Typically, when we think of exoplanets, we think of neat spheres floating directly in front of their star, which acts as a giant spotlight. From our vantage point, they would look more like two perfect two dimensional circles than spheres, though. But nature is almost never so clean-cut. A new paper published in Nature Astronomy from researchers at Cornell and the University of Bristol details the first-ever study of an exoplanet that is noticeably lopsided.
The planet, WASP-17 b, has been a favorite of exoplanet researchers since it was discovered in 2009. It’s located around 1,300 light years away in the constellation Scorpius, and is what we now call a “hot Jupiter” - but an extreme version of that category. Its year is only four days long, and while it has only about half the mass of Jupiter, it is puffed up to nearly twice its diameter, giving it a density equivalent to styrofoam. But since it is so big and hot, it provides a natural laboratory for scientists to test theories about exoplanet atmospheres.
Traditional exoplanet research focuses on analyzing the starlight that filters through the planet’s atmosphere, and averaging it over the entire planetary silhouette to create a picture of the planet’s whole atmosphere. That technique, called transmission spectroscopy, is extremely useful when attempting to understand the overall chemical makeup of an exoplanet’s atmosphere. But it treats the planet as a uniform ring, completely ignoring the separate chemistry that might be happening between the morning side, evening side, or even the poles.
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To enable the differentiation that would allow them to see the atmosphere at those specific parts of a planet, the research team, led by Lili Anderson of the University of Bristol, used a technique called transmission strings. It utilizes a specialized modeling software called Harmonica to model the planet’s silhouette like an adjustable loop rather than a static circle. When the planet enters and exits its transit, the model calculates how the atmosphere’s thickness changes around its edges, and then analyzes the wavelengths from light that pass through those different angles.
Specifically for this paper, the researchers got time on the James Webb Space Telescope, whose infrared camera was uniquely suited to the job, and which has high enough resolution to be able to differentiate the edges of the planet’s atmosphere. What they found was exactly what they expected - WASP-17 b’s atmosphere is extremely asymmetric. The planet itself is tidally locked, so the temperature difference between the day side and the night side is massive, jumping from 858 Kelvin at the planet’s morning edge to 1,522 Kelvin at its evening terminator - a 650 K difference between the two sides of the planet.
As any high school physics class will tell you, hot gas expands. And with that much of a temperature difference, that expansion is going to be noticeable. At the hotter evening terminator edge, the atmosphere expanded to 1.8 times the size of the morning side, according to the paper. From Webb’s vantage point, the planet looks more like an egg sitting on its side than a perfect sphere.
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Part of that atmospheric expansion is driven by weather. Tidally locked planets have massive jet streams that travel eastwards along the planet’s equator, transferring heated gas from the hot day side to the cooler night side. While it’s on the darker night side, it cools significantly, allowing the atmosphere to deflate.
But even with such powerful winds, it appears that the chemicals that make up the atmosphere are relatively well-balanced. The paper shows that water vapor and carbon dioxide are evenly spread throughout the planet’s perimeter, remaining steady even between the equator and the poles. However, there also appear to be some changes in the chemical composition directly at the poles when it comes to methane and sulfur dioxide, which appear to be better at surviving away from the planet’s jet streams.
This is yet another feather in our cap of understanding this well studied world. Recently, astronomers have mapped the infrared heat from its dayside, studied its nighttime glow as it shed that heat, and even caught glimpses of tiny quartz crystals floating in its upper atmosphere. Now we can add a detailed map of the atmospheric size and makeup to one of the most complete, three dimensional views of an exoplanet’s atmosphere ever.
Likely this isn’t the last planet that will get the same sort of scrutiny. While the paper represents the first time a planet has been shown to be “uneven”, Webb and the even more powerful telescopes planned for later in the century are capable of providing that insight for plenty of other exoplanets. It’s only a matter of time before we truly understand the size and shape of some of our interstellar neighbors better than we ever have before.
Learn More:
Cornell - Pulling strings: New method maps exoplanet’s true shape
L. Anderson et al - A spatial map of the atmospheric limb of a hot Jupiter