Webb data reveal a giant exoplanet's lopsided outline for the first time

Astronomers have mapped the uneven outline of a planet outside the solar system for the first time, Cornell University said. Using James Webb Space Telescope data, the team found that WASP-17b, a hot gas giant more than 1,000 light-years away, is roughly round but slightly egg-shaped, with one side bulging out further than the other. The study was published on 14 October in Nature Astronomy.

Light at the edge of the planet tells the shape

When a planet passes in front of its star, some starlight shines through the thin ring of atmosphere around its edge, called the limb. Astronomers usually average that light over the whole ring. The new method, which the team calls transmission strings, instead reads the light from different points around the edge, so it can tell where the atmosphere extends further out.

The result matched what theory predicts. WASP-17b always shows the same face to its star. Winds carry heat from the permanently lit dayside toward the nightside, so the evening edge, where the gas has just come from the day side, is hotter and puffs out further. The cooler morning edge sits lower. Water and carbon dioxide appeared consistent all the way around.

“This is proof of concept that planets are in fact 3D objects.” — Lili Alderson, Klarman Postdoctoral Fellow, Cornell University

Alderson is the first author. The data came from Webb's TST DREAMS program, led by Cornell's Nikole Lewis, and the paper has 20 co-authors, including Hannah Wakeford of the University of Bristol.

A puffy planet already known for quartz clouds

WASP-17b was discovered in 2009 and orbits its star in just 3.7 days, at a temperature of around 1,500 degrees Celsius, according to NASA. Its volume is more than seven times that of Jupiter, though its mass is less than half, which makes its atmosphere unusually extended and easier to study. In 2023, Webb detected tiny quartz crystals in its clouds, the first time quartz was found on an exoplanet.

That puffiness is part of why it was a good first target. More than 6,000 exoplanets have now been confirmed, many of them through NASA's TESS survey, which recently passed 1,000 confirmed planets of its own. Very few have been studied in three dimensions, and WASP-17b now serves as a benchmark.

From day and night to weather maps

The team plans to apply the method to many more planets and to separate conditions by latitude, not just morning and evening, as a first step toward mapping weather-scale patterns on distant worlds. Wakeford said the work gives astronomers a strong mathematical model they can apply to Webb data in general. The research was funded by NASA, the UK's Science and Technology Facilities Council and the Klarman Fellowship.