Astrophysicists discover complex weather of starless “Super-Jupiter”

Posted on: 03 March 2025

Using NASA’s James Webb Space Telescope to monitor a broad spectrum of infrared light emitted over two full rotation periods by the Super-Jupiter, which is formally known as “SIMP 0136”, the team was able to detect variations in cloud layers, temperature, and carbon chemistry that were previously hidden from view.

Detailed characterisation of objects like these is essential preparation for direct imaging of exoplanets (planets outside our solar system), with NASA’s Nancy Grace Roman Space Telescope, which is scheduled to begin operations in 2027.

Rapidly rotating, and free-floating

SIMP 0136 is a rapidly rotating, free-floating object roughly 13 times the mass of Jupiter, located in the Milky Way just 20 light-years from Earth.

This artist’s concept shows what the isolated planetary-mass object SIMP 0136 could look like based on recent observations from NASA’s James Webb Space Telescope and previous observations from Hubble, Spitzer, and numerous ground-based telescopes. Image credit: NASA, ESA, CSA, Joseph Olmsted (STScI).

Although it is not classified as a gas giant exoplanet — it doesn’t orbit a star and may instead be a brown dwarf — SIMP 0136 is an ideal target for exo-meteorology as it is the brightest object of its kind in the northern sky. Because it is isolated, it can be observed with no fear of light contamination or variability caused by a host star. And its short rotation period of just 2.4 hours makes it possible to survey very efficiently.

Prior to the Webb observations, SIMP 0136 had been studied extensively using ground-based observatories and NASA’s Hubble and Spitzer space telescopes.

“We already knew that it varies in brightness, and we were confident that there are patchy cloud layers that rotate in and out of view and evolve over time,” explained Allison McCarthy, doctoral student at Boston University and lead author of the study published today in The Astrophysical Journal Letters.

“We also thought there could be temperature variations, chemical reactions, and possibly some effects of auroral activity affecting the brightness, but we weren’t sure.”

To figure it out, the team needed Webb’s ability to measure very precise changes in brightness over a broad range of wavelengths.

Charting thousands of infrared rainbows

The team observed SIMP 0136 over two consecutive rotations, using two instruments on the JWST that together provide information across infrared wavelengths. The result was hundreds of detailed light curves, each showing the change in brightness of a very precise wavelength (colour) as different sides of the object rotated into view.

“To see the full spectrum of this object change over the course of minutes was incredible,” said principal investigator Johanna Vos, Associate Professor in Trinity’s School of Physics. “Until now, we only had a little slice of the near-infrared spectrum from Hubble, and a few brightness measurements from Spitzer.” 

The team noticed almost immediately that there were several distinct light-curve shapes. At any given time, some wavelengths were growing brighter, while others were becoming dimmer or not changing much at all. A number of different factors must be affecting the brightness variations.

“Imagine watching Earth from far away. If you were to look at each colour separately, you would see different patterns that tell you something about its surface and atmosphere, even if you couldn’t make out the individual features,” explained co-author Philip Muirhead, Boston University. “Blue would increase as oceans rotate into view. Changes in brown and green would tell you something about soil and vegetation.”

These light curves show the change in brightness of three different sets of wavelengths (colors) of near-infrared light coming from the isolated planetary-mass object SIMP 0136 as it rotated. Image Credit: NASA, ESA, CSA, Joseph Olmsted (STScI).

Patchy clouds, hot spots, and carbon chemistry

To figure out what could be causing the variability on SIMP 0136, the team used atmospheric models to show where in the atmosphere each wavelength of light was originating.  

“Different wavelengths provide information about different depths in the atmosphere,” explained McCarthy. “We started to realise that the wavelengths that had the most similar light-curve shapes also probed the same depths, which reinforced this idea that they must be caused by the same mechanism.”

One group of wavelengths originates deep in the atmosphere where there could be patchy clouds made of iron particles. A second group comes from higher clouds thought to be made of tiny grains of silicate minerals. The variations in both of these light curves are driven by patchiness of the cloud layers.  

And a third group of wavelengths originates at very high altitude, far above the clouds, and seems to track temperature. Bright “hot spots” could be related to auroras that were previously detected at radio wavelengths, or to upwelling of hot gas from deeper in the atmosphere.

Some of the light curves cannot be explained by either clouds or temperature, but instead show variations related to atmospheric carbon chemistry.

“We haven’t really figured out the chemistry part of the puzzle yet,” added Prof. Vos. “But it is clear that JWST provides us with a unique ability to study how the presence of certain molecules may change across the atmosphere and over time.”

This research was conducted as part of Webb’s General Observer (GO) Program 3548. Other authors on the study include Dr Evert Nasedkin and Cian O’Toole from Trinity College Dublin.

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