Astronomers from Cornell and the University of Bristol, using the James Webb Space Telescope, have found that WASP-17 b, a puffed-up 'hot Jupiter' about 1,300 light years away, is the first exoplanet shown to be noticeably lopsided. The evening edge of its atmosphere expands to 1.8 times the size of its morning edge, the result of a 650 kelvin temperature difference between the two sides, according to a new paper in Nature Astronomy.
Researchers from Cornell and the University of Bristol, led by Lili Anderson of Bristol, studied WASP-17 b using a technique called transmission strings. Rather than treating the planet's silhouette as a uniform circle, the method, built on modelling software called Harmonica, treats it like an adjustable loop. As the planet enters and exits its transit, the model calculates how the atmosphere's thickness changes around its edges and analyses the wavelengths of starlight passing through those different angles. Webb's infrared camera had the resolution needed to distinguish the planet's atmospheric edges.
WASP-17 b, discovered in 2009 in the constellation Scorpius, is an extreme version of the hot Jupiter category. Its year lasts just four days. It has about half the mass of Jupiter but is puffed up to nearly twice its diameter, giving it a density close to that of styrofoam. Because it is tidally locked, one side always faces its star. The paper reports a temperature jump from 858 kelvin at the morning edge to 1,522 kelvin at the evening terminator. Hot gas expands, so the evening atmosphere swells to 1.8 times the morning side. From Webb's vantage point, the planet looks like an egg resting on its side.
Powerful eastward jet streams along the equator carry heated gas from the day side to the night side, where it cools and deflates. Despite the dramatic shape difference, the atmosphere's chemistry is broadly well balanced: water vapour and carbon dioxide are evenly spread around the planet, including between the equator and the poles. Methane and sulfur dioxide, however, appear better at surviving away from the jet streams, with changes in their composition at the poles.
The finding adds to a detailed picture of a well studied world. Astronomers have previously mapped infrared heat from its dayside, studied its nighttime glow, and caught evidence of tiny quartz crystals floating in its upper atmosphere. The new paper adds a map of atmospheric size and makeup, bringing together one of the most complete three dimensional views of an exoplanet's atmosphere ever produced.
WASP-17 b may not be the last planet mapped this way. Webb and more powerful telescopes planned later in the century should be able to apply the same approach to other exoplanets.
Analysis
Why This Matters
- The finding is the first direct evidence that an exoplanet's atmosphere can be visibly lopsided, which means the common assumption that transiting planets are uniform rings is incomplete.
- The transmission string method gives researchers a way to map morning and evening edges separately, opening up more detailed atmospheric studies for other hot Jupiters.
- A fuller three dimensional picture of WASP-17 b helps test how tidal locking, heat transport and jet streams shape exoplanet weather and chemistry.
Background
Transmission spectroscopy, the standard way to study exoplanet atmospheres, works by analysing starlight that filters through the atmosphere during a transit and averaging it over the whole planetary silhouette. That approach is useful for identifying global chemistry, but masks regional differences between a planet's day side, night side and poles. WASP-17 b, a hot Jupiter, belongs to a class of gas giants that orbit very close to their stars, making them strongly heated and easy to study. Tidal locking, where one hemisphere permanently faces the star, is common among such planets and is thought to drive extreme temperature contrasts and powerful winds.
Key Perspectives
Research team: Scientists at Bristol and Cornell argue that the lopsided shape matters, because it shows that averaging over a planet's full silhouette misses real structure. Their new technique is designed to capture that structure and, they suggest, can be applied to other exoplanets.
Wider exoplanet community: The result extends a decade and a half of observations of WASP-17 b, adding shape and chemistry to existing maps of its dayside heat, nighttime glow and quartz clouds. The method could become a standard tool for Webb observations.
Skeptics: This is one planet, and the shape is inferred through modelling software rather than direct imaging. Whether the asymmetry is typical of hot Jupiters, and whether the polar chemistry signals hold up, will depend on follow-up observations and application of the technique to a larger sample.
What to Watch
- Whether Webb time is awarded to apply transmission strings to other hot Jupiters in upcoming observation cycles.
- Follow-up observations of WASP-17 b to test the methane and sulfur dioxide changes detected near the poles.
- Later generation telescopes, planned for later in the century, that could extend the method to smaller or less extremely heated exoplanets.