Webb Maps an Exoplanet's Atmosphere as Egg-Shaped, With a Puffed-Up Evening Side Hotter Than Its Morning Edge

For decades astronomers have treated the edge of an exoplanet's atmosphere as a ring of uniform height. A team using the James Webb Space Telescope has now shown it is not, on a puffy gas giant more than 1,000 light-years away.

The study, published on 14 October in Nature Astronomy, is titled "A Spatial Map of the Atmospheric Limb of a Hot Jupiter". It is led by Lili Alderson of Cornell University, with Hannah Wakeford of the University of Bristol, and the authors call it a proof of concept, according to Cornell.

What did Webb see on WASP-17b?

WASP-17b is nearly twice Jupiter's width with half its mass, and it orbits its star every 3.7 days, Earth.com reports. Webb watched it cross in front of its star in March 2023, using three instruments across 24 wavelength bands. As the planet transits, starlight filters through the atmosphere at its rim, and the new method, called transmission strings, measures how far that atmosphere extends from the planet's centre at different angles, instead of averaging the whole rim into one spectrum.

The atmosphere came out slightly egg-shaped. The evening side is more extended than the morning side, and Cornell says this fits the picture of winds carrying heat from the permanently lit dayside toward the night side. Earth.com puts the evening side at about 1,250 degrees Celsius against about 590 on the morning side, a gap larger than those measured on three other hot planets. Those figures come from the Earth.com summary rather than the paper, which Cornell does not detail.

Why does the shape of an atmosphere matter?

It turns a dot into a place. Water vapour and carbon dioxide appeared fairly uniform around the rim, as models predicted. Wakeford said the approach lets scientists track local weather, and Alderson said it shows planets are three-dimensional objects.