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NASA’s Webb Maps Weather on Planet 280 Light-Years Away

Illustration showing a hazy blue planet against the black background of space. The planet is in the left side of the frame. The axis is tilted roughly 20 degrees counter-clockwise from vertical. The eastern side (right half) is lit by a star out of view and the western side (left half) is in shadow. The terminator (the boundary between the day and night sides) is fuzzy. There are white patchy clouds visible on the dayside, near the terminator, along the equator, that appear to be originating from the nightside.

This artist’s concept shows what the hot gas-giant exoplanet WASP-43 b could look like.

Credits: NASA, ESA, CSA, Ralf Crawford (STScI)

An international team of researchers has successfully used NASA’s James Webb Space Telescope to map the weather on the hot gas-giant exoplanet WASP-43 b.

Precise brightness measurements over a broad spectrum of mid-infrared light, combined with 3D climate models and previous observations from other telescopes, suggest the presence of thick, high clouds covering the nightside, clear skies on the dayside, and equatorial winds upwards of 5,000 miles per hour mixing atmospheric gases around the planet.

The investigation is just the latest demonstration of the exoplanet science now possible with Webb’s extraordinary ability to measure temperature variations and detect atmospheric gases trillions of miles away.

Image: Hot Gas-Giant Exoplanet WASP-43 b (Artist’s Concept)

This artist’s concept shows what the hot gas-giant exoplanet WASP-43 b could look like. WASP-43 b is a Jupiter-sized planet roughly 280 light-years away, in the constellation Sextans. The planet orbits its star at a distance of about 1.3 million miles, completing one circuit in about 19.5 hours. Because it is so close to its star, WASP-43 b is probably tidally locked: Its rotation rate and orbital period are the same, such that one side faces the star at all times.

Credits: NASA, ESA, CSA, Ralf Crawford (STScI)

Tidally Locked “Hot Jupiter”

WASP-43 b is a “hot Jupiter” type of exoplanet: similar in size to Jupiter, made primarily of hydrogen and helium, and much hotter than any of the giant planets in our own solar system. Although its star is smaller and cooler than the Sun, WASP-43 b orbits at a distance of just 1.3 million miles – less than 1/25^th the distance between Mercury and the Sun.

With such a tight orbit, the planet is tidally locked, with one side continuously illuminated and the other in permanent darkness. Although the nightside never receives any direct radiation from the star, strong eastward winds transport heat around from the dayside.

Since its discovery in 2011, WASP-43 b has been observed with numerous telescopes, including NASA’s Hubble and now-retired Spitzer space telescopes.

“With Hubble, we could clearly see that there is water vapor on the dayside. Both Hubble and Spitzer suggested there might be clouds on the nightside,” explained Taylor Bell, researcher from the Bay Area Environmental Research Institute and lead author of a study published today in Nature Astronomy. “But we needed more precise measurements from Webb to really begin mapping the temperature, cloud cover, winds, and more detailed atmospheric composition all the way around the planet.”

Mapping Temperature and Inferring Weather

Although WASP-43 b is too small, dim, and close to its star for a telescope to see directly, its short orbital period of just 19.5 hours makes it ideal for phase curve spectroscopy, a technique that involves measuring tiny changes in brightness of the star-planet system as the planet orbits the star.

Since the amount of mid-infrared light given off by an object depends largely on how hot it is, the brightness data captured by Webb can then be used to calculate the planet’s temperature.

Image: Hot Gas-Giant Exoplanet WASP-43 b (MIRI Phase Curve)

Graphic titled Hot Gas-Giant Exoplanet WASP-43 b: Phase Curve 1 ¼ Orbits; MIRI Low-Resolution Spectroscopy. y-axis: Brightness of Planet + Star (5 to 12 micron emitted light), ranging from dimmer at bottom to brighter at top. x-axis: Elapsed Time (Hours) ranging from 0 to 24 in increments of 4. Thousands of orange data points form a thick, clear pattern, with no outliers and very little scatter. Curve forms a subtle sine wave with crests from 2-4 hours and 20-24 hours, and trough in the middle from 10-14 hours. Curve interrupted by 3 prominent U-shaped valleys: 2 shallow valleys at the wave crests at 2 hours and 22 hours, and a very deep valley in the middle of the trough at 12 hours. The base of the shallow valleys at 2 and 22 hours are labeled “starlight only,” with tops of valley walls on either side labeled “dayside + star.” Center of deep valley at 12 hours is labeled “nightside + partially-blocked star,” with tops of valley walls on either side labeled “nightside + star.”

This phase curve, captured by the MIRI low resolution spectrometer on NASA’s James Webb Space Telescope, shows the change in brightness of the WASP-43 system over time as the planet orbits its star. The system appears brightest when the hot dayside of the planet is facing the telescope, just before and after it passes behind the star. The system grows dimmer as the planet continues its orbits and the nightside rotates into view. It brightens again after passing in front of the star as the dayside rotates back into view. WASP-43 b is a hot Jupiter roughly 280 light-years away, in the constellation Sextans.

Credits: Science: Taylor J. Bell (BAERI); Joanna Barstow (Open University); Michael Roman (University of Leicester) Graphic Design: NASA, ESA, CSA, Ralf Crawford (STScI)

The team used Webb’s MIRI (Mid-Infrared Instrument) to measure light from the WASP-43 system every 10 seconds for more than 24 hours. “By observing over an entire orbit, we were able to calculate the temperature of different sides of the planet as they rotate into view,” explained Bell. “From that, we could construct a rough map of temperature across the planet.”

The measurements show that the dayside has an average temperature of nearly 2,300 degrees Fahrenheit (1,250 degrees Celsius) – hot enough to forge iron. Meanwhile, the nightside is significantly cooler at 1,100 degrees Fahrenheit (600 degrees Celsius). The data also helps locate the hottest spot on the planet (the “hotspot”), which is shifted slightly eastward from the point that receives the most stellar radiation, where the star is highest in the planet’s sky. This shift occurs because of supersonic winds, which move heated air eastward.

“The fact that we can map temperature in this way is a real testament to Webb’s sensitivity and stability,” said Michael Roman, a co-author from the University of Leicester in the U.K.

To interpret the map, the team used complex 3D atmospheric models like those used to understand weather and climate on Earth. The analysis shows that the nightside is probably covered in a thick, high layer of clouds that prevent some of the infrared light from escaping to space. As a result, the nightside – while very hot – looks dimmer and cooler than it would if there were no clouds.

Image: Hot Gas-Giant Exoplanet WASP-43 b (Temperature Maps)

Graphic titled “Hot Gas-giant Exoplanet WASP-43 b: Temperature Maps; MIRI Low-Resolution Spectroscopy” showing purple to yellow temperature maps of planet’s telescope-facing hemisphere at 4 orbital positions. Gray line with arrows pointing counterclockwise forms orbital path around star. Temperature scale at lower left, labeled in °F and K, grades from purple at left to yellow at right: 1,000°F is purple; 1,500°F pink; 2,000°F orange; 2,500°F yellow. 1,000 K dark pink. 1,500 K orange-yellow. Planet behind star, labeled “Permanent Dayside”: Hemisphere is yellow in center, grading to orange at edges. Planet left of star: Color grades from yellow at right edge facing star to purple at left edge facing away. Planet in front of star, labeled “Permanent Nightside” is purple slightly right of center, grading to dark pink at edges. Planet right of star: Color grades from yellow at left edge facing star to purple at right edge facing away.

This set of maps shows the temperature of the visible side of the hot gas-giant exoplanet WASP-43 b, as it orbits its star. The dayside of the planet is visible just before and after it passes behind the star. The temperatures were calculated based on more than 8,000 brightness measurements of 5- to 12-micron mid-infrared light detected from the star-planet system by MIRI (the Mid-Infrared Instrument) on NASA’s James Webb Space Telescope. In general, the hotter an object is, the more mid-infrared light it gives off.

Credits: Science: Taylor J. Bell (BAERI); Joanna Barstow (Open University); Michael Roman (University of Leicester) Graphic Design: NASA, ESA, CSA, Ralf Crawford (STScI)

Animation: Hot Gas-Giant Exoplanet WASP-43 b (Temperature Maps)

Global temperature map of the hot gas-giant exoplanet WASP-43 b. This map was made based on the brightness of 5- to 12-micron mid-infrared light detected from the planet by MIRI (the Mid-Infrared Instrument) on NASA’s James Webb Space Telescope. In general, the hotter an object is, the more mid-infrared light it gives off. Although the planet is far too close to the blinding light of its star to see on its own, it is possible to calculate its brightness by measuring the brightness of the star-planet system as a whole, and then subtracting the amount of light coming from the star (measured when the planet is behind the star). Webb was able to measure each side of the planet by observing over an entire 19.5-hour orbit. The planet is tidally locked, which means that its rotation rate is the same as its orbital period, so different sides rotate into view as the planet moves around the star. WASP-43 b has an average temperature of about 2,280°F (1,250°C) on the dayside and 1,115°F (600°C) on the nightside. The temperature map also shows that the nightside is probably covered in thick, high clouds. Clouds prevent some of the infrared energy from escaping to space, making the nightside appear cooler than it would if there were no clouds.

Thomas Muller, MPIA

Missing Methane and High Winds

The broad spectrum of mid-infrared light captured by Webb also made it possible to measure the amount of water vapor (H₂O) and methane (CH₄) around the planet. “Webb has given us an opportunity to figure out exactly which molecules we’re seeing and put some limits on the abundances,” said Joanna Barstow, a co-author from the Open University in the U.K.

The spectra show clear signs of water vapor on the nightside as well as the dayside of the planet, providing additional information about how thick the clouds are and how high they extend in the atmosphere.

Surprisingly, the data also shows a distinct lack of methane anywhere in the atmosphere. Although the dayside is too hot for methane to exist (most of the carbon should be in the form of carbon monoxide), methane should be stable and detectable on the cooler nightside.

“The fact that we don't see methane tells us that WASP-43b must have wind speeds reaching something like 5,000 miles per hour,” explained Barstow. “If winds move gas around from the dayside to the nightside and back again fast enough, there isn’t enough time for the expected chemical reactions to produce detectable amounts of methane on the nightside.”

The team thinks that because of this wind-driven mixing, the atmospheric chemistry is the same all the way around the planet, which wasn’t apparent from past work with Hubble and Spitzer.

The MIRI observation of WASP-43 b was conducted as part of the Webb Early Release Science programs, which are providing researchers with a vast set of robust, open-access data for studying a wide array of cosmic phenomena.The James Webb Space Telescope is the world's premier space science observatory. Webb is solving mysteries in our solar system, looking beyond to distant worlds around other stars, and probing the mysterious structures and origins of our universe and our place in it. Webb is an international program led by NASA with its partners, ESA (European Space Agency) and the Canadian Space Agency.

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The research results can be viewed in the Nature Astronomy.