Meet WASP-17b: A 'Puffy' Planet
Located about 1,300 light-years away, WASP-17b is one of the most peculiar planets ever discovered. It’s a type of exoplanet known as a 'hot Jupiter'—a gas giant orbiting extremely close to its star. A single year on this world lasts less than four Earth
days. But what makes WASP-17b truly bizarre is its size and density. It's nearly twice as wide as Jupiter but has only about half its mass, giving it an incredibly low density often compared to styrofoam or a marshmallow. This 'puffiness' is thought to be caused by intense heat from its nearby star, which inflates its atmosphere. Adding to its strangeness, WASP-17b is tidally locked, meaning the same side always faces its star, creating a permanent, scorching dayside and a cooler, endless nightside.
Mapping an Atmosphere From Light-Years Away
Until now, when astronomers studied exoplanet atmospheres, they typically averaged the light passing through the entire atmospheric edge, or 'limb'. This method, called transmission spectroscopy, is great for identifying chemicals but treats the atmosphere as a uniform ring. However, a team of researchers from Cornell University and the University of Bristol developed a new technique called 'transmission strings'. Using the unparalleled power of the James Webb Space Telescope (JWST), they analysed starlight filtering through different parts of WASP-17b's atmosphere as the planet passed in front of its star. This allowed them to measure atmospheric properties point-by-point around the planet's edge, moving beyond a simple ring to see its true three-dimensional shape.
The Surprise: A Lopsided Shape
The 3D map confirmed what theoretical models had long predicted: WASP-17b's atmosphere is not a perfect, symmetrical circle. Instead, it’s distorted and lopsided. The team found a massive temperature difference between the 'morning' side of the planet (the edge rotating into view) and the 'evening' side (the edge rotating out of view). The evening limb is significantly hotter, with a temperature of around 1,522 Kelvin (about 1,250 degrees Celsius), while the morning limb is a cooler 858 Kelvin (about 585 degrees Celsius). Just as hot air expands, this extreme temperature difference causes the gas on the hotter evening side to swell outwards, making that part of the atmosphere much more extended and 'puffy' than the cooler morning side. The result is an uneven, egg-like shape rather than a neat sphere.
Why It's Not a Perfect Ring
The reason for this atmospheric distortion lies in the planet’s extreme weather. Because WASP-17b is tidally locked, its permanently sun-baked dayside gets incredibly hot. This heat drives ferocious equatorial winds, creating a jet stream that carries scorching hot gas from the dayside towards the nightside. This eastward flow of energy means that by the time the gas reaches the 'evening' boundary between day and night, it is at its hottest, causing the atmosphere there to inflate. As the winds continue across the dark nightside, the gas cools and contracts, so by the time it reaches the 'morning' boundary, the atmosphere has deflated significantly. In essence, the new 3D map is not just a picture of the planet’s shape but a real-time weather map of its violent atmospheric dynamics.
A New Era for Exoplanet Weather
This study is more than just a weather report for a single, strange planet; it’s a major proof-of-concept. For the first time, scientists have moved beyond computational models to see the true 3D shape of an exoplanet's atmosphere, confirming that our theories about atmospheric dynamics on these extreme worlds are accurate. WASP-17b now serves as a benchmark for studying the more than 6,000 other confirmed exoplanets. The 'transmission strings' technique opens the door to creating similar weather maps for other worlds, and even to start distinguishing conditions by latitude, not just between day and night. This is the first step toward mapping weather-scale patterns in alien climates and understanding the complex, chaotic, and beautiful diversity of planets beyond our solar system.
