A Cornell-led astronomy team has used James Webb Space Telescope observations to map the uneven outer edge of a planet more than 1,000 light-years away. The result gives researchers a more detailed picture of how heat and powerful winds shape the atmosphere of WASP-17b, a gas giant that orbits close to its star.
Models often treat a distant planet’s outline as a circle when analyzing the light that passes through its atmosphere. The team’s method, called “transmission strings,” instead measures the distance from the planet’s center to its atmospheric edge at different points. That allowed the researchers to resolve a slightly lopsided outline rather than one average ring, according to a Cornell account of the study.
The shape is not evidence of a solid egg-shaped planet. It describes the absorbing atmosphere detected as WASP-17b crosses in front of its star. The planet’s permanent day side is heated by the star, and winds move heat toward its dark night side, changing how far the atmosphere extends at different locations.
Looking around the planet’s edge
Lili Alderson, a Cornell Klarman postdoctoral fellow and the paper’s first author, said the method replaces the assumption that the atmospheric radius is the same all the way around. The team found a more extended region on the evening side of the equator, where winds carry heat away from the illuminated side. The cooler morning side was less extended.
Those differences were broadly consistent with theoretical predictions and earlier modeling, Cornell said. The researchers also found water and carbon dioxide distributed fairly consistently around the atmospheric edge, even while temperature and shape varied. The findings were published Oct. 14 in Nature Astronomy.
The observations came from the Webb telescope’s Deep Reconnaissance of Exoplanet Atmospheres through Multi-instrument Spectroscopy program. Cornell astronomy associate professor Nikole Lewis led that observing program and is a co-author. The paper includes researchers from the University of Bristol and other institutions; Hannah Wakeford of Bristol is its second author.
A method for studying other worlds
The team says the new approach could be applied to other planets, giving astronomers a way to compare conditions across latitude as well as between day and night. It is an early demonstration on one planet, not a full weather map of the thousands of known exoplanets.
Previous Cornell-led work used the Webb telescope to study changes in light as a planet moved behind its star. The new method examines the opposite part of the orbit, when the planet crosses in front. Together, such observations can test whether atmospheric models describe conditions on distant worlds accurately.
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