcase some of the most extreme and fascinating environments in our universe. The stark contrast between their scorching dayside and relatively "cooler" nightside creates unique chemical conditions that reveal much about these planets. By examining the atmospheres of these blistering worlds, astronomers gain tremendous insights into their thermal structure and how heat moves from one half of the planet to the other. This movement influences the types of molecules present during the day and night.

As always, spectroscopy is our main tool for exploring these distant worlds, and today’s article illustrates the amazing insights we can gain from analysing exoplanet light. Moreover, the research on WASP-33b emphasises the essential contributions of ground-based telescopes in astronomy. At the end of the day, it is the combined strength of ground and space-based observations that enable us to fully uncover the complex stories behind exoplanets like WASP-33b.

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