WASP-121 b is split between extremes. One side faces its star nonstop and burns at about 2,770 Kelvin. The other stays in darkness and cools to roughly 1,000 Kelvin. Now astronomers have found that even the narrow boundary zones between those halves are not alike.<\/p>\n
Using the James Webb Space Telescope, a team led by Cyril Gapp of the Max Planck Institute for Astronomy<\/a> detected clear differences between the planet\u2019s morning and evening terminators, the regions where day turns to night. The result gives researchers one of their sharpest looks yet at how temperature and chemistry shift across an exoplanet\u2019s atmosphere.<\/p>\n The signal appeared during transit, when WASP-121 b crossed in front of its host star. As starlight passed through the planet\u2019s atmosphere, the gases there filtered specific infrared wavelengths. By tracking how that filtering changed over the course of the transit, the team could watch different longitudes rotate into view.<\/p>\n \u201cWith its unprecedented observational quality, JWST gives us the most detailed glimpses into distant planets to date: By measuring how star light absorption changes as WASP-121 b<\/a> rotates, we probe its atmosphere longitude by longitude,\u201d Gapp said.<\/p>\n Top view of the orbit of the exoplanet WASP-121 b around its star. The planet\u2019s rotation is synchronized to its orbit, both taking about 30 hours to complete. As a result, the planet constantly faces the star with the same side producing distinct day and night sides. The transition zones between those hemispheres are the morning and evening regions. Due to the planet\u2019s proximity to the central star of only 1.9 stellar diameters, the planet rotates by about 30 degrees during its transit. (CREDIT: MPIA (CC BY 4.0)) <\/p>\n Because the planet is so close to its star, tidal forces have locked its rotation. One spin takes as long as one orbit, leaving the same hemisphere always facing the star. During a transit, though, the planet turns enough for astronomers to sample different slices of its atmosphere, including the dawn side leading its orbit and the dusk side trailing behind.<\/p>\n A planet that grows larger at dusk<\/p>\n What the team saw was not symmetrical. The evening terminator absorbed more light than the morning side, especially later in the transit. That points to an atmosphere that is hotter and more expanded at dusk.<\/p>\n The interpretation fits a long-standing picture of fierce eastward winds sweeping heat from the blazing dayside toward the cooler nightside. Those winds would warm the evening terminator, puffing up the atmosphere<\/a> there and making the planet appear slightly larger in cross-section. A bigger cross-section blocks and absorbs more of the star\u2019s light.<\/p>\n WASP-121 b is an ideal place to look for this effect because it rotates by about 30 degrees during a full transit. That is enough to separate the atmospheric signal from the two terminators with unusual precision.<\/p>\n Astronomers usually average transit data across the full event to strengthen the signal. Here, the team did the opposite. They let the signal vary over time and used statistical tests to see whether that changing model fit the observations better than a standard, time-invariant one. It did.<\/p>\n Visualization of WASP-121 b\u2019s orbital motion and phase-dependent transmission spectrum. (CREDIT: Nature Astronomy) <\/p>\n The measurements came from JWST\u2019s NIRSpec instrument, with supporting evidence from NIRISS. In the NIRSpec<\/a> data, the overall brightness dropped slightly more toward the end of the transit, matching the idea that the planet\u2019s absorbing area was increasing as hotter regions rotated into view.<\/p>\n Molecules tell a different part of the story<\/p>\n The pattern was not just about size. It also appeared in the spectrum itself.<\/p>\n A carbon monoxide feature between 4.3 and 5.2 micrometers grew stronger as the transit progressed. The researchers do not think that means carbon monoxide suddenly becomes more abundant on the evening side. Instead, they argue it is mainly a temperature effect. Carbon monoxide remains stable across the atmosphere, so hotter, more expanded gas can strengthen its signal even without adding more molecules.<\/p>\n Water behaved differently. Its signal stayed flat or shrank slightly, which the team interprets as a genuine drop in water molecules on the hotter side. In the upper atmosphere of such an extreme planet<\/a>, temperatures can become high enough to tear water apart into its constituent parts. As hotter gas rotates into view on the trailing limb, the amount of intact water drops.<\/p>\n That contrast matters because it turns the transmission spectrum into more than a heat map. It becomes a way to watch chemistry change from one longitude to another.<\/p>\n The data also hinted at a slight rise in silicon monoxide absorption, though the evidence there is weaker. The spectral feature is narrow, leaving less room for a confident interpretation.<\/p>\n Known transiting exoplanets\u2019 TSMs and rotations during transit estimated assuming synchronous rotation and zero eccentricity. (CREDIT: Nature Astronomy) A better view, but not a complete one<\/p>\n The team compared the observations with atmospheric models designed to simulate how heat spreads through a gas giant\u2019s upper layers. Those models did predict an asymmetry between the two terminators, supporting the broad explanation. But the observed signal was stronger than expected.<\/p>\n That mismatch suggests something may be missing from the standard picture. One likely candidate is clouds.<\/p>\n Not water clouds, but mineral clouds made of materials such as silicates. Earlier work has suggested such clouds may form in these harsh atmospheres. If clouds are more important near the morning terminator, they could block infrared light from hotter layers below and make that region look cooler than simple models predict.<\/p>\n Cloud physics is notoriously difficult to simulate in a changing atmosphere. Condensation, evaporation, and transport all matter, and many exoplanet models<\/a> still treat those effects only crudely or ignore them. When the researchers adjusted their simulations to approximate cloud effects, the results moved closer to the observations. Even so, they stopped short of claiming cloud detection. More sophisticated modeling will be needed.<\/p>\n The new measurements also fit with earlier ground-based work. Previous observations had traced phase-dependent behavior in atoms and ions at optical wavelengths, and infrared data from Gemini-South had suggested that carbon monoxide and water trace different parts of the atmosphere during transit. The new JWST results support that picture while extending it into molecular measurements from space.<\/p>\n Equatorial cross-sections of WASP-121b\u2019s temperature field in the SPARC\/MITgcm. (CREDIT: Nature Astronomy) A method that could spread beyond one world<\/p>\n WASP-121 b is among the most extreme known ultrahot Jupiters, but it may not be unique in offering this kind of atmospheric view. The researchers say other planets with the right temperatures and rotation geometry could also reveal phase-dependent signals during transit.<\/p>\n That matters because transmission spectroscopy has long treated an exoplanet\u2019s limb as if it were mostly uniform. This work shows that assumption can miss important structure. Dawn and dusk on a distant planet may tell different stories.<\/p>\n