Astronomers have mapped the atmospheric outline of the hot Jupiter WASP-17b in new detail, finding that it is not a uniform circular halo around the planet. Instead, the planet’s transmitting atmosphere is asymmetrical: one side extends farther than the other.
The result comes from observations with the James Webb Space Telescope and a method known as transmission strings. The research was co-led by scientists at Cornell University and the University of Bristol and was published in Nature Astronomy.
Looking beyond a simple planetary silhouette
WASP-17b is an extremely hot gas giant roughly 1,300 light-years from Earth. Like other “hot Jupiters,” it orbits close enough to its star that one hemisphere is permanently exposed to stellar radiation while the other remains in continual darkness.
Planetary atmospheres are often represented as neat circular outlines in transit observations, when a planet passes in front of its star. The new work treats that outline as something that can vary around the planet rather than as a single, fixed radius. That distinction allowed the team to measure the shape of the atmosphere across angles spanning the planet’s poles.
The measurements indicate a broadly spherical atmosphere with an uneven contour. The side associated with gas arriving from the permanently illuminated dayside is more extended, while the side associated with gas that has crossed the cooler nightside is less extended.
Heat and wind leave a visible imprint
The proposed explanation is atmospheric circulation. WASP-17b has a broad equatorial jet with high wind speeds that moves material and heat from the dayside toward the nightside. Gas passing across the evening limb of the planet is still hot after coming from the irradiated hemisphere, making that part of the atmosphere more extended.
On the morning limb, the observed gas has traveled through the nightside before returning toward daylight. It has cooled during that journey and appears less extended. The contrast gives researchers a direct view of how radiation, temperature and winds combine to shape an exoplanet atmosphere.
Rather than only measuring which molecules may be present, the approach addresses a different question: what is the physical geometry of the layer absorbing starlight during a transit? The answer matters because a changing atmospheric shape can affect how scientists interpret transit signals.
A method for more detailed exoplanet weather studies
The transmission-strings technique was developed at the University of Bristol by astrophysicists David Grant and Hannah Wakeford. Its application to WASP-17b is presented as the first time an exoplanet’s uneven atmospheric contours have been unraveled in this way using JWST data.
The finding does not mean the planet itself has become distorted into a radically different object. The mapped effect concerns its extended, light-absorbing atmosphere. But it offers an observational route to studying conditions at different positions around a distant world’s limb.
For hot Jupiters, where stellar heating is extreme and atmospheric circulation is vigorous, that spatial detail can help connect transit measurements to the underlying physics. WASP-17b’s larger hot-side atmospheric extent and smaller cooled-side extent provide a concrete example of a planet’s circulation pattern appearing in its observed silhouette.
From averaged atmospheres to mapped worlds
Exoplanet research has increasingly moved from detecting worlds to characterizing them. This result pushes that characterization further by treating a planetary atmosphere as a dynamic, three-dimensional system rather than a single averaged layer.
WASP-17b remains an unusually favorable target because its atmosphere is large and intensely heated. Even so, the work demonstrates that JWST observations can reveal more than an overall transit depth. They can expose differences across an atmosphere that arise from the movement of heat and gas around a planet.
As similar techniques are applied to other planets, astronomers may be able to compare atmospheric circulation across a broader range of worlds. For now, WASP-17b provides a clear case in which a distant planet’s atmosphere is visibly shaped by the division between its permanent day and permanent night.
This article is based on reporting by Phys.org. Read the original article.
Originally published on phys.org
