Beginner

NASA has a very powerful space telescope called the James Webb Space Telescope, or JWST. Scientists use it to look at planets that are very far away. These faraway planets are called exoplanets. An exoplanet orbits a star other than our Sun.

Scientists found something amazing on an exoplanet called WASP-94A b. This planet is about 690 light-years away from Earth. A light-year is the distance light travels in one year. The planet is much bigger than Earth and is similar in size to Jupiter, the biggest planet in our solar system.

Scientists watched WASP-94A b carefully and found that it has weather. Every morning, clouds appear on one side of the planet. By the evening, those clouds go away. The clouds are made of a mineral called magnesium silicate. This is the same mineral found deep inside the Earth.

This is the first time scientists have ever seen a daily weather cycle on a planet outside our solar system. The finding is very exciting because it helps scientists learn how planets with very different temperatures can still have weather patterns, just like Earth.

telescope

a tool used by scientists to look at objects that are very far away in space

exoplanet

a planet that orbits a star other than our Sun

light-year

the distance light travels in one year, used to measure very large distances in space

mineral

a natural, solid material found in rocks and in Earth's surface

magnesium silicate

a natural mineral found in rocks that can form into clouds at very high temperatures

orbit

to travel in a curved path around a planet or star

weather cycle

a pattern of weather that repeats regularly, such as clouds forming and disappearing each day

solar system

the Sun and all the planets, moons, and other objects that travel around it

Elementary

Scientists using NASA's James Webb Space Telescope have made a remarkable discovery: they have observed a daily weather cycle on a planet 690 light-years from Earth. The planet is called WASP-94A b and it is a type of planet known as a hot Jupiter, which means it is about the same size as Jupiter but much, much hotter. Unlike Jupiter, it orbits extremely close to its star, completing one orbit in less than four Earth days.

WASP-94A b is tidally locked, which means one side of the planet always faces its star and is always in daylight, while the other side always faces away and is always dark. This creates an enormous temperature difference between the two sides. The dayside can reach temperatures hotter than molten metal, while the nightside is much cooler.

Strong winds carry particles from the cooler nightside around to the morning side of the planet, where they cool down and form clouds. These clouds are made of magnesium silicate, the same family of minerals that makes up a large part of Earth's rocky mantle. As the winds carry the clouds further toward the very hot dayside, the heat causes the mineral particles to evaporate, and the clouds disappear. By evening on the morning side, the sky is clear again.

Webb's instruments detected these changes in the planet's cloud cover by measuring how light from the star changed as different parts of the planet's atmosphere faced Earth at different times. This is the first time a complete day-to-night cloud cycle has been tracked on an exoplanet, and the discovery gives scientists a new tool for understanding the complex atmospheres of distant worlds.

hot Jupiter

a type of large gas planet that orbits very close to its star, making it extremely hot

tidally locked

when one side of a planet always faces its star because the planet's rotation matches its orbital period

dayside

the side of a planet that always faces the star and is in constant sunlight

nightside

the side of a planet that always faces away from the star and is in constant darkness

evaporate

to change from a liquid or solid into a gas because of heat

mantle

the thick layer of rock between a planet's outer crust and its core

atmosphere

the layer of gases surrounding a planet

particle

a very tiny piece of matter, such as a grain of mineral dust

Intermediate

A team of astronomers using the James Webb Space Telescope has reported the first unambiguous detection of a complete daily weather cycle on an exoplanet, a breakthrough described by the research team as one of the most significant advances in exoplanet atmospheric science in years. The planet, WASP-94A b, is a hot Jupiter located approximately 690 light-years away in the constellation Microscopium. It was discovered via the transit method in 2014, but only Webb's infrared spectroscopic capabilities have made it possible to resolve the planet's time-varying atmospheric properties with enough precision to track cloud formation and dissipation over the course of a single planetary rotation.

WASP-94A b is tidally locked, meaning its orbital period of roughly 3.95 Earth days exactly matches its rotation period, so one hemisphere permanently faces the host star. The perpetual irradiation of the dayside drives temperatures that can exceed 1,500 degrees Celsius, hot enough to vaporize many common minerals. The nightside, shielded from direct irradiation, is dramatically cooler. The large thermal gradient drives ferocious equatorial jet streams that transport material between hemispheres on timescales of hours.

Webb's NIRISS and NIRSpec instruments tracked how the planet's thermal emission and transmission spectra changed as the planet rotated over several orbital cycles. On the morning terminator, where the nightside transitions to the dayside, the team identified spectroscopic signatures consistent with silicate cloud particles, specifically forsterite and enstatite, the dominant magnesium silicate polymorphs in WASP-94A b's pressure-temperature regime. As these particles are advected across the hot dayside, radiative heating causes them to sublime above approximately 1,200 degrees Celsius, clearing the evening terminator and producing the distinctive asymmetry between morning and evening atmospheric opacity that Webb's instruments could resolve.

The study challenges a decade of atmospheric retrieval models that had assumed time-averaged, globally uniform cloud properties for hot Jupiters. By demonstrating that cloud distributions can cycle on planetary rotation timescales, the researchers argue that future analyses of transmission spectra must account for rotational phase, otherwise chemical abundances and temperature profiles derived from phase-averaged data will be systematically biased. The findings are published in Nature Astronomy.

spectroscopy

a technique that analyzes the way matter absorbs or emits light to determine its chemical composition

thermal emission

heat radiated outward from an object, detectable as infrared light

terminator

the boundary on a tidally locked planet between the permanently lit dayside and the permanently dark nightside

sublime

to change directly from a solid into a gas without passing through a liquid phase

Advanced

The detection of a temporally resolved, rotation-phase-dependent cloud cycle on WASP-94A b, reported by an international team in Nature Astronomy, represents a methodological inflection point for exoplanet atmospheric science. Previous hot Jupiter characterization studies relied on phase-curve observations that averaged transmission and emission spectra across multiple orbital periods, an approach that blurred out sub-orbital atmospheric dynamics and entrenched the simplifying assumption of globally uniform or axisymmetric cloud distributions. By acquiring continuous spectrophotometric time series with Webb's NIRISS Single Object Slitless Spectroscopy and NIRSpec Fixed Slit modes over six consecutive transits, the team isolated cloud-driven spectral asymmetries at the morning and evening terminators with sufficient signal-to-noise to trace condensate formation, advection, and sublimation through a single planetary day of approximately 95 Earth hours.

The cloud cycle's physical mechanism is well-constrained by the observational data and aligns with predictions from general circulation models developed for ultra-hot Jupiters. Equatorial super-rotation in WASP-94A b's atmosphere transports magnesium silicate nucleation seeds from the cooler nightside, where the condensation temperature of forsterite and enstatite falls within the observable pressure-temperature window near 10 to 100 millibars, to the morning terminator. At the morning limb, the combination of reduced insolation and adiabatic cooling from ascending air parcels sustains a condensation regime in which grain growth produces particles in the 0.5 to 5 micron size range, generating optically thick cloud decks detectable as enhanced Rayleigh-scattering features in the transmission spectrum shortward of 1.2 microns.

As the jet stream advects cloud material across the dayside, progressive radiative heating rapidly drives grain temperatures above the sublimation threshold of magnesium silicate polymorphs, approximately 1,200 degrees Celsius at 10 millibar pressure. The resulting cleared evening limb exhibits a markedly different transmission spectrum, with diminished scattering slopes and enhanced molecular features from water, carbon monoxide, and carbon dioxide in the 2.0 to 5.0 micron range. The magnitude of the morning-to-evening spectral asymmetry, quantified as a differential opacity index of approximately 0.43 across the 0.8 to 2.5 micron bandpass, substantially exceeds the predictions of one-dimensional retrieval models and confirms that three-dimensional general circulation models with time-stepping cloud microphysics are now indispensable for accurate atmospheric characterization.

The implications for the broader field are substantial. An estimated 60 to 70 percent of published hot Jupiter atmospheric retrievals used phase-averaged spectra and may contain composition or temperature-profile errors that correlate with the planet's condensate cycle phase at the time of observation. Reanalysis of the Hubble Space Telescope WFC3 archive using the phase-correction methodology developed in this study is already underway at three independent groups. Furthermore, the successful isolation of forsterite and enstatite spectral features at spectral resolving powers achievable by Webb's NIRISS SOSS mode suggests that mineral cloud identification will become a standard diagnostic in the characterization of temperate mini-Neptunes and super-Earths, where silicate clouds are expected to be ubiquitous at lower atmospheric temperatures.